1. THE ASTROPHYSICAL JOURNAL, 465 : L5–L8, 1996 July 1
᭧ 1996. The American Astronomical Society. All rights reserved. Printed in U.S.A.
HUBBLE SPACE TELESCOPE OBSERVATIONS OF OBSCURATION RINGS IN HERCULES A: IMPLICATIONS FOR
ENERGY TRANSPORT IN POWERFUL RADIO GALAXIES
STEFI A. BAUM, CHRISTOPHER P. O’DEA, SIGRID DE KOFF,1 WILLIAM SPARKS,
JEFFREY J. E. HAYES, MARIO LIVIO, AND DANIEL GOLOMBEK
Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218
Received 1996 March 4; accepted 1996 April 12
ABSTRACT
We have used the Hubble Space Telescope to obtain snapshot images of Hercules A, the host galaxy to the
powerful radio source 3C 348, through a broadband red filter. We report the discovery of interlocking,
kiloparsec-scale rings of obscuration aligned near the radio axis and slightly offset from the galaxy’s nucleus. We
discuss possible models for these rings and their implications for models of energy transport in extragalactic radio
jets.
Subject headings: dust, extinction — galaxies: active — galaxies: individual (Hercules A) —
galaxies: interactions — galaxies: ISM — galaxies: jets
1. INTRODUCTION the host galaxy of 3C 348 (Sadun & Hayes 1993; Hayes et al.
1996) contrasts sharply with the low surface brightness of the
The astrophysics of the launching, collimation, and propa-
galaxy itself as seen by HST. Two faint, dark, interlocking rings
gation of jets is one of the outstanding problems in the study
with centers offset 11"5 from the nucleus of Her A are
of powerful radio-loud active galactic nuclei. In this Letter, we
apparent at low S/N in both of these images. The measured
report the discovery with the Hubble Space Telescope (HST) of
geometrical properties of the rings are summarized in Table 1,
a phenomenon that has the potential to shed new light on the
and an idealized sketch based on these properties is presented
transport of energy in powerful radio jets. We present HST
in Figure 2 (Plate L3), where we also indicate the location of
Wide Field Planetary Camera 2 (WFPC2) observations, taken
the nucleus of 3C 348, the orientation of the radio source’s
through the F702W broadband red filter, of Hercules A, the
axis, and the companion galaxy.
host galaxy of the powerful radio source 3C 348. These images
The two rings are slightly elliptical and exhibit different
show two laterally unresolved, interlocking rings of obscura-
morphologies. The eastern, smaller, ring is oriented with its
tion slightly offset from the nucleus, roughly along the radio
center and its long axis directly along the radio jet’s axis and
jet’s axis. We discuss possible origins of these rings and their
has a characteristic width of 1"5 (3 kpc). It appears to be
implications for the nature of the central engine and radio jets
centered on a small but resolved optically emitting feature that
in Her A. We adopt H0 ϭ 75 and q0 ϭ 0.5, which yields a scale
is elongated along the radio axis. The nature of this feature is
of 12 kpc arcsec Ϫ1 at the redshift of Her A ( z ϭ 0.154).
unknown—it could be optical synchrotron emission from a
2. OBSERVATIONS AND REDUCTION knot in the jet, a region of star formation, or an emission-line
region. By contrast, the western, larger, ring is oriented with its
We obtained two 300 s exposures of Her A, with the target center 130Њ from the radio source’s axis and its major axis
centered in the planetary camera of WFPC2, using the broad- virtually perpendicular to the radio axis. This ring has a
band red F702W filter. These observations were obtained characteristic width of 2"25 (5.5 kpc).
during the course of the HST 3CR Snapshot Survey (de Koff The dark rings appear at low S/N in our snapshot images,
et al. 1994). In de Koff et al. (1996), the snapshot data are and it is clear that longer integration, multicolor images are
presented for the sources in the 3CR with redshifts warranted to confirm their existence and to further study their
0.5 Ͼ z Ͼ 0.1, including 3C 348, and we refer the reader to nature. Nevertheless we believe the features are likely to be
that paper for a more detailed description of the observations real and not an artifact of the observations or our eyes. We
and data reduction. have, to date, examined similar WFPC2 images of over 200
The two 300 s frames were combined to reject cosmic rays. 3CR galaxies, and in this source alone have we identified such
Two dark, interlocking rings are visible in the single images as rings. Her A has long been known to be one of only two
well as the combined, cosmic-ray–rejected image, but in all powerful radio sources that show closed loops or bubbles of
cases they are seen at very low signal-to-noise ratio (S/N) synchrotron-emitting plasma in its large-scale radio structure
(near the limit of our ability to detect them). We found that (Dreher & Feigelson 1984; van Breugel & Fomalont 1984).
smoothing the images by a 0"08 Gaussian produced the best Thus, Her A has now been shown to be blowing both dark
visual representation of the rings. optical rings and radio bubbles.
3. RESULTS
4. DISCUSSION
In Figure 1 (Plate L2), we show a gray-scale representation
of our HST images. The bright, high surface brightness, The rings appear as dark regions in which the underlying
elliptical companion located 14Љ northwest of the nucleus of stellar light from the host galaxy is not seen. Thus they appear
to be rings or shells of absorbing material. Below, we first
1 Leiden University. discuss several possible mechanisms for the absorption and
L5

2. L6 BAUM ET AL. Vol. 465
TABLE 1
PROPERTIES OF OBSCURATION RINGS a
Major Axis Minor Axis P.A. P.A. to Nucleus
Ring (kpc) (kpc) (deg) (deg)
East . . . . . . 2.8 2.3 90 90
West. . . . . . 4.8 4.2 0 120
a
Estimated from the HST images.
then discuss possible origins for the rings and their implica-
tions for models of jets in extragalactic radio sources.
4.1. Origin of the Obscuration
We consider two potential mechanisms for the absorption of
the underlying stellar light, (1) dust and (2) Thomson scatter-
ing by electrons.
The contrast ratio between the obscured ring and the
surrounding stellar light is roughly 1Ϻ4. If the absorption is
exponential, then
␶ ϭ Ϫln 1 Ϫ ͩ ⌬S
S ͪ , (1)
FIG. 3.—Plot of the ring/bubble diameter (perpendicular to the jet’s axis) as
where S is the background flux and ⌬S is the depth of the a function of distance from the nucleus.
absorption. This yields an optical depth of ␶ 1 1.4. The change
in magnitude of the background light is ⌬m ϭ Ϫ2.5 ϫ log
(1 Ϫ ⌬S/S) Ϫ1 2 1.5 mag. At the wavelength of the F702W 4.2. Origin of the Rings and Their Association
filter, A(F702W) 3 2E(B Ϫ V ). This implies E(B Ϫ V ) 2 with the Radio Source
0.75. If the dust-to-gas ratio is approximately the Galactic We consider several possible origins for the optical obscu-
value, then, from Burstein & Heiles (1978), ration rings. First, they may, of course, bear no relation to the
N͑H͒ 2 5.0 ϫ 1021 ͓E͑B Ϫ V ͒ ϩ 0.06͔ cm Ϫ2 , (2) radio jets or the nuclear activity but represent, for example,
Ϫ2
remnants from a merger or resonant structures in the host
which yields N(H) 2 4.1 ϫ 10 cm . 21
galaxy. It is clear that the optical obscuring rings are not in an
While, in principle, we should be able to determine whether equilibrium configuration, however; they must be transient or
the obscuring material is distributed in a bubble or a true ring evolving features. Since 3C 348 is a radio-loud, active source
by looking for obscuration that is internal to the ring, the S/N with clear radio jets and lobes (a phenomenon present in only
in the current data is too low to allow an investigation of this 11% of all galaxies at Her A’s absolute magnitudes), and since
question. Regardless of the true distribution of the obscuring the radio structure of 3C 348 is distinguished even within that
material, we can approximate the path length through it at the class of select sources by the presence of closed loops or
observed ring to be half the diameter of the ring. Assuming, bubbles or radio synchrotron– emitting plasma, it is certainly
therefore, a path length of 1Љ (2 kpc), we derive an average gas worthwhile to seek origins for the rings that are related to that
density in the ring of nH 2 0.7 cm Ϫ3 for gas with a filling factor activity.
of unity. If the gas is clumpy, the average density will be higher. In Figure 3, we plot the diameter perpendicular to the radio
If the absorption is instead due to Thomson scattering by axis of the observed optical and radio rings as a function of
electrons with density ne and path length r, the optical depth radius from the nucleus. Any model that seeks to explain the
is given by optical and radio rings with a common mechanism must be
␶ ϭ ne ␴ T r , (3) able to explain the relation shown; the optical and radio rings
follow a roughly linear relation between diameter and dis-
where ␴ T is the Thomson scattering cross section. For a path tance. We consider three possibilities below.
length of 2 kpc and an optical depth of 1.4 we thus have One possibility is that the rings are dusty molecular clouds
ne 2 340 cm Ϫ3 . Depending on the temperature of the gas in that have been entrained and transported along the jets or
that instance, we might expect significant line emission from within a turbulent sheath around the jet. The clouds might
such a dense ionized gas. Therefore spectroscopy of the rings have been pulled into ringlike structures by turbulent eddies in
should be obtained. the propagating jet flow. Those same turbulent eddies might
A priori, then, with the current observation, we cannot manifest themselves as the radio bubbles seen on the large
distinguish between a dust or an electron-scattering origin for scale. Thus this model would suggest a common origin for the
the observed obscuring rings since, in both cases, the derived small-scale (1kpc) optical rings and the large-scale (10s of
densities are certainly within the realm of feasibility. Interest- kpc) radio bubbles.
ingly, the HST images do not show any other evidence of dust A second possibility is that the rings might be produced by
in the galaxy in the form of dust disks or filaments. We also expanding bubbles of hot gas, which either produce the opacity
find no strong evidence for distorted optical isophotes in the by compressing dust along their outer edges or via electron
host or companion galaxy. scattering off the hot gas itself. To explain the two bubbles on

3. No. 1, 1996 OBSCURATION RINGS IN HERCULES A L7
alternate sides of the nucleus, we would posit that hot gas timescale involved have changed with time, since, while there
bubbles were ejected roughly along the radio axis and at are dramatic wiggles apparent in the large-scale lobe structure
roughly (though perhaps not exactly) the same times. If these of Her A, the inner radio jet (within 115 kpc of the nucleus)
are hot, expanding bubbles, since the bubbles are roughly as is remarkably straight; (2) there has been a wide-angle nuclear
large (in diameter) as their distance from the nucleus, their outflow that has swept dense cold gas into a bipolar structure
expansion and ejection velocities must be roughly equal. This in the inner few kiloparsecs of 3C 348. To explain the
suggests that the dynamics of the rings are “bubble-like,” asymmetry of the two optical rings with the wobbling, etching
rather than “jetlike.” The possibility that optical line– emitting jet model, one must presume that either the wide-angle
gas might be associated with the optical rings should be outflow was asymmetric on opposite sides of the nucleus or
explored, as such gas would allow a direct measurement of the that the jets themselves have slightly different orientations on
bubbles’ propagation and expansion velocities. the two sides of the source. This model would also need to
If the small-scale optical rings are expanding hot gas bub- explain why the rings are seen in obscuration; presumably, the
bles, do they share a common origin with the large-scale radio role of the jets must be to compress that gas and/or dust in the
bubbles? Early models of radio jets hypothesized that, rather conical wind, thereby increasing its density.
than being continuously ejected in hydromagnetic flows, they Of the three models for the origin of the optical and radio
might be composed of a series of plasmons— clouds of hot, rings presented here, the plasmon model seems the most
radio-emitting plasmons pinched off in periodic ejections from straightforward, and it most naturally explains the roughly
the nucleus (e.g., De Young & Axford 1967; Christiansen linear relationship observed between the rings’ diameters and
1969; Jaffe & Perola 1973; Pacholczyk & Scott 1976; Chris- distance from the nucleus. Given the present data, however,
tiansen, Pacholczyk, & Scott 1981). Plasmon models fell into other models cannot be excluded.
disfavor when Very Large Array observations showed radio
jets to be linear and continuous. Plasmon models are energet-
ically unfavorable compared to continuous or jet-flow models, 5. SUMMARY
as a result of the large adiabatic losses suffered during the
We have presented HST WFPC2 broadband red images of
expansion of the bubbles as they move outward into the lobes.
the powerful radio galaxy Hercules A and reported the
From our observed optical rings to the radio bubbles, expan-
detection of two faint dark rings of obscuration with an
sion factors of only 14 in radius are seen, implying adiabatic
interlocking, “bipolar” appearance. These rings are 12Љ
energy losses of a factor of 14. Plasmon models require
(4 kpc) in diameter and are offset by 11"5 (3 kpc), roughly
continuous reacceleration of synchrotron particles by shocks
along the radio axis. The morphologies and orientations of the
along the expanding plasma bubble or in instabilities along the
two rings are distinct; one is oriented along the radio axis and
surfaces of those bubbles as they interact with the ambient
appears to be centered on a small optical feature that is itself
medium. Deep radio observations of the inner few kiloparsecs
oriented along the radio axis. The other, located on the
of Her A should be undertaken to determine if the obscuring
opposite side of the nucleus, has its long axis roughly perpen-
optical rings are visible in the radio as bubbles or plasmons. If
dicular to the radio axis and its center offset by 130Њ from that
a relationship between the dust bubble and visible radio
axis. The radio source associated with Her A, 3C 348, was
structure is discovered, this would support a model for the
already known from VLA imaging to have a unique radio
radio source in terms of ejection of discrete plasmons rather
structure with radio bubbles/close loops in its eastern radio jet
than a continuous jet (Dreher & Feigelson 1984). We note
and lobe. We suggest that the optical rings may be caused by
that sporadic relativistic ejection of radio-emitting plasmoids
dust obscuration or by electron scattering. We have considered
in a double-jet geometry has been observed in the Galactic
scenarios in which they are related to the radio jet. These
superluminal transient sources GRS 1915ϩ105 (Mirabel &
obscuring rings may trace the interaction of precessing radio
Rodrı ´guez 1994) and GRO I1655Ϫ40 (Hjellming & Rupen
jets with the ambient medium or may be due to shells swept up
1995).
by expanding radio plasmons. The plasmon model explains the
A third possibility is that the optical obscuring rings in Her
observed roughly linear relationship between bubble diameter
A are produced in a fashion similar to the way in which
and distance from the nucleus most naturally.
optically emitting rings and bubbles are produced in stellar
Further, deeper, multicolor optical observations and deeper
outflows in Galactic objects, e.g., planetary nebulae (Livio
high-resolution VLA radio images will be required to deter-
1996) and luminous blue variables (Nota et al. 1995). In these
mine the nature of these fascinating optical structures and
objects it is believed that a wind from a central object is shaped
their relationship to the activity in Her A. It will also be
via a density contrast in the ambient medium into a bipolar
important to determine whether Her A (and possibly 3C 310)
outflow. The interaction of a precessing or wobbling jet with
are “special” in their energy transport properties, or whether
this preexisting surface can cause a ring to be traced out (see,
they tell us that episodic ejections rather than continuous
e.g., Livio 1996). Such a model has been discussed in the
outflow are common in powerful radio galaxies.
context of the offset rings in SN 1987A (Burrows et al. 1995).
Precession or wobbling of the radio jet’s axis in Her A is
supported by the observed wiggles in the large-scale radio jets This work was supported by NASA through grant GO-
and the overall point symmetry of the envelope of the radio 5476.01 from the Space Telescope Science Institute, which is
structure (see Dreher & Feigelson 1984). If this scenario is operated by the Association of Universities for Research in
correct, it would imply that (1) the precession cone and Astronomy, Inc., under NASA contract NAS 5-26555.
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5. PLATE L2
FIG. 1.—Gray-scale image of the planetary camera F702W image of Her A
BAUM et al. (see 465, L5)

6. PLATE L3
FIG. 2.—Idealized sketch of the interlocking “rings” superposed on the gray-scale image of Her A. Arrows indicate the locations of the nucleus of 3C 348, the
companion galaxy, and the radio source’s axis.
BAUM et al. (see 465, L5)