The radial velocities of members of Shakhbazian 4 are determined. It is found that the dispersion of the radial velocities is 440 km/s. The apparent and absolute magnitudes of galaxies in V color are obtained. It is found that the M/L ratio of the group is about 220 solar mass/solar luminosity. The crossing time for the Shakhbazian 4 group is equal to 47 Myr

Amirkhanian, A. S.

Khachikian, E. YE.

Lynds, C. R.

English

NASA Technical Reports Server

-1 
A STUDY OF THE CONPACT GROUP OF GALAXIES SHAHBAZIAN 4 
C. R. Lyndsl, E. Ye. Khachikian', and A. S .  Amirkhanian' 
ABSTRACT. F o r  the members of the compact group of galaxies 
Shahbazian 4 the radial velocities are defined. The velocity 
dispersion is 4 4 0  km s'l. 
magnitudes of galaxies in the V band as well as the mass-to- 
luminosity ratio are obtained. The latter is approximately 220 
M,/L,. The "crossing time" for the group Shahbazian 4 is equal 
to 4 . 7  x lo7 years. It is one of the densest group of galaxies 
with n - 10 
Moreover the appparent and absolute 
4 galaxies Mpce3. 
'Kitt Peak National Observatory, operated by AURA# Inca, under 
contract with the National Science Foundation 
*Byurakan Astrophysical Observatory 
https://ntrs.nasa.gov/search.jsp?R=19910007564 2020-03-19T19:50:57+00:00Z
'c 
Considerable attktion has been given recently to the so- 
called Shahbazian's compact groups of the compact galaxies 
(Shahbazian 1973; Ambartsumian et al., 1975). This is one of the 
interesting classes of groups of galaxies. At present a large 
number of these groups are known and moreover some of them have 
been studied morphologically (Shahbazian and Amirkhanian, 1978) 
and photometrically (Shahbazian, 1978; Amirkhanian and Egikian, 
1987; Amirkhanian, 1988; Amirkhanian et al., 1990). Because of 
the considerable distance of the majority of the compact groups 
and the faintness of their members spectral studies of these 
objects are not numerous. We should like to note pioneering work 
by Robinson and Wampler (1973) concerned with the spectral study 
of the group Shahbazian 1. 
At this time interest in compact groups of galaxies has been 
stimulated for two principal reasons: 
1. These are the most compact objects among the known 
groups of galaxies; the space density of galaxies in these groups 
is very high. 
2. The first determination of the mass-to-luminosity (M/L) 
ratio for the group Shahbazian 1 (Robinson and Wampler, 1973; see 
also Shahbazian, 1978) has given a value one order of magnitude 
less than M/L for double galaxies and close to that of globular 
clusters. 
However, later, a small value of the M/L ratio for the group 
Shahbazian 1 has not been confirmed (Kirshner and Malumuth, 
1980). For other groups the mass-to-luminosity ratio has been 
I34 
found greater as well, from 30 to 260 M,/L, (see Amirkhanian and 
Egikian, 1987; Amirkhanian, 1989; Amirkhanian, et al., 1990). A 
detailed study of the compact groups of galaxies in the lists of 
Shahbazian et al. is of great interest from the point of view of 
the evolution of galaxy groups, too. 
In this work results of spectral and photometric study of 
one of the most remarkable groups in Shahbazian's lists (No. 4) 
are presented. The late publication of the spectral observations 
is due both to objective and subjective reasons. 
In June 1973 spectral observations of five galaxies from the 
compact group Shahbazian 4 were carried out with the 84-inch 
telescope at Kitt Peak National Observatory (USA) in order to 
obtain their radial velocities. The spectra were acquired with 
the grating spectrograph at the Cassegrain focus of the 
telescope. The six-camera secondary electron image intensifier 
of type P829D supplied with a "Lynds mask'' (with the 
photoemulsion extension in the direction perpendicular to the 
dispersion during the exposure) was used as the photodetector. 
The image intensification was great enough to detect individual 
photoelectrons in the spectrum image. 
The wavelength range 3500 - 7500 A with a dispersion of 240 
A/m and a spectral resolution of about 10 A was covered during 
one exposure. In Shahbazian 4 the radial velocities of five 
galaxies (out of seven) are defined by a correlation analysis 
method. The root-mean-square error in determination of radial 
velocities is about 100 km s-l. The radial velocities of group 
member galaxies Vo are given in Table 1. These velocities were 
corrected for solar motion by the formula: 
I35 
I1 A Vo (km s-') = 300 s ing"  cosg 
TABLE 1 
Galaxy C o r r e c t e d  R a d i a l  V e l o c i t y  ' Vo (km s") 
C. G. 4 ( 1 )  
( 2 )  29 110 
( 3 )  30 220 
( 4 )  30 100 
(5) 29 220 
, 
I '  
From t h e  d a t a  i n  Table  1 t h e  mean v e l o c i t y  <Vo> = 29 630 km 
s'l, t h a t  c o r r e s p o n d s  to  a group d i s t a n c e  of 395 Mpc ( h e r e  and 
t h r o u g h o u t  Ho = 75 km s-l Plpc-'). 
between t h e  g a l a x i e s  is < R - l > - l  = 37 kpc. 
d i s p e r s i o n  <AV > 
s o  
The mean harmonic d i s t a n c e  
The r a d i a l  v e l o c i t y  
1/2 
c o r r e c t e d  f o r  o b s e r v a t i o n a l  errors is 440 km 2 
-1 
Taking  f o r  t h e  v i r i a l  mass d e f i n i t i o n  an e x p r e s s i o n  
a c c o r d i n g  t o  Karachentsev  and Karachentseva  (1975)  w e  have: 
1 3  -1 M = 3rG - < b V 2 >  < R - l >  = 1.8 10 Ma I 
n-1 
I36 
where n - t h e  number of g a l a x i e s  i n  group.  
A p i c t u r e  of t h i s  group o b t a i n e d  by one of t h e  a u t h o r s  ( E .  
Y e .  Khach ik ian )  on h y p e r s e n s i t i z e d  emuls ion  Kodak I I IaJ  emuls ion  
(GG385 f i l t e r  w i t h  45 minu tes  exposure  a t  the  prime f o c u s  of t h e  
4 meter t e l e s c o p e  a t  K i t t  Peak O b s e r v a t o r y )  is g i v e n  i n  F i g u r e  
1. The p l a t e  l i m i t i n g  magnitude is a b o u t  23m* 5 ,  
The a p p a r e n t  magni tudes  of the  g a l a x i e s  i n  V have been 
de te rmined  by d e t a i l e d  s u r f a c e  photometry of plates  obtained a t  
t h e  prime f o c u s  of t h e  2.6 meter t e l e s c o p e  of t h e  Byurakan 
A s t r o p h y s i c a l  Obse rva to ry  u s i n g  e x t r a f a c a l  images of stars for 
c a l i b r a t i o n .  The p l a t e  scale is abou t  21  "/mm, The p la tes  have 
been measured w i t h  t h e  S h n e l l  microphotometer ,  The s i z e  of t h e  
scann ing  aperture corresponds to 100 um x 100 urn. The t o t a l  
a p p a r e n t  and c o r r e s p o n d i n g  absolute magnitudes (corrected for 
a b s o r p t i o n  i n  o u r  Galaxy and for r e d s h i f t  a c c o r d i n g  to P e t e r s o n  
(1970)  and Whi t ford  ( 1 9 7 1 ) )  are g i v e n  i n  Table! 2.  
137 
TABLE 2 
Galaxy Apparent Magnitude mv Absolute Elagnitude M, 
17.04 
17.72 
18 . 19 
17.87 
18.46 
19.5 
19 . 73 
~~~~ 
- 21.34 
- 20.66 
- 20.19 
- 20.51 
- 19.92 
- 18.88 
- 18.65 
The total luminosity of the seven galaxies 8.1 x 1O1O I, and 
the mass-to-luminosity ratio is about 220 M,/L,. 
The estimate of the “crossing time” for this group is of 
interest as well. According to Gott, Wrixon and Wanner (1973) we 
get an expression for the crossing time 
(the quantity <R> is equal to 0.045 Mpc for the compact group 
Shahbazian 4 ) .  
Such a small crossing time corresponding to approximately 
1/300 of the age of the Universe age may be interpreted in two 
ways : 
I38 
a. The group h a s  a ve ry  small age (of o r d e r  l o 8  y e a r s )  and ,  
p d s s i b l y ,  it is n o t  s t a b l e :  
b. I f  t h e  group h a s  an age of o r d e r  H - l  = 13 x l o 9  y e a r s  it 
is undoubtedly g r a v i t a t i o n a l l y  connec ted .  
I n  t h e  second case one can suppose  t h a t  t h e  o b s e r v a b l e  group was 
a c o n s i d e r a b l y  r i c h e r  cluster i n  t h e  p a s t  which l o s t  most of i ts 
" l i g h t e r "  ( i .e . ,  f a i n t e r )  members i n  t h e  p r o c e s s  of dynamical  
e v o l u t i o n .  I n  t h e  f a v o r  of t h i s  view t h e  f a c t  speaks  t h a t  f i v e  
g a l a x i e s  from seven  ones  have t h e  h i g h e r  l u m i n o s i t i e s  from - 20m 
up to  - 21m.3. 
I t  is impor t an t  t o  c a l l  a t t e n t i o n  t o  a h igh  s p a c e  d e n s i t y  of 
g a l a x i e s  i n  t h e  group of o r d e r  l o 4  g a l a x i e s  Mpcm3 which is 
c o n s i d e r a b l y  h i g h e r  t h a n  t h a t  of t h e  c e n t r a l  r e g i o n s  of t h e  
r e g u l a r  c l u s t e r s  of g a l a x i e s .  According t o  B a h c a l l  (1975)  t h e  
s p a c e  d e n s i t y  of t h e  g a l a x i e s  amounts from 60 to  300 g a l a x i e s  
M ~ c ' ~  f o r  15 c l u s t e r s  w i t h  t h e  r i c h n e s s  classes 0 ,  1, and 2 f rom 
t h e  A b e l l ' s  l ist (1958) .  The g roup  c o n s i s t s  e x c l u s i v e l y  of E and 
SO/a g a l a x i e s .  According t o  Postman and Geller (1984)  t h e  
overwhelming m a j o r i t y  of t h e  g a l a x i e s  i n  t h e  d e n s e s t  r e g i o n s  of 
t h e  groups  and c l u s t e r s  ( n  ? 600 g a l a x i e s  M P C ' ~ ]  belong t o  E and 
SO types .  T h i s  f a c t  is i n t e r p r e t e d  by them as a r e s u l t  of 
e f f e c t s  i n  t h e  o u t e r  r e g i o n s  of g a l a x i e s  of s t r i p p i n g  mechanisms 
e f f e c t i v e  a t  such  h igh  s p a c e  d e n s i t i e s .  Q u i t e  p o s s i b l y  t h e  same 
mechanism acts i n  t h e  case of compact group Shahbazian  4. 
I39 
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Amirkhanian, A. S . I  and Egikian, A. G. 1987, Astrofizika, 27, 
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I40 
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141 
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I42 


A study of the compact group of galaxies Shahbazian 4

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