1. Photometrical investigation of the shell galaxy NGC 474
Nikola Karavasilev1 , Nikolay Kacharov
Abstract: NGC 474 is a standard lenticular galaxy, surrounded by shells of matter, which origin is still discussed. There are two
possibilities for their formation: merging of two galaxies of low mass or gravitational interaction with the satellite galaxy NGC 470,
without merging. In our lecture we present the results of the surface photometry of NGC 474. According to our measurements, the
surface brightness profile of the galaxy is typical for a lenticular one, even in its central parts, and the color indices if the shells are
consistent with those of NGC 474. This implies that their origin is not due to merging of two galaxies, but to the interaction with
NGC 470.
Keywords: shell galaxies, shell formation, peculiar galaxies, surface photometry, NGC 470, NGC 474
1. Introduction
What is a shell galaxy?
The shell galaxies are a subclass of the peculiar galaxies.
This term was introduced by Malin and Carter in 1980. In
the majority of cases shells are formed around galaxies of
low mass (elliptical or lenticular). According to the Sloan
Digital Sky Survey (SDSS) one sixth of the elliptical and
one third of the lenticular galaxies have shell-like features Fig. 2 – NGC 7600
(Darling), which means that they are a common feature of
low mass galaxies. Consequently, it is important to un- Models for shell formation
derstand the processes of shell formation, in order to un-
derstand the whole picture of galaxy formation. Actually, When the shells have been discovered, their origin has
there are questions, concerning the origin of the shells, been completely unclear. The numerical simulations (N
which are still not answered. bodies simulations) indicate that there are two possible
scenarios, able to explain, what is observed. In each case
of shell galaxy one of them is realized. The first of them is
Variety of their structure merging of two low mass galaxies. In this case the shells
consist of stars, which have been peeled away, during the
The shells are very different by their structure(Darling). merging. They belong to both the galaxies. The second
On 1 and 2 two examples, illustrating this are shown. On possible scenario is close gravitational interaction, with-
the left picture, the galaxy NGC 474 has shells, chaotically out complete merging. Now, the shells are gravitation-
distributed around it, but on the right image, the shells ally liberated stars, belonging to one of the galaxies. They
of NGC 7600 (Turnbull) are almost strictly aligned at one shouls be chaotically distributed arround one of the galax-
straight line. This variety of their structure implies that ies. This is possible, especially, when one galaxy has very
their origin is different. close satellite, which is the case of NGC 474.
Exploration of shell galaxies
Mainly, the shell galaxies are observed in the optical band
and are explored with the methods of the surface pho-
tometry. The shells have extremely low surface bright-
ness, and this is why, obtaining spectra is very hard task.
Actually, during the interaction between the galaxies, the
interstellar gas is heated up to temperatures equal to sev-
eral hundred thousand degrees. Because of this, during
the last years, they are observed also in the X-rays band.
2. NGC 470 and 474 - general information
Fig. 1 – NGC 474
In the table below, the main characteristics of NGC 474
and NGC 470 are presented. They are located in the con-
stellation Pisces and the most appropriate period for ob-
servations from August to November. The galaxies are
1 karavasilev@gmail.com nearly at the celestial Equator, and it is why they can be
ICYA2009 proceedings 1
2. 2 Karavasilev & Kacharov
observed from both the hemispheres of the Earth. NGC
474 is a normal lenticular galaxy, but it is classified as a
peculiar, because of its shells. Actually, it has the bright-
est known shell, all around the sky. Its satellite, NGC 470
is a spiral galaxy, but it has a disturbed structure, due to
the gravitational interaction with NGC 474. It also has an
obvious tidal tails.
Fig. 4 – Frame of NGC 474 and NGC 470
√
average of its major and minor semi-axis ( ab). The pro-
Fig. 3 – General information about NGC 470 and NGC 474 file is typical for a lenticular galaxy and it is not disturbed
even in its central parts. This is important hint for the shell
formation procceses.
3. Observations and data reduction
The observations of NGC 474 were taken during the
night 6-7.11.2008 with the (50 cm) Schmidt telescope at
the National Astronomical Observatory Rozhen and a
CCD camera SBIG STL 11000M, owned by the Astronom-
ical Department at ”St. Kliment Ochridski” University
of Sofia. The camera has a resolution of 4008x2672px
and a field of 48 arcminx70 arcmin, which corresponds to
1, 07 arcsec/px. Three 10-minute exposures were made
in standart B, V and R filters. The seeing was about
3 arcsec. The dark subtraction and dividing by a flat
field were made with the IRIS software. For the surface
photometry we used procedures, written in IDL, by Dr. Fig. 5 – Surface brightness profile of NGC 474 in R band
Boyko Mihov, a research fellow in the Institute of As-
tronomy at the Bulgarian Academy of Sciences. The first
step of the photometry was determination of the back-
Photometrical decomposition
ground level in each filter. The measured values are: fil-
ter B : 21, 39m / arcsec2 ,filter V : 20, 95m / arcsec2 , filter
NGC 474 consists of two main photometrically distin-
R : 19, 79m / arcsec2 . After that, we used another proce-
guishable components: disk and bulge, because it is a
dure to calculate the seeing, using the background stars.
lenticular galaxy. The surface brightness of the disk, as
The third step is to locate the center of the galaxy. Finally,
a function of the distance from the center, is described by
when the previous three steps are completed, we apply
the linear law:
the procedure, which plots the isophotes of the galaxy,
approximating them with ellipses.
1.08r
µ (r ) = µ0 + ( Reshetnikov, 2001), (1)
h
4. Results
where h is a scale factor and µ0 is the surface bright-
On 4 we present the obtained image of the couple. It is a ness in the center. The bulge should obey the Cersic’s law:
combination of the images in the three filters.
r 1/n
µ(r ) = µ0 + 1.0857ν(n)( ) ( Reshetnikov, 2001), (2)
re
Surface photometry
where: µ0 is the surface brightness in the center, re
On 5, the surface brightness (µ) profile of NGC 474 in filter is the effective radius of an aperture, containing half the
R is presented (µ is expressed in magnitudes per square lumonisity of the galaxy, n is an integer, between 3 and 10,
arcsecond (m / arcsec2 ) ). In this case, r is the effective ra- and ν(n) is a positive number, which value depends on
dius of the isophote i.e. it is equal to the geometrically n. In the majority of cases n = 4, and the Cersic’s law is
3. shell galaxies 3
transformed to the law of de Vaucouleurs:
r 1/4
µ(r ) = µe + 8.3268(( ) − 1)( Reshetnikov, 2001) (3)
re
In this case µe is the surface brightness at effective dis-
tance re . To decompose the galaxy into these two compo-
nents means, to find the exact functions, describing them.
First we fitted a linear law to the outer parts of the pro-
file, because there we have only disk. After that, we sub-
tracted the obtained linear equation from the total surface
brightness profile in order to obtain the law, describing
the bulge. According to our measurements, it obeys very
precisely the law of de Vaucouleurs. Here we list the for-
mulas, best describing the surface brightness of the two Fig. 7 – B-R color index as a function of the distance from
components in the three used filters. We also provide the the center of NGC 474
calculated B/D, which is the ratio of luminosity of the
disk to the luminosity of the bulge: graph it is obvious, that the color index of the shell fol-
B filter: lows the trend of the central galaxy (NGC 474). This will
be discussed in the next part of the lecture in the context
disk : µ(r ) = 0.052r [ arcsec] + 21.48 (4)
of shell formation.
bulge : µ(r ) = 6.593r [ arcsec]1/4 + 10.23 (5) On the next plot 8 a two-color diagram, comparing
B-V and V-R color indices of the galaxy and its shells, is
B/D = 0.75 (6)
shown. According to our measurements, the color indices
V filter: of the galaxy are consistent with those of the shells.
disk : µ(r ) = 0.051r [ arcsec] + 20.92 (7)
bulge : µ(r ) = 7.871r [ arcsec]1/4 + 7.487 (8)
B/D = 1.08 (9)
R filter:
disk : µ(r ) = 0.049r [ arcsec] + 19.89 (10)
bulge : µ(r ) = 6.491r [ arcsec]1/4 + 6.686 (11)
B/D = 0.73 (12)
On 6 we present the photometrical decomposition of
NGC 474 in filter R.
Fig. 8 – Two-color diagram
5. Discussion
Based on our photometrical results we can make the con-
clusion, that the more probable scenario for shell forma-
tion in the case of NGC 474 is close gravitational inter-
action, without merging. First, as we mentioned in the
previous part, the profile of the surface brightness is not
disturbed in its central parts. It implies that the galaxy has
never undergone a merging with another galaxy. There-
fore, the merging model can be ruled out. The color index
Fig. 6 – Photometrical decomposition of NGC 474 in R band
B-R of the shells follows the trend of this index of the cen-
tral galaxy. Consequently, it is possible the shells to origi-
nate from NGC 474 (Schombert), and their formation to be
a result of re-destibution of matter, because of the gravita-
The color indices tion from NGC 470. Actually, B-V color index of the outer
shells is about 0.9m . The same index of the outer parts of
The next step of our investigation was to obtain the varia- NGC 470 is approximatelly 0.75m , slightly bluer, than the
tion of the color indices of the galaxy and the shells with outer shells. It is possible to consider them as a gravita-
the distance from the center. On 7 we present the obtained tionally liberated stars from the satellite galaxy and they
result for B − R. Each point represents one shell. On the have reddened, because of their evolution (it is reasonable
4. 4 Karavasilev & Kacharov
to consider that the interstellar dust and gas in the shells
is peeled away and young stars are not formed there, so
the should increase their B-V index) (Turnbull). Therefore,
it is possible the outer shells of NGC 474 to belong to the
outer parts of NGC 470. Finally, we will discuss the struc-
ture of the shells. They are chaotically located around the
main galaxy. According all numerical simulations, it is
possible only if they are not formed as a result of merging
of two galaxies.
6. Conclusion
In general we have two models of shell formation around
low-mass galaxies: merging of two elliptical or lenticular
galaxies and close gravitational interaction, but without
merging. In the case of NGC 474 the more probable seems
to be the second scenario. The profile of the surface bright-
ness of the central galaxy and colors of the shells are, mea-
sured by us confirm that the inner shells belong to NGC
474, but it is possible the outer to be liberated from the
satellite NGC 470. Their chaotical structure is in agree-
ment with the results from the numerical simulations of
the non-merging scenario.
References
Darling, D.
Reshetnikov, V., 2001, Surface photometry of galaxies.
http://www.astro.spbu.ru/staff/resh/Book/index.html.
Schombert, , AJ, 94, 300.
Turnbull, , MNRAS, 307, 967.
