Cosmological simulations of structure formation predict that galaxy clusters continue to grow and evolve through ongoing
mergers with group-scale systems. During these merging events, the ram pressure applied by the intracluster medium acts to strip
the gas from the infalling groups, forming large tails of stripped gas, which eventually become part of the main cluster. In this
work, we present a detailed analysis of our new deep Chandra observations of the NGC 4839 group falling into the nearby Coma
cluster, providing a unique opportunity to explore the way galaxy clusters in the local universe continue to grow. Our analysis
reveals a cold front feature at the leading head of the group, preceded by a bow shock of hot gas in front with a Mach number
of ∼ 1.5. The power spectrum of surface brightness fluctuations in the tail shows that the slope gets less steep as the distance
from the leading head increases, changing from −2.35+0.07
−0.06 at the inner part of the tail to −1.37+0.09
−0.07 at the outermost part of
the tail. These values are shallower than the slope of the Kolmogorov 2D power spectrum, indicating that thermal conduction is
being suppressed throughout the tail, enabling long-lived small-scale turbulence, which would typically be washed out if thermal
conduction was not inhibited. The characteristic amplitude of surface brightness fluctuations in the tail suggests a mild level of
turbulence with a Mach number in the range of 0.1–0.5, agreeing with that found for the infalling group in Abell 2142.
Detection of anisotropic satellite quenching in galaxy clusters up to z ∼ 1Sérgio Sacani
Satellite galaxies in the cluster environment are more likely to be quenched than galaxies in the general field. Recently, it has
been reported that satellite galaxy quenching depends on the orientation relative to their central galaxies: satellites along the
major axis of centrals are more likely to be quenched than those along the minor axis. In this paper, we report a detection
of such anisotropic quenching up to z ∼ 1 based on a large optically selected cluster catalogue constructed from the Hyper
Suprime-Cam Subaru Strategic Program. We calculate the quiescent satellite galaxy fraction as a function of orientation angle
measured from the major axis of central galaxies and find that the quiescent fractions at 0.25 < z < 1 are reasonably fitted
by sinusoidal functions with amplitudes of a few per cent. Anisotropy is clearer in inner regions (<r200m) of clusters and not
significant in cluster outskirts (>r200m). We also confirm that the observed anisotropy cannot be explained by differences in
local galaxy density or stellar mass distribution along the two axes. Quiescent fraction excesses between the two axes suggest
that the quenching efficiency contributing to the anisotropy is almost independent of stellar mass, at least down to our stellar
mass limit of M∗ = 1 × 1010 M. Finally, we argue that the physical origins of the observed anisotropy should have shorter
quenching time-scales than ∼ 1 Gyr, like ram-pressure stripping, because, for anisotropic quenching to be observed, satellites
must be quenched before their initial orientation angles are significantly changed.
Solving the Multimessenger Puzzle of the AGN-starburst Composite Galaxy NGC 1068Sérgio Sacani
Multiwavelength observations indicate that some starburst galaxies show a dominant nonthermal contribution from
their central region. These active galactic nuclei (AGN)-starburst composites are of special interest, as both
phenomena on their own are potential sources of highly energetic cosmic rays and associated γ-ray and neutrino
emission. In this work, a homogeneous, steady-state two-zone multimessenger model of the nonthermal emission
from the AGN corona as well as the circumnuclear starburst region is developed and subsequently applied to the
case of NGC 1068, which has recently shown some first indications of high-energy neutrino emission. Here, we
show that the entire spectrum of multimessenger data—from radio to γ-rays including the neutrino constraint—can
be described very well if both, starburst and AGN corona, are taken into account. Using only a single emission
region is not sufficient.
Detection of anisotropic satellite quenching in galaxy clusters up to z ∼ 1Sérgio Sacani
Satellite galaxies in the cluster environment are more likely to be quenched than galaxies in the general field. Recently, it has
been reported that satellite galaxy quenching depends on the orientation relative to their central galaxies: satellites along the
major axis of centrals are more likely to be quenched than those along the minor axis. In this paper, we report a detection
of such anisotropic quenching up to z ∼ 1 based on a large optically selected cluster catalogue constructed from the Hyper
Suprime-Cam Subaru Strategic Program. We calculate the quiescent satellite galaxy fraction as a function of orientation angle
measured from the major axis of central galaxies and find that the quiescent fractions at 0.25 < z < 1 are reasonably fitted
by sinusoidal functions with amplitudes of a few per cent. Anisotropy is clearer in inner regions (<r200m) of clusters and not
significant in cluster outskirts (>r200m). We also confirm that the observed anisotropy cannot be explained by differences in
local galaxy density or stellar mass distribution along the two axes. Quiescent fraction excesses between the two axes suggest
that the quenching efficiency contributing to the anisotropy is almost independent of stellar mass, at least down to our stellar
mass limit of M∗ = 1 × 1010 M. Finally, we argue that the physical origins of the observed anisotropy should have shorter
quenching time-scales than ∼ 1 Gyr, like ram-pressure stripping, because, for anisotropic quenching to be observed, satellites
must be quenched before their initial orientation angles are significantly changed.
Solving the Multimessenger Puzzle of the AGN-starburst Composite Galaxy NGC 1068Sérgio Sacani
Multiwavelength observations indicate that some starburst galaxies show a dominant nonthermal contribution from
their central region. These active galactic nuclei (AGN)-starburst composites are of special interest, as both
phenomena on their own are potential sources of highly energetic cosmic rays and associated γ-ray and neutrino
emission. In this work, a homogeneous, steady-state two-zone multimessenger model of the nonthermal emission
from the AGN corona as well as the circumnuclear starburst region is developed and subsequently applied to the
case of NGC 1068, which has recently shown some first indications of high-energy neutrino emission. Here, we
show that the entire spectrum of multimessenger data—from radio to γ-rays including the neutrino constraint—can
be described very well if both, starburst and AGN corona, are taken into account. Using only a single emission
region is not sufficient.
The physical conditions_in_a_pre_super_star_cluster_molecular_cloud_in_the_an...Sérgio Sacani
Artigo descreve estudo feitos pelos astrônomos utilizando o ALMA para descobrir um proto-aglomerado globular de estrelas gigantes se formando no interior das galáxias Antenas, o famoso par de galáxias em interação. É a primeira vez que os astrônomos conseguem observar um objeto desse tipo nos seus estágios iniciais de vida e com o ambiente ao redor inalterado.
Observations of a pre-merger shock in colliding clusters of galaxiesSérgio Sacani
Clusters of galaxies are the largest known gravitationally bound
structures in the Universe. When clusters collide, they create
merger shocks on cosmological scales, which transform most
of the kinetic energy carried by the cluster gaseous halos into
heat1–3
. Observations of merger shocks provide key information
on the merger dynamics, and enable insights into the formation and thermal history of the large-scale structures. Nearly
all of the merger shocks are found in systems where the clusters have already collided4–12; knowledge of shocks in the premerger phase is a crucial missing ingredient13,14. Here, we report
on the discovery of a unique shock in a cluster pair 1E 2216.0-
0401 and 1E 2215.7-0404. The two clusters are observed at an
early phase of major merger. Contrary to all the known merger
shocks observed ubiquitously on merger axes, the new shock
propagates outward along the equatorial plane of the merger.
This discovery uncovers an important epoch in the formation
of massive clusters, when the rapid approach of the cluster
pair leads to strong compression of gas along the merger axis.
Current theoretical models15,16 predict that the bulk of the shock
energy might be dissipated outside the clusters, and eventually
turn into heat of the pristine gas in the circum-cluster space.
A rare case of FR I interaction with a hot X-ray bridge in the A2384 galaxy c...Sérgio Sacani
Clusters of varying mass ratios can merge and the process significantly disturbs
the cluster environments and alters their global properties. Active radio galaxies are
another phenomenon that can also affect cluster environments. Radio jets can interact
with the intra-cluster medium (ICM) and locally affect its properties. Abell 2384
(hereafter A2384) is a unique system that has a dense, hot X-ray filament or bridge
connecting the two unequal mass clusters A2384(N) and A2384(S). The analysis of its
morphology suggests that A2384 is a post-merger system where A2384(S) has already
interacted with the A2384(N), and as a result hot gas has been stripped over a ∼ 1
Mpc region between the two bodies. We have obtained its 325 MHz GMRT data,
and we detected a peculiar FR I type radio galaxy which is a part of the A2384(S).
One of its radio lobes interacts with the hot X-ray bridge and pushes the hot gas in
the opposite direction. This results in displacement in the bridge close to A2384(S).
Based on Chandra and XMM-Newton X-ray observations, we notice a temperature and
entropy enhancement at the radio lobe-X-ray plasma interaction site, which further
suggests that the radio lobe is changing thermal plasma properties. We have also
studied the radio properties of the FR I radio galaxy, and found that the size and
radio luminosity of the interacting north lobe of the FR I galaxy are lower than those
of the accompanying south lobe.
The xmm newton-view_of_the_central_degrees_of_the_milk_waySérgio Sacani
Novas imagens do Observatório de Raios-X XMM-Newton da ESA revelaram alguns dos processos mais intensos que acontecem no coração da nossa Via Láctea.
As fontes brilhantes e pontuais que se destacam por toda imagem indicam os sistemas estelares binários onde uma das estrelas atingiu o final de sua vida, desenvolvendo para um objeto compacto e denso – uma estrela de nêutrons ou um buraco negro.
A região central da Via Láctea também contém jovens estrelas e aglomerados estelares e algumas dessas fontes são visíveis como pontos brancos e vermelhos brilhando na imagem, que se espalha por 1000 anos-luz.
A maior parte da ação ocorre no centro, onde nuvens difusas de gás estão sendo cavadas por ventos poderosos soprados por estrelas jovens, bem como por supernovas.
The Expansion of the X-Ray Nebula Around η CarSérgio Sacani
The massive colliding wind binary system η Car is embedded in an X-ray emitting region having a characteristic
temperature of a few million degrees, associated with ejecta produced during the 1840s, and in earlier outbursts.
We use CHANDRA X-ray imaging observations obtained over the past two decades to directly measure the
expansion of the X-ray nebula for the first time. A combined CHANDRA/ACIS image shows a faint, nearly
uniform elliptic structure. This faint elliptical “shell” has a similar orientation and shape as the Homunculus nebula
but is about 3 times larger. We measure proper motions of brighter regions associated with the X-ray emitting ring.
We compare spectra of the soft X-ray emitting plasma in CHANDRA/ACIS and XMM-Newton PN observations
and show that the PN observations indicate a decline in X-ray flux which is comparable to that derived from
NICER observations. We associate the diffuse elliptical emission surrounding the bright X-ray “ring” with the blast
wave produced during the Great Eruption. We suggest that the interaction of this blast wave with pre-existing
clumps of ejecta produces the bright, broken X-ray emitting ring. We extrapolate the trend in X-ray energy back to
the time of the Great Eruption using a simple model and show that the X-ray energy was comparable to the kinetic
energy of the Homunculus, suggesting equipartition of energy between fast, low-density ejecta and slower, dense
ejecta.
The binding of cosmological structures by massless topological defectsSérgio Sacani
Assuming spherical symmetry and weak field, it is shown that if one solves the Poisson equation or the Einstein field
equations sourced by a topological defect, i.e. a singularity of a very specific form, the result is a localized gravitational
field capable of driving flat rotation (i.e. Keplerian circular orbits at a constant speed for all radii) of test masses on a thin
spherical shell without any underlying mass. Moreover, a large-scale structure which exploits this solution by assembling
concentrically a number of such topological defects can establish a flat stellar or galactic rotation curve, and can also deflect
light in the same manner as an equipotential (isothermal) sphere. Thus, the need for dark matter or modified gravity theory is
mitigated, at least in part.
EWOCS-I: The catalog of X-ray sources in Westerlund 1 from the Extended Weste...Sérgio Sacani
Context. With a mass exceeding several 104 M⊙ and a rich and dense population of massive stars, supermassive young star clusters
represent the most massive star-forming environment that is dominated by the feedback from massive stars and gravitational interactions
among stars.
Aims. In this paper we present the Extended Westerlund 1 and 2 Open Clusters Survey (EWOCS) project, which aims to investigate
the influence of the starburst environment on the formation of stars and planets, and on the evolution of both low and high mass stars.
The primary targets of this project are Westerlund 1 and 2, the closest supermassive star clusters to the Sun.
Methods. The project is based primarily on recent observations conducted with the Chandra and JWST observatories. Specifically,
the Chandra survey of Westerlund 1 consists of 36 new ACIS-I observations, nearly co-pointed, for a total exposure time of 1 Msec.
Additionally, we included 8 archival Chandra/ACIS-S observations. This paper presents the resulting catalog of X-ray sources within
and around Westerlund 1. Sources were detected by combining various existing methods, and photon extraction and source validation
were carried out using the ACIS-Extract software.
Results. The EWOCS X-ray catalog comprises 5963 validated sources out of the 9420 initially provided to ACIS-Extract, reaching a
photon flux threshold of approximately 2 × 10−8 photons cm−2
s
−1
. The X-ray sources exhibit a highly concentrated spatial distribution,
with 1075 sources located within the central 1 arcmin. We have successfully detected X-ray emissions from 126 out of the 166 known
massive stars of the cluster, and we have collected over 71 000 photons from the magnetar CXO J164710.20-455217.
Más contenido relacionado
Similar a A deep dive: Chandra observations of the NGC 4839 group falling into the Coma cluster
The physical conditions_in_a_pre_super_star_cluster_molecular_cloud_in_the_an...Sérgio Sacani
Artigo descreve estudo feitos pelos astrônomos utilizando o ALMA para descobrir um proto-aglomerado globular de estrelas gigantes se formando no interior das galáxias Antenas, o famoso par de galáxias em interação. É a primeira vez que os astrônomos conseguem observar um objeto desse tipo nos seus estágios iniciais de vida e com o ambiente ao redor inalterado.
Observations of a pre-merger shock in colliding clusters of galaxiesSérgio Sacani
Clusters of galaxies are the largest known gravitationally bound
structures in the Universe. When clusters collide, they create
merger shocks on cosmological scales, which transform most
of the kinetic energy carried by the cluster gaseous halos into
heat1–3
. Observations of merger shocks provide key information
on the merger dynamics, and enable insights into the formation and thermal history of the large-scale structures. Nearly
all of the merger shocks are found in systems where the clusters have already collided4–12; knowledge of shocks in the premerger phase is a crucial missing ingredient13,14. Here, we report
on the discovery of a unique shock in a cluster pair 1E 2216.0-
0401 and 1E 2215.7-0404. The two clusters are observed at an
early phase of major merger. Contrary to all the known merger
shocks observed ubiquitously on merger axes, the new shock
propagates outward along the equatorial plane of the merger.
This discovery uncovers an important epoch in the formation
of massive clusters, when the rapid approach of the cluster
pair leads to strong compression of gas along the merger axis.
Current theoretical models15,16 predict that the bulk of the shock
energy might be dissipated outside the clusters, and eventually
turn into heat of the pristine gas in the circum-cluster space.
A rare case of FR I interaction with a hot X-ray bridge in the A2384 galaxy c...Sérgio Sacani
Clusters of varying mass ratios can merge and the process significantly disturbs
the cluster environments and alters their global properties. Active radio galaxies are
another phenomenon that can also affect cluster environments. Radio jets can interact
with the intra-cluster medium (ICM) and locally affect its properties. Abell 2384
(hereafter A2384) is a unique system that has a dense, hot X-ray filament or bridge
connecting the two unequal mass clusters A2384(N) and A2384(S). The analysis of its
morphology suggests that A2384 is a post-merger system where A2384(S) has already
interacted with the A2384(N), and as a result hot gas has been stripped over a ∼ 1
Mpc region between the two bodies. We have obtained its 325 MHz GMRT data,
and we detected a peculiar FR I type radio galaxy which is a part of the A2384(S).
One of its radio lobes interacts with the hot X-ray bridge and pushes the hot gas in
the opposite direction. This results in displacement in the bridge close to A2384(S).
Based on Chandra and XMM-Newton X-ray observations, we notice a temperature and
entropy enhancement at the radio lobe-X-ray plasma interaction site, which further
suggests that the radio lobe is changing thermal plasma properties. We have also
studied the radio properties of the FR I radio galaxy, and found that the size and
radio luminosity of the interacting north lobe of the FR I galaxy are lower than those
of the accompanying south lobe.
The xmm newton-view_of_the_central_degrees_of_the_milk_waySérgio Sacani
Novas imagens do Observatório de Raios-X XMM-Newton da ESA revelaram alguns dos processos mais intensos que acontecem no coração da nossa Via Láctea.
As fontes brilhantes e pontuais que se destacam por toda imagem indicam os sistemas estelares binários onde uma das estrelas atingiu o final de sua vida, desenvolvendo para um objeto compacto e denso – uma estrela de nêutrons ou um buraco negro.
A região central da Via Láctea também contém jovens estrelas e aglomerados estelares e algumas dessas fontes são visíveis como pontos brancos e vermelhos brilhando na imagem, que se espalha por 1000 anos-luz.
A maior parte da ação ocorre no centro, onde nuvens difusas de gás estão sendo cavadas por ventos poderosos soprados por estrelas jovens, bem como por supernovas.
The Expansion of the X-Ray Nebula Around η CarSérgio Sacani
The massive colliding wind binary system η Car is embedded in an X-ray emitting region having a characteristic
temperature of a few million degrees, associated with ejecta produced during the 1840s, and in earlier outbursts.
We use CHANDRA X-ray imaging observations obtained over the past two decades to directly measure the
expansion of the X-ray nebula for the first time. A combined CHANDRA/ACIS image shows a faint, nearly
uniform elliptic structure. This faint elliptical “shell” has a similar orientation and shape as the Homunculus nebula
but is about 3 times larger. We measure proper motions of brighter regions associated with the X-ray emitting ring.
We compare spectra of the soft X-ray emitting plasma in CHANDRA/ACIS and XMM-Newton PN observations
and show that the PN observations indicate a decline in X-ray flux which is comparable to that derived from
NICER observations. We associate the diffuse elliptical emission surrounding the bright X-ray “ring” with the blast
wave produced during the Great Eruption. We suggest that the interaction of this blast wave with pre-existing
clumps of ejecta produces the bright, broken X-ray emitting ring. We extrapolate the trend in X-ray energy back to
the time of the Great Eruption using a simple model and show that the X-ray energy was comparable to the kinetic
energy of the Homunculus, suggesting equipartition of energy between fast, low-density ejecta and slower, dense
ejecta.
The binding of cosmological structures by massless topological defectsSérgio Sacani
Assuming spherical symmetry and weak field, it is shown that if one solves the Poisson equation or the Einstein field
equations sourced by a topological defect, i.e. a singularity of a very specific form, the result is a localized gravitational
field capable of driving flat rotation (i.e. Keplerian circular orbits at a constant speed for all radii) of test masses on a thin
spherical shell without any underlying mass. Moreover, a large-scale structure which exploits this solution by assembling
concentrically a number of such topological defects can establish a flat stellar or galactic rotation curve, and can also deflect
light in the same manner as an equipotential (isothermal) sphere. Thus, the need for dark matter or modified gravity theory is
mitigated, at least in part.
EWOCS-I: The catalog of X-ray sources in Westerlund 1 from the Extended Weste...Sérgio Sacani
Context. With a mass exceeding several 104 M⊙ and a rich and dense population of massive stars, supermassive young star clusters
represent the most massive star-forming environment that is dominated by the feedback from massive stars and gravitational interactions
among stars.
Aims. In this paper we present the Extended Westerlund 1 and 2 Open Clusters Survey (EWOCS) project, which aims to investigate
the influence of the starburst environment on the formation of stars and planets, and on the evolution of both low and high mass stars.
The primary targets of this project are Westerlund 1 and 2, the closest supermassive star clusters to the Sun.
Methods. The project is based primarily on recent observations conducted with the Chandra and JWST observatories. Specifically,
the Chandra survey of Westerlund 1 consists of 36 new ACIS-I observations, nearly co-pointed, for a total exposure time of 1 Msec.
Additionally, we included 8 archival Chandra/ACIS-S observations. This paper presents the resulting catalog of X-ray sources within
and around Westerlund 1. Sources were detected by combining various existing methods, and photon extraction and source validation
were carried out using the ACIS-Extract software.
Results. The EWOCS X-ray catalog comprises 5963 validated sources out of the 9420 initially provided to ACIS-Extract, reaching a
photon flux threshold of approximately 2 × 10−8 photons cm−2
s
−1
. The X-ray sources exhibit a highly concentrated spatial distribution,
with 1075 sources located within the central 1 arcmin. We have successfully detected X-ray emissions from 126 out of the 166 known
massive stars of the cluster, and we have collected over 71 000 photons from the magnetar CXO J164710.20-455217.
The debris of the ‘last major merger’ is dynamically youngSérgio Sacani
The Milky Way’s (MW) inner stellar halo contains an [Fe/H]-rich component with highly eccentric orbits, often referred to as the
‘last major merger.’ Hypotheses for the origin of this component include Gaia-Sausage/Enceladus (GSE), where the progenitor
collided with the MW proto-disc 8–11 Gyr ago, and the Virgo Radial Merger (VRM), where the progenitor collided with the
MW disc within the last 3 Gyr. These two scenarios make different predictions about observable structure in local phase space,
because the morphology of debris depends on how long it has had to phase mix. The recently identified phase-space folds in Gaia
DR3 have positive caustic velocities, making them fundamentally different than the phase-mixed chevrons found in simulations
at late times. Roughly 20 per cent of the stars in the prograde local stellar halo are associated with the observed caustics. Based
on a simple phase-mixing model, the observed number of caustics are consistent with a merger that occurred 1–2 Gyr ago.
We also compare the observed phase-space distribution to FIRE-2 Latte simulations of GSE-like mergers, using a quantitative
measurement of phase mixing (2D causticality). The observed local phase-space distribution best matches the simulated data
1–2 Gyr after collision, and certainly not later than 3 Gyr. This is further evidence that the progenitor of the ‘last major merger’
did not collide with the MW proto-disc at early times, as is thought for the GSE, but instead collided with the MW disc within
the last few Gyr, consistent with the body of work surrounding the VRM.
Observation of Io’s Resurfacing via Plume Deposition Using Ground-based Adapt...Sérgio Sacani
Since volcanic activity was first discovered on Io from Voyager images in 1979, changes
on Io’s surface have been monitored from both spacecraft and ground-based telescopes.
Here, we present the highest spatial resolution images of Io ever obtained from a groundbased telescope. These images, acquired by the SHARK-VIS instrument on the Large
Binocular Telescope, show evidence of a major resurfacing event on Io’s trailing hemisphere. When compared to the most recent spacecraft images, the SHARK-VIS images
show that a plume deposit from a powerful eruption at Pillan Patera has covered part
of the long-lived Pele plume deposit. Although this type of resurfacing event may be common on Io, few have been detected due to the rarity of spacecraft visits and the previously low spatial resolution available from Earth-based telescopes. The SHARK-VIS instrument ushers in a new era of high resolution imaging of Io’s surface using adaptive
optics at visible wavelengths.
Earliest Galaxies in the JADES Origins Field: Luminosity Function and Cosmic ...Sérgio Sacani
We characterize the earliest galaxy population in the JADES Origins Field (JOF), the deepest
imaging field observed with JWST. We make use of the ancillary Hubble optical images (5 filters
spanning 0.4−0.9µm) and novel JWST images with 14 filters spanning 0.8−5µm, including 7 mediumband filters, and reaching total exposure times of up to 46 hours per filter. We combine all our data
at > 2.3µm to construct an ultradeep image, reaching as deep as ≈ 31.4 AB mag in the stack and
30.3-31.0 AB mag (5σ, r = 0.1” circular aperture) in individual filters. We measure photometric
redshifts and use robust selection criteria to identify a sample of eight galaxy candidates at redshifts
z = 11.5 − 15. These objects show compact half-light radii of R1/2 ∼ 50 − 200pc, stellar masses of
M⋆ ∼ 107−108M⊙, and star-formation rates of SFR ∼ 0.1−1 M⊙ yr−1
. Our search finds no candidates
at 15 < z < 20, placing upper limits at these redshifts. We develop a forward modeling approach to
infer the properties of the evolving luminosity function without binning in redshift or luminosity that
marginalizes over the photometric redshift uncertainty of our candidate galaxies and incorporates the
impact of non-detections. We find a z = 12 luminosity function in good agreement with prior results,
and that the luminosity function normalization and UV luminosity density decline by a factor of ∼ 2.5
from z = 12 to z = 14. We discuss the possible implications of our results in the context of theoretical
models for evolution of the dark matter halo mass function.
THE IMPORTANCE OF MARTIAN ATMOSPHERE SAMPLE RETURN.Sérgio Sacani
The return of a sample of near-surface atmosphere from Mars would facilitate answers to several first-order science questions surrounding the formation and evolution of the planet. One of the important aspects of terrestrial planet formation in general is the role that primary atmospheres played in influencing the chemistry and structure of the planets and their antecedents. Studies of the martian atmosphere can be used to investigate the role of a primary atmosphere in its history. Atmosphere samples would also inform our understanding of the near-surface chemistry of the planet, and ultimately the prospects for life. High-precision isotopic analyses of constituent gases are needed to address these questions, requiring that the analyses are made on returned samples rather than in situ.
Multi-source connectivity as the driver of solar wind variability in the heli...Sérgio Sacani
The ambient solar wind that flls the heliosphere originates from multiple
sources in the solar corona and is highly structured. It is often described
as high-speed, relatively homogeneous, plasma streams from coronal
holes and slow-speed, highly variable, streams whose source regions are
under debate. A key goal of ESA/NASA’s Solar Orbiter mission is to identify
solar wind sources and understand what drives the complexity seen in the
heliosphere. By combining magnetic feld modelling and spectroscopic
techniques with high-resolution observations and measurements, we show
that the solar wind variability detected in situ by Solar Orbiter in March
2022 is driven by spatio-temporal changes in the magnetic connectivity to
multiple sources in the solar atmosphere. The magnetic feld footpoints
connected to the spacecraft moved from the boundaries of a coronal hole
to one active region (12961) and then across to another region (12957). This
is refected in the in situ measurements, which show the transition from fast
to highly Alfvénic then to slow solar wind that is disrupted by the arrival of
a coronal mass ejection. Our results describe solar wind variability at 0.5 au
but are applicable to near-Earth observatories.
Gliese 12 b: A Temperate Earth-sized Planet at 12 pc Ideal for Atmospheric Tr...Sérgio Sacani
Recent discoveries of Earth-sized planets transiting nearby M dwarfs have made it possible to characterize the
atmospheres of terrestrial planets via follow-up spectroscopic observations. However, the number of such planets
receiving low insolation is still small, limiting our ability to understand the diversity of the atmospheric
composition and climates of temperate terrestrial planets. We report the discovery of an Earth-sized planet
transiting the nearby (12 pc) inactive M3.0 dwarf Gliese 12 (TOI-6251) with an orbital period (Porb) of 12.76 days.
The planet, Gliese 12 b, was initially identified as a candidate with an ambiguous Porb from TESS data. We
confirmed the transit signal and Porb using ground-based photometry with MuSCAT2 and MuSCAT3, and
validated the planetary nature of the signal using high-resolution images from Gemini/NIRI and Keck/NIRC2 as
well as radial velocity (RV) measurements from the InfraRed Doppler instrument on the Subaru 8.2 m telescope
and from CARMENES on the CAHA 3.5 m telescope. X-ray observations with XMM-Newton showed the host
star is inactive, with an X-ray-to-bolometric luminosity ratio of log 5.7 L L X bol » - . Joint analysis of the light
curves and RV measurements revealed that Gliese 12 b has a radius of 0.96 ± 0.05 R⊕,a3σ mass upper limit of
3.9 M⊕, and an equilibrium temperature of 315 ± 6 K assuming zero albedo. The transmission spectroscopy metric
(TSM) value of Gliese 12 b is close to the TSM values of the TRAPPIST-1 planets, adding Gliese 12 b to the small
list of potentially terrestrial, temperate planets amenable to atmospheric characterization with JWST.
Gliese 12 b, a temperate Earth-sized planet at 12 parsecs discovered with TES...Sérgio Sacani
We report on the discovery of Gliese 12 b, the nearest transiting temperate, Earth-sized planet found to date. Gliese 12 is a
bright (V = 12.6 mag, K = 7.8 mag) metal-poor M4V star only 12.162 ± 0.005 pc away from the Solar system with one of the
lowest stellar activity levels known for M-dwarfs. A planet candidate was detected by TESS based on only 3 transits in sectors
42, 43, and 57, with an ambiguity in the orbital period due to observational gaps. We performed follow-up transit observations
with CHEOPS and ground-based photometry with MINERVA-Australis, SPECULOOS, and Purple Mountain Observatory,
as well as further TESS observations in sector 70. We statistically validate Gliese 12 b as a planet with an orbital period of
12.76144 ± 0.00006 d and a radius of 1.0 ± 0.1 R⊕, resulting in an equilibrium temperature of ∼315 K. Gliese 12 b has excellent
future prospects for precise mass measurement, which may inform how planetary internal structure is affected by the stellar
compositional environment. Gliese 12 b also represents one of the best targets to study whether Earth-like planets orbiting cool
stars can retain their atmospheres, a crucial step to advance our understanding of habitability on Earth and across the galaxy.
The importance of continents, oceans and plate tectonics for the evolution of...Sérgio Sacani
Within the uncertainties of involved astronomical and biological parameters, the Drake Equation
typically predicts that there should be many exoplanets in our galaxy hosting active, communicative
civilizations (ACCs). These optimistic calculations are however not supported by evidence, which is
often referred to as the Fermi Paradox. Here, we elaborate on this long-standing enigma by showing
the importance of planetary tectonic style for biological evolution. We summarize growing evidence
that a prolonged transition from Mesoproterozoic active single lid tectonics (1.6 to 1.0 Ga) to modern
plate tectonics occurred in the Neoproterozoic Era (1.0 to 0.541 Ga), which dramatically accelerated
emergence and evolution of complex species. We further suggest that both continents and oceans
are required for ACCs because early evolution of simple life must happen in water but late evolution
of advanced life capable of creating technology must happen on land. We resolve the Fermi Paradox
(1) by adding two additional terms to the Drake Equation: foc
(the fraction of habitable exoplanets
with significant continents and oceans) and fpt
(the fraction of habitable exoplanets with significant
continents and oceans that have had plate tectonics operating for at least 0.5 Ga); and (2) by
demonstrating that the product of foc
and fpt
is very small (< 0.00003–0.002). We propose that the lack
of evidence for ACCs reflects the scarcity of long-lived plate tectonics and/or continents and oceans on
exoplanets with primitive life.
A Giant Impact Origin for the First Subduction on EarthSérgio Sacani
Hadean zircons provide a potential record of Earth's earliest subduction 4.3 billion years ago. Itremains enigmatic how subduction could be initiated so soon after the presumably Moon‐forming giant impact(MGI). Earlier studies found an increase in Earth's core‐mantle boundary (CMB) temperature due to theaccumulation of the impactor's core, and our recent work shows Earth's lower mantle remains largely solid, withsome of the impactor's mantle potentially surviving as the large low‐shear velocity provinces (LLSVPs). Here,we show that a hot post‐impact CMB drives the initiation of strong mantle plumes that can induce subductioninitiation ∼200 Myr after the MGI. 2D and 3D thermomechanical computations show that a high CMBtemperature is the primary factor triggering early subduction, with enrichment of heat‐producing elements inLLSVPs as another potential factor. The models link the earliest subduction to the MGI with implications forunderstanding the diverse tectonic regimes of rocky planets.
Climate extremes likely to drive land mammal extinction during next supercont...Sérgio Sacani
Mammals have dominated Earth for approximately 55 Myr thanks to their
adaptations and resilience to warming and cooling during the Cenozoic. All
life will eventually perish in a runaway greenhouse once absorbed solar
radiation exceeds the emission of thermal radiation in several billions of
years. However, conditions rendering the Earth naturally inhospitable to
mammals may develop sooner because of long-term processes linked to
plate tectonics (short-term perturbations are not considered here). In
~250 Myr, all continents will converge to form Earth’s next supercontinent,
Pangea Ultima. A natural consequence of the creation and decay of Pangea
Ultima will be extremes in pCO2 due to changes in volcanic rifting and
outgassing. Here we show that increased pCO2, solar energy (F⨀;
approximately +2.5% W m−2 greater than today) and continentality (larger
range in temperatures away from the ocean) lead to increasing warming
hostile to mammalian life. We assess their impact on mammalian
physiological limits (dry bulb, wet bulb and Humidex heat stress indicators)
as well as a planetary habitability index. Given mammals’ continued survival,
predicted background pCO2 levels of 410–816 ppm combined with increased
F⨀ will probably lead to a climate tipping point and their mass extinction.
The results also highlight how global landmass configuration, pCO2 and F⨀
play a critical role in planetary habitability.
Constraints on Neutrino Natal Kicks from Black-Hole Binary VFTS 243Sérgio Sacani
The recently reported observation of VFTS 243 is the first example of a massive black-hole binary
system with negligible binary interaction following black-hole formation. The black-hole mass (≈10M⊙)
and near-circular orbit (e ≈ 0.02) of VFTS 243 suggest that the progenitor star experienced complete
collapse, with energy-momentum being lost predominantly through neutrinos. VFTS 243 enables us to
constrain the natal kick and neutrino-emission asymmetry during black-hole formation. At 68% confidence
level, the natal kick velocity (mass decrement) is ≲10 km=s (≲1.0M⊙), with a full probability distribution
that peaks when ≈0.3M⊙ were ejected, presumably in neutrinos, and the black hole experienced a natal
kick of 4 km=s. The neutrino-emission asymmetry is ≲4%, with best fit values of ∼0–0.2%. Such a small
neutrino natal kick accompanying black-hole formation is in agreement with theoretical predictions.
Detectability of Solar Panels as a TechnosignatureSérgio Sacani
In this work, we assess the potential detectability of solar panels made of silicon on an Earth-like
exoplanet as a potential technosignature. Silicon-based photovoltaic cells have high reflectance in the
UV-VIS and in the near-IR, within the wavelength range of a space-based flagship mission concept
like the Habitable Worlds Observatory (HWO). Assuming that only solar energy is used to provide
the 2022 human energy needs with a land cover of ∼ 2.4%, and projecting the future energy demand
assuming various growth-rate scenarios, we assess the detectability with an 8 m HWO-like telescope.
Assuming the most favorable viewing orientation, and focusing on the strong absorption edge in the
ultraviolet-to-visible (0.34 − 0.52 µm), we find that several 100s of hours of observation time is needed
to reach a SNR of 5 for an Earth-like planet around a Sun-like star at 10pc, even with a solar panel
coverage of ∼ 23% land coverage of a future Earth. We discuss the necessity of concepts like Kardeshev
Type I/II civilizations and Dyson spheres, which would aim to harness vast amounts of energy. Even
with much larger populations than today, the total energy use of human civilization would be orders of
magnitude below the threshold for causing direct thermal heating or reaching the scale of a Kardashev
Type I civilization. Any extraterrrestrial civilization that likewise achieves sustainable population
levels may also find a limit on its need to expand, which suggests that a galaxy-spanning civilization
as imagined in the Fermi paradox may not exist.
Jet reorientation in central galaxies of clusters and groups: insights from V...Sérgio Sacani
Recent observations of galaxy clusters and groups with misalignments between their central AGN jets
and X-ray cavities, or with multiple misaligned cavities, have raised concerns about the jet – bubble
connection in cooling cores, and the processes responsible for jet realignment. To investigate the
frequency and causes of such misalignments, we construct a sample of 16 cool core galaxy clusters and
groups. Using VLBA radio data we measure the parsec-scale position angle of the jets, and compare
it with the position angle of the X-ray cavities detected in Chandra data. Using the overall sample
and selected subsets, we consistently find that there is a 30% – 38% chance to find a misalignment
larger than ∆Ψ = 45◦ when observing a cluster/group with a detected jet and at least one cavity. We
determine that projection may account for an apparently large ∆Ψ only in a fraction of objects (∼35%),
and given that gas dynamical disturbances (as sloshing) are found in both aligned and misaligned
systems, we exclude environmental perturbation as the main driver of cavity – jet misalignment.
Moreover, we find that large misalignments (up to ∼ 90◦
) are favored over smaller ones (45◦ ≤ ∆Ψ ≤
70◦
), and that the change in jet direction can occur on timescales between one and a few tens of Myr.
We conclude that misalignments are more likely related to actual reorientation of the jet axis, and we
discuss several engine-based mechanisms that may cause these dramatic changes.
The solar dynamo begins near the surfaceSérgio Sacani
The magnetic dynamo cycle of the Sun features a distinct pattern: a propagating
region of sunspot emergence appears around 30° latitude and vanishes near the
equator every 11 years (ref. 1). Moreover, longitudinal flows called torsional oscillations
closely shadow sunspot migration, undoubtedly sharing a common cause2. Contrary
to theories suggesting deep origins of these phenomena, helioseismology pinpoints
low-latitude torsional oscillations to the outer 5–10% of the Sun, the near-surface
shear layer3,4. Within this zone, inwardly increasing differential rotation coupled with
a poloidal magnetic field strongly implicates the magneto-rotational instability5,6,
prominent in accretion-disk theory and observed in laboratory experiments7.
Together, these two facts prompt the general question: whether the solar dynamo is
possibly a near-surface instability. Here we report strong affirmative evidence in stark
contrast to traditional models8 focusing on the deeper tachocline. Simple analytic
estimates show that the near-surface magneto-rotational instability better explains
the spatiotemporal scales of the torsional oscillations and inferred subsurface
magnetic field amplitudes9. State-of-the-art numerical simulations corroborate these
estimates and reproduce hemispherical magnetic current helicity laws10. The dynamo
resulting from a well-understood near-surface phenomenon improves prospects
for accurate predictions of full magnetic cycles and space weather, affecting the
electromagnetic infrastructure of Earth.
Extensive Pollution of Uranus and Neptune’s Atmospheres by Upsweep of Icy Mat...Sérgio Sacani
In the Nice model of solar system formation, Uranus and Neptune undergo an orbital upheaval,
sweeping through a planetesimal disk. The region of the disk from which material is accreted by
the ice giants during this phase of their evolution has not previously been identified. We perform
direct N-body orbital simulations of the four giant planets to determine the amount and origin of solid
accretion during this orbital upheaval. We find that the ice giants undergo an extreme bombardment
event, with collision rates as much as ∼3 per hour assuming km-sized planetesimals, increasing the
total planet mass by up to ∼0.35%. In all cases, the initially outermost ice giant experiences the
largest total enhancement. We determine that for some plausible planetesimal properties, the resulting
atmospheric enrichment could potentially produce sufficient latent heat to alter the planetary cooling
timescale according to existing models. Our findings suggest that substantial accretion during this
phase of planetary evolution may have been sufficient to impact the atmospheric composition and
thermal evolution of the ice giants, motivating future work on the fate of deposited solid material.
Exomoons & Exorings with the Habitable Worlds Observatory I: On the Detection...Sérgio Sacani
The highest priority recommendation of the Astro2020 Decadal Survey for space-based astronomy
was the construction of an observatory capable of characterizing habitable worlds. In this paper series
we explore the detectability of and interference from exomoons and exorings serendipitously observed
with the proposed Habitable Worlds Observatory (HWO) as it seeks to characterize exoplanets, starting
in this manuscript with Earth-Moon analog mutual events. Unlike transits, which only occur in systems
viewed near edge-on, shadow (i.e., solar eclipse) and lunar eclipse mutual events occur in almost every
star-planet-moon system. The cadence of these events can vary widely from ∼yearly to multiple events
per day, as was the case in our younger Earth-Moon system. Leveraging previous space-based (EPOXI)
lightcurves of a Moon transit and performance predictions from the LUVOIR-B concept, we derive
the detectability of Moon analogs with HWO. We determine that Earth-Moon analogs are detectable
with observation of ∼2-20 mutual events for systems within 10 pc, and larger moons should remain
detectable out to 20 pc. We explore the extent to which exomoon mutual events can mimic planet
features and weather. We find that HWO wavelength coverage in the near-IR, specifically in the 1.4 µm
water band where large moons can outshine their host planet, will aid in differentiating exomoon signals
from exoplanet variability. Finally, we predict that exomoons formed through collision processes akin
to our Moon are more likely to be detected in younger systems, where shorter orbital periods and
favorable geometry enhance the probability and frequency of mutual events.
Emergent ribozyme behaviors in oxychlorine brines indicate a unique niche for...Sérgio Sacani
Mars is a particularly attractive candidate among known astronomical objects
to potentially host life. Results from space exploration missions have provided
insights into Martian geochemistry that indicate oxychlorine species, particularly perchlorate, are ubiquitous features of the Martian geochemical landscape. Perchlorate presents potential obstacles for known forms of life due to
its toxicity. However, it can also provide potential benefits, such as producing
brines by deliquescence, like those thought to exist on present-day Mars. Here
we show perchlorate brines support folding and catalysis of functional RNAs,
while inactivating representative protein enzymes. Additionally, we show
perchlorate and other oxychlorine species enable ribozyme functions,
including homeostasis-like regulatory behavior and ribozyme-catalyzed
chlorination of organic molecules. We suggest nucleic acids are uniquely wellsuited to hypersaline Martian environments. Furthermore, Martian near- or
subsurface oxychlorine brines, and brines found in potential lifeforms, could
provide a unique niche for biomolecular evolution.
Continuum emission from within the plunging region of black hole discsSérgio Sacani
The thermal continuum emission observed from accreting black holes across X-ray bands has the potential to be leveraged as a
powerful probe of the mass and spin of the central black hole. The vast majority of existing ‘continuum fitting’ models neglect
emission sourced at and within the innermost stable circular orbit (ISCO) of the black hole. Numerical simulations, however,
find non-zero emission sourced from these regions. In this work, we extend existing techniques by including the emission
sourced from within the plunging region, utilizing new analytical models that reproduce the properties of numerical accretion
simulations. We show that in general the neglected intra-ISCO emission produces a hot-and-small quasi-blackbody component,
but can also produce a weak power-law tail for more extreme parameter regions. A similar hot-and-small blackbody component
has been added in by hand in an ad hoc manner to previous analyses of X-ray binary spectra. We show that the X-ray spectrum
of MAXI J1820+070 in a soft-state outburst is extremely well described by a full Kerr black hole disc, while conventional
models that neglect intra-ISCO emission are unable to reproduce the data. We believe this represents the first robust detection of
intra-ISCO emission in the literature, and allows additional constraints to be placed on the MAXI J1820 + 070 black hole spin
which must be low a• < 0.5 to allow a detectable intra-ISCO region. Emission from within the ISCO is the dominant emission
component in the MAXI J1820 + 070 spectrum between 6 and 10 keV, highlighting the necessity of including this region. Our
continuum fitting model is made publicly available.
hematic appreciation test is a psychological assessment tool used to measure an individual's appreciation and understanding of specific themes or topics. This test helps to evaluate an individual's ability to connect different ideas and concepts within a given theme, as well as their overall comprehension and interpretation skills. The results of the test can provide valuable insights into an individual's cognitive abilities, creativity, and critical thinking skills
Seminar of U.V. Spectroscopy by SAMIR PANDASAMIR PANDA
Spectroscopy is a branch of science dealing the study of interaction of electromagnetic radiation with matter.
Ultraviolet-visible spectroscopy refers to absorption spectroscopy or reflect spectroscopy in the UV-VIS spectral region.
Ultraviolet-visible spectroscopy is an analytical method that can measure the amount of light received by the analyte.
ANAMOLOUS SECONDARY GROWTH IN DICOT ROOTS.pptxRASHMI M G
Abnormal or anomalous secondary growth in plants. It defines secondary growth as an increase in plant girth due to vascular cambium or cork cambium. Anomalous secondary growth does not follow the normal pattern of a single vascular cambium producing xylem internally and phloem externally.
Remote Sensing and Computational, Evolutionary, Supercomputing, and Intellige...University of Maribor
Slides from talk:
Aleš Zamuda: Remote Sensing and Computational, Evolutionary, Supercomputing, and Intelligent Systems.
11th International Conference on Electrical, Electronics and Computer Engineering (IcETRAN), Niš, 3-6 June 2024
Inter-Society Networking Panel GRSS/MTT-S/CIS Panel Session: Promoting Connection and Cooperation
https://www.etran.rs/2024/en/home-english/
Phenomics assisted breeding in crop improvementIshaGoswami9
As the population is increasing and will reach about 9 billion upto 2050. Also due to climate change, it is difficult to meet the food requirement of such a large population. Facing the challenges presented by resource shortages, climate
change, and increasing global population, crop yield and quality need to be improved in a sustainable way over the coming decades. Genetic improvement by breeding is the best way to increase crop productivity. With the rapid progression of functional
genomics, an increasing number of crop genomes have been sequenced and dozens of genes influencing key agronomic traits have been identified. However, current genome sequence information has not been adequately exploited for understanding
the complex characteristics of multiple gene, owing to a lack of crop phenotypic data. Efficient, automatic, and accurate technologies and platforms that can capture phenotypic data that can
be linked to genomics information for crop improvement at all growth stages have become as important as genotyping. Thus,
high-throughput phenotyping has become the major bottleneck restricting crop breeding. Plant phenomics has been defined as the high-throughput, accurate acquisition and analysis of multi-dimensional phenotypes
during crop growing stages at the organism level, including the cell, tissue, organ, individual plant, plot, and field levels. With the rapid development of novel sensors, imaging technology,
and analysis methods, numerous infrastructure platforms have been developed for phenotyping.
Richard's aventures in two entangled wonderlandsRichard Gill
Since the loophole-free Bell experiments of 2020 and the Nobel prizes in physics of 2022, critics of Bell's work have retreated to the fortress of super-determinism. Now, super-determinism is a derogatory word - it just means "determinism". Palmer, Hance and Hossenfelder argue that quantum mechanics and determinism are not incompatible, using a sophisticated mathematical construction based on a subtle thinning of allowed states and measurements in quantum mechanics, such that what is left appears to make Bell's argument fail, without altering the empirical predictions of quantum mechanics. I think however that it is a smoke screen, and the slogan "lost in math" comes to my mind. I will discuss some other recent disproofs of Bell's theorem using the language of causality based on causal graphs. Causal thinking is also central to law and justice. I will mention surprising connections to my work on serial killer nurse cases, in particular the Dutch case of Lucia de Berk and the current UK case of Lucy Letby.
BREEDING METHODS FOR DISEASE RESISTANCE.pptxRASHMI M G
Plant breeding for disease resistance is a strategy to reduce crop losses caused by disease. Plants have an innate immune system that allows them to recognize pathogens and provide resistance. However, breeding for long-lasting resistance often involves combining multiple resistance genes
This presentation explores a brief idea about the structural and functional attributes of nucleotides, the structure and function of genetic materials along with the impact of UV rays and pH upon them.
Professional air quality monitoring systems provide immediate, on-site data for analysis, compliance, and decision-making.
Monitor common gases, weather parameters, particulates.
2. 2 M. S. Mirakhor et al.
04:00.0 02:00.0 13:00:00.0 58:00.0 12:56:00.0
29:00:00.0
30:00.0
28:00:00.0
30:00.0
27:00:00.0
Right ascension
Declination
58:00.0 50.0 40.0 30.0 20.0 10.0 12:57:00.0 50.0 40.0 30.0 56:20.0
40:00.0
35:00.0
27:30:00.0
25:00.0
20:00.0
Right ascension
Declination
Chandra
5 arcmin
150 kpc
Leading Head
Kelvin-Helmholtz Instabilities?
Ridge
Inner Tail
Outer Tail
Shock
Cold Front
XMM-Newton
N
E
20 arcmin
600 kpc
Primary Shock
Secondary Shock
Radio Relic
Figure 1. Left: Voronoi tessellated, XMM–Newton mosaicked image of the Coma cluster in the 0.7–1.2 keV energy band (taken from Mirakhor & Walker 2020),
showing the infalling group NGC 4839 to the southwest of the cluster. The white box highlights the spatial extent of our Chandra observation of the group. The
white dashed circle marks the radius of 𝑟500 from the cluster centre. The green curves are the presumed positions of the primary and secondary shocks (taken
from Fig. 11 in Churazov et al. 2021), and the cyan contour marks the location of the radio relic (taken from Bonafede et al. 2022). These features are discussed
in Section 6.2. Right: Exposure-corrected, background-subtracted Chandra image of the infalling group NGC 4839, smoothed heavily to highlight the diffuse
emission from the group. The morphological features discussed in the main text are labelled in the image. The green curves mark the positions of cold and shock
fronts detected in our Chandra data.
an X-ray bright head-like structure. The XMM–Newton image also
reveals the group falling into the Coma core with its tail pointing
away from the cluster, coinciding with where a cosmic web filament
joins the Coma cluster to Abell 1367 (Malavasi et al. 2020).
It has long been debated whether the NGC 4839 group is falling
into Coma for the first time (e.g. Neumann et al. 2001; Akamatsu
et al. 2013) or it has already passed the Coma centre and is now on
its second infall (e.g. Burns et al. 1994; Oh et al. 2023). Lyskova
et al. (2019) have compared the observed X-ray geometry of the
NGC 4839 group with a series of numerical simulations of different
infall scenarios, and have found the best match to be a scenario in
which the NGC 4839 group is on its second infall. In this case, the
group would have entered the Coma cluster from the northeast, before
passing close by the core and heading out to the southwest, where
it has turned around and begun to fall back towards the core. These
simulations indicate that the group has spent an enormous amount
of time in the ICM of the cluster (∼ 2.5 Gyr), yet its tail has still
survived against thermal conduction.
In this paper, we examine several X-ray properties of the infalling
group NGC 4839 and probe the suppression of thermal conduction
throughout its tail using our new deep Chandra observations. The
NGC 4839 group shares a similar morphology to that of Abell 2142,
providing a rare opportunity to explore the way galaxy clusters in
the local universe continue to grow. Furthermore, the large-spatial
extent of the NGC 4839 group, afforded by the low redshift of the
system, allows us to explore the microphysics of the merging process
on unprecedentedly small scales. The rest of the paper is structured
as follows: Section 2 goes over the observations, imaging, and spec-
tral analyses; Sections 3 and 4 analysis the main features observed
in our Chandra data; Section 5 examines the power spectrum of
surface brightness fluctuations in the tail to probes turbulence and
transport processes; our findings are discussed in Section 6; and our
conclusions are presented in Section 7.
Throughout the paper, for consistency with Eckert et al. (2017),
we adopt a ΛCDM cosmology with ΩΛ = 0.7, Ωm = 0.3, and
𝐻0 = 70 km s−1 Mpc−1. At the redshift of NGC 4839 (𝑧 = 0.0245;
Ahn et al. 2012), 1 arcsec corresponds to 0.496 kpc. Uncertainties
are at the 68 per cent confidence level unless otherwise stated.
2 ANALYSIS PROCEDURE
2.1 Imaging
We used our new seven Chandra observations taken in 2020 (PI:
S. A. Walker) and one archival observation from 2010 for a total
observation time of 232 ks. Table 1 gives a summary of the Chan-
dra observations used in this work. All data were reprocessed using
the chandra_repro script of CIAO 4.13 and CALDB 4.9.1. Back-
ground light curves were analyzed to filter out any time periods
affected by flares. Counts, exposure maps, and flux images were
produced from the combined observations using the CIAO script
merge_obs. Point sources were detected and excluded from the anal-
ysis using CIAO wavdetect with wavelet scales of 1, 2, 4, 8, and 16
pixels.
Following the same procedure as described in Hickox & Marke-
vitch (2006), we used the stowed ACIS images with a total observa-
tion time of 367 ks in order to model and subtract instrumentation
contribution to the background. The stowed images were scaled so
that the 9.5–12 keV count rates matched the observations.
The right-hand panel of Fig. 1 shows our exposure-corrected,
MNRAS 000, 1–10 (2015)
3. NGC 4839 group 3
Table 1. Summary of the Chandra observations for the NGC 4839 group.
Obs. ID Date RA Dec. Exposure (ks)
22648 2020-03-03 12 57 11.32 +27 27 40.14 33
22649 2020-03-04 12 57 11.32 +27 27 40.14 34
22930 2020-08-11 12 57 00.26 +27 28 00.59 37
23182 2020-03-04 12 57 11.32 +27 27 40.14 31
23361 2020-11-02 12 57 00.26 +27 28 00.59 19
24853 2020-11-03 12 57 00.26 +27 28 00.59 27
24854 2020-11-08 12 57 00.26 +27 28 00.59 8
12887 2010-11-11 12 57 24.10 +27 29 47.00 43
background-subtracted Chandra image of the NGC 4839 group in
the 0.5–7.0 keV band, smoothed heavily to highlight the diffuse emis-
sion from the group. An incredible variety of structures are revealed
by our deep Chandra observations. It shows an elongated tail of
ram-pressure-stripped gas pointing away from the Coma centre, pre-
ceded by an X-ray bright structure, the head. The extended tail can
be divided into two main regions: inner and outer. The northern side
of the inner tail exhibits a ripple feature around 25 kpc wide that
resembles a KHI feature (this is labelled on the right-hand panel of
Fig. 1 as ‘Kelvin-Helmholtz Instabilities?’). We discuss this feature
further in Section 4. There is also a curved ridge of enhanced X-ray
surface brightness in front and to the southeast of the head, which
appears separated from it by a dark trough. In Lyskova et al. (2019),
this emission was referred to as the "sheath" and is explained by the
interaction between the re-infalling group and the gas stripped from
its tail, which is now mixed with the surrounding ICM.
2.2 Spectral analysis
To study the spectral properties of the gas in various regions of the
NGC 4839 group, it is necessary to choose spatial regions for X-
ray spectroscopy. To do that, we used the contour binning software
CONTBIN version 1.6 (Sanders 2006) with a threshold for the signal-
to-noise ratio of 50. This signal-to-noise ratio was used to accommo-
date the fainter parts of the field. The software chooses regions using
contours from adaptively-smoothed, background-subtracted images
in such a way that the generated regions closely follow the X-ray
surface brightness distribution. For each selected region, we run the
CIAO tool specextract to create the X-ray spectrum for the source
along with the response matrix files (RMFs) and the ancillary re-
sponse files (ARFs).
For the spectral fitting and background subtraction we followed
the methods discussed in Wang & Walker (2014) and Walker et al.
(2018). First, stowed backgrounds1 were used, scaled to match the
9.5–12 keV count rate, to subtract the particle background. To model
the soft X-ray background, we used ROSAT All Sky Survey data
for a background annulus around the Coma cluster extending from
2–3 degrees. The soft background was modelled as consisting of
an absorbed APEC model originating from the Galactic Halo with a
temperature of 0.23 ± 0.01 keV and normalisation of 2.7+0.3
−0.3
× 10−3,
and an unabsorbed APEC component from the local hot bubble with
a temperature of 0.11 ± 0.001 keV and normalisation of 1.7+0.4
−0.4
×
10−3 (the quoted normalisations are in units of 10−14
4𝜋[𝐷A (1+𝑧)]2 cm−5
per 400𝜋 square arcmin area, where 𝐷A is the angular diameter
1 https://cxc.harvard.edu/contrib/maxim/acisbg/
5 arcmin
Shock
Cold Front
150 kpc
0
1
2
3
4
5
6
7
Figure 2. Temperature map of the infalling group NGC 4839 created using
the contour binning technique with a signal-to-noise ratio of 50. The green
curves mark the positions of cold and shock fronts discussed in the main text.
White contours are the X-ray emission shown in Fig. 1.
distance). These values are consistent with those found in Simionescu
et al. (2013). For the cosmic X-ray background component from
unresolved point sources, this was modelled as an absorbed power
law of index 1.4 and allowed to be a free parameter in the spectral
fitting. After modelling the background, the emission from the galaxy
group was fitted with an absorbed APEC model. The Galactic column
density was fixed at 𝑁H = 8.79 × 1019 cm−2 (Dickey & Lockman
1990), the metallicity fixed at 𝑍 = 0.3𝑍 (as was done in Simionescu
et al. 2013), and the redshift fixed at 𝑧 = 0.0245.
The temperature map of the NGC 4839 group is shown in Fig.
2. Some interesting features can be seen in the temperature map. A
region of cold gas (∼ 2 keV) can be seen within the leading head,
preceded by what appears to be a region of shock-heated gas (> 5
keV) in front and to the southeast. Interestingly, this hot gas extends
along the southern side of the outer tail. The gas temperature in
the outer tail is ∼3.5 keV, agreeing with that obtained using Suzaku
(Akamatsu et al. 2013). In the following sections, we discuss in detail
the properties of the features reported here.
3 THE HEAD AND RIDGE FEATURES
As shown in Fig. 1 (right) and Fig. 2, there are sharp jumps in
the surface brightness and the gas temperature in front and to the
southeast of the head structure, suggesting the presence of a leading
edge. To investigate this possibility further, we extracted the surface
brightness and temperature profiles from a region shown in Fig. 3
(labelled F1). The extracted surface brightness profile was then fit-
ted with a broken power-law model projected along the line of sight
(Markevitch & Vikhlinin 2007) to determine a possible density dis-
continuity across the selected region. Assuming spherical symmetry,
the density distribution can be given by
𝑛(𝑟) =
(
𝑛0
𝑟
𝑟sh
𝛼1
if 𝑟 ≤ 𝑟sh,
1
𝐶 𝑛0
𝑟
𝑟sh
𝛼2
if 𝑟 𝑟sh,
(1)
MNRAS 000, 1–10 (2015)
4. 4 M. S. Mirakhor et al.
F2
F1
T5
T4
T3
T2
T1
5 arcmin
150 kpc
Figure 3. Exposure-corrected Chandra X-ray image of the NGC 4839 group,
showing the locations of regions used to analyse its properties. The green an-
nuli mark the regions where the properties of the head and ridge features were
studied, while the white boxes mark the regions where the power spectrum of
surface brightness fluctuations in the tail was extracted. The arrows indicate
the direction in which the surface brightness and temperature profiles were
extracted.
where 𝑛0 is the density normalisation, 𝛼1 and 𝛼2 are the power-law
indices, and 𝑟sh is the shock putative distance where the surface
brightness discontinuity is located. At the discontinuity location, the
post-shock density, 𝑛2, is higher than the pre-shock density, 𝑛1, by
a factor of 𝐶 = 𝑛2/𝑛1. This factor is related to the Mach number
through the Rankine-Hugoniot jump conditions (Landau Lifshitz
1987). The Mach number can be calculated either using the density
discontinuity:
M =
2𝐶
𝛾 + 1 − 𝐶 (𝛾 − 1)
1
2
, (2)
or temperature discontinuity:
M =
(𝛾 + 1)2(𝑇2
𝑇1
− 1)
2𝛾(𝛾 − 1)
1
2
, (3)
where 𝑇1 and 𝑇2 are, respectively, the pre-shock and post-shock
temperatures at the location of the temperature discontinuity, and
𝛾 = 5/3 for a non-relativistic monatomic gas.
The surface brightness profile and the best-fitting model are shown
in the upper panel of Fig. 4. In Table 2, the best-fitting parameters of
the broken power-law model are presented. There is a clear disconti-
nuity in the surface brightness profile at 2.6 arcmin. Our best-fitting
model indicates a density drop by a factor of 𝐶 = 1.6, corresponding
to a Mach number of M = 1.4+0.3
−0.2
. At this surface brightness dis-
continuity, the gas temperature drops by a factor of 𝑇2/𝑇1 = 1.7 (see
Fig. 4, lower panel), resulting in a Mach number of M = 1.5+0.8
−0.5
, in
agreement with what we found using the gas density.
We also note a sharp drop in the surface brightness in front of the
head structure (Fig. 1, right). To investigate the nature of this edge,
we extracted the surface brightness and temperature profiles from the
Figure 4. Top: Surface brightness profile extracted from the green annuli
labelled F1 in Fig. 3. The best fit to the data using a broken power-law model
is shown as a solid blue line. The inset shows the gas density profile, where
its x-axis is in units of arcmin and the y-axis is in unis of cm−3. Bottom:
Temperature profile for the same region as noted above. The dotted vertical
line shows the location of the shock edge.
Figure 5. Top: Surface brightness profile extracted from the green annuli
labelled F2 in Fig. 3. The best fit to the data using a broken power-law model
is shown as a solid blue line. The inset shows the gas density profile, where
its x-axis is in units of arcmin and the y-axis is in unis of cm−3. The surface
brightness profile drops off beyond 2 arcmin, indicating the flat plateau outside
the cold front is not the background level. Bottom: Temperature profile for
the same region as noted above. The dotted vertical line show the location of
the cold front.
region labelled F2 in Fig. 3. Fitting the surface brightness profile of
the region with the broken power-law model (equation 1), we found a
very sharp drop in the gas density by a factor of 4.1+1.4
−1.0
at a distance
of ∼ 0.4 arcmin (Fig. 5, upper panel). The surface brightness profile
along this direction drops off at large radii, indicating the flat plateau
outside the cold front is not the background level. The best-fitting
MNRAS 000, 1–10 (2015)
5. NGC 4839 group 5
Table 2. Best-fitting parameters of the broken power-law model.
Region 𝑛0 (10−3 cm−3) 𝑟sh (arcmin) 𝛼1 𝛼2 𝐶 Reduced 𝜒2
F1 1.11+0.09
−0.07
2.59+0.18
−0.17
−0.09+0.01
−0.01
−0.52+0.05
−0.04
1.60+0.28
−0.26
1.05
F2 4.03+0.35
−0.31
0.39+0.04
−0.04
−0.30+0.03
−0.02
−0.35+0.05
−0.04
4.07+1.36
−0.97
1.08
parameters of this fit are shown in Table 2. The location of this
sharp density edge coincides with the location of the temperature
jump within the head structure (Fig. 5, lower panel). Given that
the gas temperature within the head is significantly lower than its
surrounding, we conclude that this feature is a cold front. At this
cold front, the gas temperature jumps by a factor of 2.7. This sharp
edge is similar to that reported at the leading edge of the bullet in the
Bullet cluster (Markevitch et al. 2002). Eckert et al. (2017) also found
a cold front feature associated with a sharp density discontinuity of
4.5+1.6
−1.0
at the leading edge of the infalling group in Abell 2142.
4 KHI-LIKE FEATURE
Cosmological simulations (e.g. Roediger et al. 2015a,b) showed that
KHIs can form to the sides of infalling groups in clusters of galax-
ies. The simulations also indicate that these KHIs introduce surface
brightness fluctuations and horn-like features in the ICM. The pres-
ence of a sufficiently viscous or magnetised plasma suppresses these
KHIs, and hence turbulent mixing. Consequently, under these dy-
namical conditions, one expects to see an unmixed, bright tail on
scales of tens or hundreds of kiloparsecs in the wake behind the
infalling group. On the other hand, for an inviscid or weekly magne-
tised plasma, the stripped gas in the wake is expected to mix quickly
with the surrounding ICM via KHIs, suppressing the discontinuity
in the gas density and the surface brightness.
As shown in our Chandra image (Fig. 1, right-hand panel), the
NGC 4839 group has a straight inner tail along its southern side
and a KHI-like feature along its northern side. If this is the case,
this feature should appear in the surface brightness profile. For this
purpose, we extracted surface brightness profiles from the northern
and southern sides of the inner tail. Fig. 6 shows the regions in which
surface brightness profiles were extracted: three in the north and two
in the south. The surface brightness profiles are presented in Fig. 7.
While the southern regions show an edge as indicated by a steep
drop in the surface brightness profiles, the northern regions show
no clear edge. We also applied the Gaussian Gradient Magnitude
(GGM) filter (Sanders et al. 2016; Walker et al. 2016) to the Chandra
data of the group to enhance surface brightness edges in the X-ray
image. Fig. 8 shows the Chandra image of the group along with the
GGM-filtered images using five different width scales. In the filtered
images with width scales of 2 and 4 pixels, an edge can be clearly
seen along the southern side of the inner tail, while a very faint
ripple in surface brightness, which coincides with the location of the
KHI-like feature, is visible along the northern side of the inner tail.
In the image filtered with a scale of 8 pixels, the ripple along the
northern side of the inner tail becomes more visible, but its gradient
is not as sharp as that along the southern side. Using scales of 16 and
32 pixels, the filtered images become more sensitive to structures at
larger radii, including the shock front discussed in Section 3.
The above findings might suggest that the stripped gas along the
northern side of the inner tail is mixed with the surrounding ICM
via KHIs, which implies an inviscid or weekly magnetised plasma.
On the other hand, the X-ray bright tail that extends for several
S1
N1
S2
N2 N3
2 arcmin
60 kpc
Figure 6. Zoomed-in region of the Chandra image of the NGC 4839 group,
showing the locations of regions used to probe for surface brightness fluctua-
tions in the ICM. The white arrows indicate the direction in which the surface
brightness profiles were extracted.
6.0
8.0
10.0
12.0
14.0
16.0
S
X
(10
3
cts
s
1
arcmin
2
)
Region N1
Region N2
Region N3
0.5 1.0 1.5 2.0
r (arcmin)
6.0
8.0
10.0
12.0
14.0
16.0
18.0
S
X
(10
3
cts
s
1
arcmin
2
)
Region S1
Region S2
Figure 7. Surface brightness profiles for the northern and southern sides of
the infalling group NGC 4839, following the directions of the white arrows
shown in Fig. 6. While the southern regions show a clear edge as indicated
by a steep drop in the surface brightness profiles, the northern regions show
no clear edge.
MNRAS 000, 1–10 (2015)
6. 6 M. S. Mirakhor et al.
Shock
2 4
16
8 32
Southern edge is sharper than
northern side
Ripple in northern side
Chandra image
Figure 8. Chandra image of the NGC 4839 group along with the GGM filtered images with width scales of 2, 4, 8, 16, 32 pixels. The white bar has a length of
5 arcmin (corresponding to 150 kpc).
hundred kiloparsecs in the wake behind the group argues against
the presence of KHIs. This apparently unmixed tail might indicate
significant viscosity or draped magnetic fields. Alternatively, this
structure could be due to the complex dynamic history of the NGC
4839 group. Since the group is heading back to its second infall as
suggested by several recent studies (e.g. Lyskova et al. 2019), this
peculiar geometry of the tail could be the result of a turbulent motion
as the group is moving through its own gas. However, given that this
KHI-like feature has a small-spatial extent with a length of ∼ 25
kpc, the existing data do not allow us to conclusively constrain its
properties and thus its nature, as the smallest spatial scales on which
we can map the temperature with the current depth of Chandra data
is around 50 × 50 kpc.
5 THE TAIL
5.1 Morphology
A diversity of structures can be seen in the tail of the group from our
deep Chandra observations (Fig. 1, right-hand panel). The inner tail,
the region closest to the head, is relatively straight and extends up to
∼150 kpc from the head. However, the northern side of the inner tail
does not appear to be as straight as the southern side and exhibits
some ripples in the X-ray surface brightness that resemble KHI rolls.
Beyond 150 kpc from the head, the inner tail flares outwards into a
much larger, turbulent outer tail, which extends over several hundred
kiloparsecs. The outermost regions of the tail display an irregular
and patchy morphology, and the gas that is associated with the tail is
likely to be mixed with the surrounding ICM.
5.2 Power spectrum
Previous studies (e.g. Churazov et al. 2012; Gaspari Churazov
2013; Zhuravleva et al. 2014; Gaspari et al. 2014; Hofmann et al.
2016) showed that the characteristic amplitude of the power spec-
trum of X-ray surface brightness fluctuations in the ICM is linearly
proportional to the level of turbulent motion. The slope of the power
spectrum, on the other hand, is sensitive to the level of diffusion
in the ICM, dominated by conduction (Gaspari Churazov 2013).
Therefore, by measuring the power spectrum of surface brightness
fluctuations, we can trace the effects of turbulence and transport
processes in the ICM. Due to the relatively flat surface brightness
distribution and the remarkable level of dynamical activity, the tail
of the NGC 4839 group is an ideal target for investigating the X-ray
surface brightness fluctuations to highlight the underlying physical
properties of the gas associated with the tail. Furthermore, Eckert
et al. (2017) have shown that the shape of the power spectrum is es-
sentially unaffected by projection effects when the motion is mostly
in the plane of the sky, which is the case for the NGC 4839 group
(Colless Dunn 1996).
To extract the power spectrum of the X-ray surface brightness
fluctuations, we defined five regions (shown in Fig. 3) along the tail
of the NGC 4839 group. The goal is to determine how the power
spectrum changes with increasing distance from the head of the
group. For this purpose, region T1 is set to trace the gas of the inner
tail, while the other regions are set to trace the gas of the outer tail.
Below, we provide a detailed description of the analytical procedures
that were followed to calculate the power spectrum of the surface
brightness fluctuations in the tail.
MNRAS 000, 1–10 (2015)
7. NGC 4839 group 7
5.2.1 Method
Following the method described in Churazov et al. (2012), Gaspari
Churazov (2013), and Eckert et al. (2017), the power spectrum of
surface brightness fluctuations was extracted using the modified Δ-
variance method introduced by Arévalo et al. (2012). The raw image
(𝐼) was smoothed by two Gaussian filters with widths that differ by a
small amount. To correct for gaps in the data, the smoothed images
were divided by the smoothed exposure map (𝐸). The difference
between these two images resulted in a convolved image that is
dominated by fluctuations at scales of ∼𝜎,
𝐼𝜎 =
𝐺𝜎1 ◦ 𝐼
𝐺𝜎1 ◦ 𝐸
−
𝐺𝜎2 ◦ 𝐼
𝐺𝜎2 ◦ 𝐸
× 𝐸, (4)
where 𝐺𝜎1 and 𝐺𝜎2 represent, respectively, Gaussian filters of
widths 𝜎1 = 𝜎/
√
1 + 𝜀 and 𝜎2 = 𝜎
√
1 + 𝜀, with 𝜀 1. Following
Arévalo et al. (2012), we fixed 𝜀 at 10−3. The variance of the con-
volved image (𝐼𝜎) for a given wave number 𝑘 is 𝑉𝑘 =
Í
𝐼2
𝜎, and is
linearly proportional to the power spectrum (Churazov et al. 2012).
We convolved the count image by the modified filter (equation 4)
on varying scales from 5–120 pixels. Following Eckert et al. (2017),
we binned the Chandra data by a factor of 2 so that 1 bin = 0.984
arcsec. We calculated the mean surface brightness, 𝜇𝑆, in each region
so that the scaled 2D power spectrum can be given by the following
(Eckert et al. 2017):
𝑃2D(𝑘) =
1
𝜀2𝜋𝑘2𝜇2
𝑆
𝑉𝑘
Í
𝐸2
, (5)
where the characteristic 2D amplitude of the fluctuations is defined
as 𝐴2D =
√︁
𝑃2D(𝑘)2𝜋𝑘2.
5.2.2 Poisson noise and uncertainties
The Poisson noise contribution is expected to be given by a flat
power spectrum. This power can be calculated from the raw image
as following (Eckert et al. 2017):
𝑃Poisson(𝑘) =
1
𝜇2
𝑆
Í
𝐼
Í
𝐸2
. (6)
As discussed in Churazov et al. (2012) and Eckert et al. (2017),
there is a correlation among the powers calculated at different scales,
which introduces a covariance between data points that needs to be
taken into account. In order to estimate the uncertainties and create
the covariance matrix, we calculated the variance in each of the given
regions. We then generated 𝑁r = 10, 000 realizations of the power
spectrum by randomly shuffling the pixel values within each region.
The covariance matrix was calculated by
Σ2
𝑖, 𝑗 =
1
𝑁r
𝑁r
∑︁
ℓ=1
(𝑃ℓ (𝑘𝑖) − ¯
𝑃(𝑘𝑖))(𝑃ℓ (𝑘 𝑗) − ¯
𝑃(𝑘 𝑗)), (7)
where 𝑃ℓ (𝑘𝑖) and 𝑃ℓ (𝑘 𝑗) represent the ℓth realization of the power
spectra at scales 𝑘𝑖 and 𝑘 𝑗, respectively. ¯
𝑃(𝑘𝑖) and ¯
𝑃(𝑘 𝑗) are the
mean power spectra.
The 2D power spectrum and characteristic amplitude of the surface
brightness fluctuations for each selected region in the tail of the NGC
4839 group are shown in Fig. 9, with Poisson noise being subtracted.
The uncertainties are taken from the square root of the diagonal
elements of the covariance matrix. The figure shows that the power
at smaller wave numbers (i.e. larger scale distances) decreases with
increasing distance from the head of the group, while the power at
Table 3. Best-fitting parameters of the power-law fit.
Region 𝛼 𝑃0 Reduced 𝜒2
T1 −2.35+0.07
−0.06
4.68+0.14
−0.12
× 10−3 1.01
T2 −2.16+0.10
−0.08
3.98+0.11
−0.11
× 10−3 1.03
T3 −1.98+0.10
−0.09
1.07+0.08
−0.09
× 10−2 1.10
T4 −1.67+0.04
−0.04
3.47+0.16
−0.16
× 10−2 1.12
T5 −1.37+0.09
−0.07
1.05+0.10
−0.11
× 10−1 1.09
higher wave numbers remains relatively the same. On the other hand,
the characteristic amplitude of surface brightness fluctuations (Fig. 9,
bottom) peaks at a small wave number of ∼ 4 × 10−3 arcsec−1 in the
inner tail (region T1), and as we move to the furthest part of the outer
tail (such as regions T4 and T5), the amplitude peaks at a high wave
number of ∼ 6 × 10−2 arcsec−1.
5.2.3 Slope of the power spectrum
As shown in Fig. 9 and by Eckert et al. (2017), the shape of the power
spectrum closely follows a power law. We, therefore, fitted the power
spectrum for each selected region in the tail with a simple power
law, 𝐹(𝑘) = 𝑃0𝑘𝛼, where 𝑃0 and 𝛼 are the normalisation and slope,
respectively. We used a likelihood function that takes the covariance
between data points into account,
logℒ = −
1
2
𝑁
∑︁
𝑖, 𝑗=1
(𝑃𝑖 − 𝐹(𝑘𝑖))(𝑃𝑗 − 𝐹(𝑘 𝑗))(Σ2
)−1
𝑖, 𝑗, (8)
where (Σ2)−1
𝑖, 𝑗 is the inverse of the covariance matrix. For the fitting
process, we used the affine-invariant ensemble sampler for Markov
chain Monte Carlo implemented in the EMCEE package (Foreman-
Mackey et al. 2013). We chose a burn-in phase of 1,000 steps, fol-
lowed by 10,000 MCMC steps.
The best-fitting power-law models to the extracted power spectra
are shown as blue solid lines in the upper panel of Fig. 9. The best-
fitting parameters and their 68 per cent uncertainties are presented
in Table 3. Overall, the slope gets less steep as the distance from
the head of the group increases. Near the head (region T1), it is the
steepest with a slope of 𝛼 = −2.35+0.07
−0.06
and steadily flattens with
increasing distance to a value of 𝛼 = −1.37+0.09
−0.07
at the furthest part
of the tail (region T5). For all regions, our estimated slope of the
power spectrum is shallower than the slope of the Kolmogorov 2D
power spectrum (∝ 𝑘−8/3).
6 DISCUSSION
6.1 Transport processes
Based on our findings presented in Section 5, the slope of the power
spectrum for the surface brightness fluctuations in the tail of the NGC
4839 group gets less steep as the distance from the head of the group
increases. The power at smaller wave numbers decreases as we move
further away from the head, while the power at higher wave numbers
stays relatively the same. This implies that the power from larger
turbulent eddies decreases with increasing distance from the head,
where the bulk of the turbulence is being generated as the group
falls into the Coma cluster. In keeping with the energy cascade of
turbulent motion (i.e. large eddies feeding small eddies), we should
also see a decrease in contribution from larger turbulent eddies and
MNRAS 000, 1–10 (2015)
8. 8 M. S. Mirakhor et al.
10 2
k (arcsec 1)
100
101
102
103
P
2D
(k)
Region T1
= 2.35+0.07
0.06
10 2 10 1
k (kpc 1)
10 2
k (arcsec 1)
Region T2
= 2.16+0.10
0.08
10 2 10 1
k (kpc 1)
10 2
k (arcsec 1)
Region T3
= 1.98+0.10
0.09
10 2 10 1
k (kpc 1)
10 2
k (arcsec 1)
Region T4
= 1.67+0.04
0.04
10 2 10 1
k (kpc 1)
10 2
k (arcsec 1)
Region T5
= 1.37+0.09
0.07
10 2 10 1
k (kpc 1)
10 2
k (arcsec 1)
10 1
A
2D
(k)
Region T1
10 2 10 1
k (kpc 1)
10 2
k (arcsec 1)
Region T2
10 2 10 1
k (kpc 1)
10 2
k (arcsec 1)
Region T3
10 2 10 1
k (kpc 1)
10 2
k (arcsec 1)
Region T4
10 2 10 1
k (kpc 1)
10 2
k (arcsec 1)
Region T5
10 2 10 1
k (kpc 1)
Figure 9. Top: 2D power spectrum of the surface brightness fluctuations (blue data points) for the five regions in the tail of the NGC 4839 group, with Poisson
noise being subtracted. In each panel, the red data points are the subtracted Poisson noise, and the solid blue line is the best-fitting power-law model to the data,
with covariance taken into account. The best-fitting values of the slope 𝛼 with the 68 per cent confidence interval are shown in the upper-right corner. For all
regions, the slope of the power spectrum is shallower than that of the Kolmogorov 2D power spectrum (green solid line), indicating suppression of thermal
conduction throughout the tail. Bottom: Characteristic 2D amplitude of the surface brightness fluctuations (blue data points). The red data points are Poisson
noise. The observed amplitude is in the approximate range of 0.03–0.10, depending on the scale, suggesting a mild level of turbulence throughout the tail.
an increase from smaller turbulent eddies as the distance from the
head increases. The characteristic amplitude of surface brightness
fluctuations (Fig. 9, bottom) does indeed show this feature. In region
T1, the amplitude peaks at a small wave number, and as one moves to
the outermost region, region T5, the amplitude peaks at a high wave
number.
Using Chandra observations, Eckert et al. (2017) determined the
power spectrum of surface brightness fluctuations in the whole tail
of the infalling group in Abell 2142 to reveal the microphysics of its
gas. In order to have a proper comparison to the power spectrum of
the infalling group in Abell 2142, we constructed a power spectrum
for the entire tail of the NGC 4839 group (Fig. 10). We found a
slope of 𝛼 = −2.33 ± 0.03, which is in agreement with the slope
of 𝛼 = −2.28+0.19
−0.22
for the tail of the infalling group in Abell 2142.
In both cases, the slope is flatter than the Kolmogorov slope. Eckert
et al. (2017) attributed this to the suppression of conductivity within
the bulk of the plasma (Gaspari Churazov 2013). Such suppression
results in the slow mixing of the merging plasma and the long survival
of small-scale fluctuations. The NGC 4839 group exhibits both of
these qualities: the temperature within the tail stays above that of the
surrounding ICM for several hundred kiloparsecs (see Fig. 2) and the
power of small-scale fluctuations is relatively constant throughout
the tail. If thermal conduction was not inhibited, Spitzer-like thermal
conduction would wash out small-scale perturbations in the ICM,
resulting in steeper power spectrum (Gaspari et al. 2014). Therefore,
like the infalling group in Abell 2142, our findings suggest that
thermal conduction is suppressed throughout the tail of the NGC
4839 group.
As can be seen in our Chandra image of the group (Fig. 1, right-
hand panel), the morphology of the tail is complex, but overall can be
described by a 3D conical geometry. For a conical geometry and an
inclination angle of 30 degree or smaller with respect to the plane of
the sky, Eckert et al. (2017) have shown that the normalization of the
3D power spectrum, 𝑃3D(𝑘), changes by a factor of 4 when convert-
MNRAS 000, 1–10 (2015)
9. NGC 4839 group 9
10 2
k (arcsec 1)
10 1
100
101
102
103
Scaled
P
2D
(k)
= 2.33+0.03
0.03 (This work)
= 2.28+0.19
0.22 (Eckert et al. 2017)
Figure 10. Scaled 2D power spectrum of the surface brightness fluctuations
for the entire tail of the NGC 4839 group, compared to that of the infalling
group in Abell 2142 (Eckert et al. 2017). Our estimated slope, which is
in agreement with that estimated for the infalling group in Abell 2142, is
flatter than the Kolmogorov slope, indicating that thermal conduction is being
suppressed throughout the tail.
ing from the projected 2D power spectrum, 𝑃2D(𝑘). As shown in the
bottom panel of Fig. 9, the characteristic 2D amplitude, 𝐴2D(𝑘), is
in the approximate range of 0.03–0.10 throughout the entire range of
wave numbers. Using these values and the conversion factor between
𝑃2D(𝑘) and 𝑃3D(𝑘), we found that the characteristic 3D amplitude,
𝐴3D =
√︁
𝑃3D(𝑘)4𝜋𝑘3, is in the range of 0.02–0.07, suggesting a
mild level of turbulence with a Mach number in the range of 0.1–0.5
(Gaspari Churazov 2013).
6.2 Group dynamics
Cosmological simulations (Lyskova et al. 2019; Sheardown et al.
2019; Zhang et al. 2019) showed that the NGC 4839 group initially
passed through the Coma centre from the northeast and reversed its
direction of motion after reaching the apocentre, and is now heading
back for its second infall. Recent observations of the Coma cluster
with eROSITA (Churazov et al. 2021) and LOFAR (Bonafede et al.
2021) reveal evidence of a faint bridge connecting the NGC 4839
group with the Coma cluster, indicating that the group has already
passed the main cluster. The presence of a shock (e.g. Simionescu
et al. 2013; Akamatsu et al. 2013; Churazov et al. 2022) at the outer
edge of the radio relic to the southwest of the group (e.g. Brown
Rudnick 2011; Bonafede et al. 2022) further supports the post-merger
scenario. In the left-hand panel of Fig. 1, we show the locations of
this shock (labelled as primary shock) and the radio relic.
Previous X-ray studies (e.g. Neumann et al. 2003; Simionescu et al.
2013; Mirakhor Walker 2020) identified a sharp edge to the west
of the Coma core, extending to the region connecting the group to
the Coma cluster (see Fig. 1, left). Using X-ray data from eROSITA,
Churazov etal. (2021) interpreted thisedge as a secondaryor mini-
accretion shock driven by the first passage of the NGC 4839 group
through the Coma core. Based on the numerical simulations of Zhang
et al. (2021), Churazov et al. (2021) argued that this secondary
shock and the one at the outer edge of the radio relic are caused by
the NGC 4839 group. According to this scenario, the gas displaced
by the first passage of the NGC 4839 group eventually falls back
and reaches hydrostatic equilibrium, forming a secondary shock.
While the primary merger shock continues to propagate outwards
and is currently located at the position of the radio relic.
Our Chandra observations of the group reveal the presence of cold
and shock fronts at the interface connecting the group to the Coma
cluster, indicating that the group is heading radially towards the Coma
centre. Based on our spatial and spectral analyses of the leading head,
we can determine the infall velocity of the group from the bow shock.
Using an ambient gas temperature of 3 keV, the local speed of sound
is 𝑐𝑠 = 1480(𝑇/108K)1/2 = 875 km s−1. For a Mach number of
1.5+0.8
−0.5
estimated using the gas temperature, the infall velocity of
the group is 𝜎𝑣 = 𝑐𝑠M = 1315+700
−440
km s−1 relative to the Coma
cluster. The estimated infall velocity drops to 1230+265
−180
km s−1 if
we adopt a Mach number of 1.4+0.3
−0.2
estimated using the gas density.
These values are consistent with the infall velocity of 1700+350
−500
km
s−1 estimated for the NGC 4839 group based on galaxy redshifts and
a dynamical two-body model (Colless Dunn 1996).
7 CONCLUSIONS
Based on our new deep Chandra observations, we have carried out
a detailed analysis of the NGC 4839 group falling into the Coma
cluster. The Chandra observations of the infalling group reveal a
very complex morphology with a long tail of ram-pressure stripped
gas extending for several hundred kiloparsecs, preceded by an X-ray
bright head-like structure. The main findings of this work can be
summarised as follows:
(i) Ouranalysisreveals acoldfrontfeaturewith averyhighdensity
jump at the leading head of the group, preceded by a bow shock of
hot gas to the southeast following the ridge of emission. At this shock
front, the gas temperature drops by a factor of 1.7, corresponding to a
Mach number of 1.5+0.8
−0.5
, statistically consistent with a Mach number
of 1.4+0.3
−0.2
estimated using the gas density.
(ii) The power spectrum of surface brightness fluctuations in the
tail shows that the power at larger scale distances, i.e. smaller wave
numbers, decreases with increasing distance from the head of the
group, while the power at smaller scale distances remains relatively
the same. This implies that the power from larger turbulent eddies de-
creases with distance from the head, where the bulk of the turbulence
is generated as the group falls into the main cluster.
(iii) The slope of the power spectrum of surface brightness fluctu-
ations in the tail of the NGC 4839 group gets less steep as the distance
from the head increases, changing from 𝛼 = −2.35+0.07
−0.06
at the inner
part of the tail to 𝛼 = −1.37+0.09
−0.07
at the outermost part of the tail. The
power spectrum for the entire tail has a slope of 𝛼 = −2.33+0.03
−0.03
, con-
sistent with the slope found in the tail of the infalling group in Abell
2142 (𝛼 = −2.28+0.19
−0.22
; Eckert et al. 2017). Our estimated slopes
are shallower than the slope of the Kolmogorov 2D power spectrum,
indicating that thermal conduction is being suppressed throughout
the tail.
(iv) The characteristic 2D amplitude of surface brightness fluctu-
ations in the tail peaks at large-scale distances in the inner tail, while
as we move to the outermost part of the tail, the amplitude peaks at
small-scale distances. This implies an increase in contribution from
smaller turbulent eddies and a decrease from larger eddies as the
distance from the head increases. We found that the characteristic 2D
MNRAS 000, 1–10 (2015)
10. 10 M. S. Mirakhor et al.
amplitude is in the approximate range of 0.03–0.10 throughout the
entire range of wave numbers, suggesting a mild level of turbulence
with a Mach number in the range of 0.1–0.5 (Gaspari Churazov
2013).
(v) Our Chandra observations of the group reveal a KHI-like
feature along the northern side of the inner tail. The GGM-filtered
images of the group show the gradient in surface brightness along
this side is not as sharp as that along the southern side of the inner tail,
which could indicate the presence of KHI rolls that mix the stripped
gas with the ambient ICM. On the other hand, the X-ray bright tail that
extends for several hundred kiloparsecs in the wake argues against
the existence of KHI rolls, which might indicate significant viscosity
or draped magnetic fields. As an alternative, this peculiar structure
could be due to the complex dynamic history of the group. However,
given the small spatial extent of this structure, the existing data do
not allow us to conclusively determine its nature.
ACKNOWLEDGEMENTS
We thank the referee for their report. MSM, SAW and JR acknowl-
edge support from Chandra grant GO0-21120X. This work is based
on observations obtained with the Chandra observatory, a NASA
mission.
DATA AVAILABILITY
The Chandra Data Archive stores the data used in this paper. The
Chandra data were processed using the Chandra Interactive Anal-
ysis of Observations (CIAO) software. The XMM–Newton Science
Archive (XSA) stores the archival data used in this paper, from which
the data are publicly available for download. The XMM data were
processed using the XMM–Newton Science Analysis System (SAS).
The software packages HEASoft and XSPEC were used, and these can
be downloaded from the High Energy Astrophysics Science Archive
Research Centre (HEASARC) software web page.
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