Over the last 20 years, a number of workers have studied the multiple nuclei cD galaxy in the rich Abell cluster 1775, trying to discover its nature. In all the cases though, very little has been published concerning its morphology. The majority of arguments about the nature of this object have been based on the relative radial velocities of the 2 components with each other and with the other galaxies in the cluster, or its radio morphology. Very little work has been done on the optical morphology. To rectify that lack of data, the authors have obtained charge coupled device (CCD) images of the cD. The authors find from the CCD data that the cD is unlikely to be a bound object and that there is strong evidence for a collision

Bhattacharya, B.

Hayes, J. J. E.

English

NASA Technical Reports Server

N - 1  
The c D  Galaxy in Abell Cluster 1775 
J. J. E. Hayes and B. Bhattacharya 
Space Telescope Science Institute 
$700 San Martin Drive 
Baltimore, MD 21 21 8 
ABSTRACT: Over the last 20 years, a number of workers have studied the multiple nuclei cD 
galaxy in the rich Abell cluster 1775, trying to discover its nature. In all the cases though, 
very little has been published concerning its morphology. The majority of arguments about the 
nature of the this object have been based on the relative radial velocities of the 2 components 
with each other and with the other galaxies in the cluster, or its radio morphology. Very little 
work has been done on the optical morphology. To rectify that lack of data, we have obtained 
BVRI CCD images of the cD. We find from our CCD data that the cD is unlikely to be a bound 
object and that there is strong evidence for an collision. 
1 Introduction 
The cD system in Abell cluster 1775 has been studied by a number of workers in the past. 
Chincarini et al. (1971), using Palomar Schmidt plates and image-tube spectra, argued that 
this object was a gravitationally bound pair, formjng a stable system having a mass of a few 
times 1013 solar masses. They noted that the pair was in a sparse region of the cluster and that 
the colours of both components were “normal” for ellipticals. The conclusion was that such an 
isolated system which appeared to be undisturbed would be a stable, bound system. However, 
Hintzen (1979) using his original spectroscopy confirmed that the relative velocities of the two 
galaxies were M 1700 kms-l and noted the discovery by Miley and Harris (1977) of a 300 
kpc long stmight radio-tail emerging from the SE galaxy (assuming H, = 75 kms“ Mpc-l). 
From these data Hintzen concluded that the A1775 system could not be a stable bound system, 
but either a superposition of two galaxies in two different clusters, or some sort of interaction. 
The latter conclusion seems to be supported by the discovery of O’Dea and Owen (1985) that 
the NW component is a wide-angle-tail source. Hayes (1982), using Palomar Schmidt plates 
modeled the cD system using the “standard” King (1966) models and found that both the 
stable bound model and the superposition ideas were both unlikely, leaving the collision model. 
We have now obtained broadband BVRI CCD images of the cD, and it is this data that we 
present here. 
2 Observations 
The BVRI CCD images of the A1775 cD were obtained 10 June 1986 under the KPNO Request 
Observing Program. The #1-0.9 m telescope was used in conjunction with the TI2 CCD. The 
f-ratio used was f/7.5, giving an image scale of 0.43n pixele1. The seeing was mostly near 2”. 
Initial flattening, etc., was done on the mountain while the actual reductions were done at 
STScI using the UNIX-based version of the “Vista” reduction package. The filters used are the 
KPNO standard broadband set. 
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https://ntrs.nasa.gov/search.jsp?R=19910007573 2020-03-19T19:51:16+00:00Z
3 Discussion 
The most remarkable feature of the A1775 cD is the unusual shape of the common envelope 
which surrounds the two nuclei. In all of our images it shows up as having a very square or 
boxy appearance - indeed, truncated. In Figure 1 we plot the R isophotes. 
Figure 1: Half-magnitude R 
isophotes of the A1775 cD. North 
is at the top, East to the left 
Indeed, there is a very steep fall off of ligA in the outer regions c the SE component which 
shows up in the brightness profiles of the system. Figure 2 is the V brightness profile at 
position angle = 125.3'. The profile shows that there is 30% more light between the two than 
one would expect for a simple superposition of two ellipticals, thus confirming one of Hayes' 
(1982) conclusions that the system is not a superpostion of two ellipticals. 
Figure 2: V band brightness pro- 
file of A1775. SE is to the left. 
Position angle of the cut is 125.3'. 
In addition, the fainter (NW) component shows very strong isophotal twists and radial varia- 
tions in ellipticity. This would seem to indicate that this component at least is suffering from 
some sort of perturbation. The brighter (SE) component also shows isophotal twists and ellip- 
ticity changes, but not to the same extent. We plot the isophotal twists and the variations in 
ellipticity in Figures 3 and 4 below. 
I96 
.a 
1 
I 
0 
0 1 4 a a  
Radius (are-second.) 
Figure 3: Average isophotal twists. 
The open symbols and solid line are 
for the SE (brighter) component, 
while the solid symbols and dotted 
line refer to the NW component. 
Figure 4: Average ellipticity of the 
cD system. The symbols have the 
same meaning as in Figure 3. 
We have also looked at the (B-V) colours of the two components. We find that for both 
components, (B-V) decreases (as expected for ellipticah) from 1.06 to about 0.80. However, 
the gradient of the index is sharp; the colour drops B 0.2 magnitudes in only a few arc-seconds, 
supporting our notion that the profiles are being truncated by a collision. We see no evidence 
for star formation. 
Other data come from looking at the brightness profiles and comparing them to the deVau- 
couleurs (1948) r1I4 law. In Figure 5 we plot the surface brightness profiles for the two com- 
ponents in each of BVRI (in relative magnitudes) and a scaled deVaucou1eur.s law in each of 
the frames. One notices immediately that none of the profiles are well-fit by the deVaucouleurs 
model. In the outskirts of most of the profiles, the model is too faint by a factor of 0.2 magni- 
tudes. 
Our last data comes from looking at the residuals of models constructed using our photometric 
fits subtracted from theobserved data. We see that the SE component is fairly well modeled 
(except for the core, which is expected as we have qot deconvolved the seeing from the images), 
but there is a great deal of low luminosity diffuse brightness left over from the NW component, 
indicating that this component is not being well-modeled after all. 
I97 
a8 
8 
1.6 
1 
A a 2 1  u 1.4 u u 
W 
Y 
a 
a8 
C 
1.6 
1 L4 1.D 1. I I 
Y 
3A 
Y 
s 
u 
84 
U 
U 
1 11 14 1. 
Figure 5: Brightness profiles of A1775. 
The dotted line is the SE component; 
the dashed line the NW component, and 
the solid line is the rl/* law. We see that 
in virtually all cases, the observed pro- 
files are brighter than the model. Sub- 
figure (a) is the B data; (b) the V data; 
(c )  is the R data, and (d) is the I data. 
Overall the data fits the model quite 
poorly, something expected for a colli- 
sion. 
We believe that this is caused by the effects of a collision, which is being seen in the NW 
component more because it is about half the mass of the SE component (see Conclusions 
below). In Figure 6 we plot the residual contours for the I filter data. 
Figure 6: Residual contours of I filter data. 
Note the large, diffuse area near the posi- 
tion of the NW component, showing that 
the model is not adequate, and we believe is 
indicative of a collision. Both cores are not 
well-modeled as we have not deconvolved the 
seeing from the data. 
20 
10 
-20 
I I I I I I I I  I I I , I 1 I , , I  
20 10 0 -10 -20 
R* (arcsec) 
4 Conclusions 
From the isophotal structure, previous measures of the relative velocities of the two compo- 
nents (Chincarini et al. (1971), Jenner (1974) and Huchra (1982)), and the radio morphology 
mentioned above, we believe that the bound stable model proposed by Chincarini et al. is not 
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tenable. Also, looking at the the (B-V) colour index we see that the colour gradient is steep, but 
that the range in (B-V) (from about 1.06 to about 0.8) is typical, for normal elliptical galaxies. 
The brightness profile of the two galaxies is also diagnostic, in that the amount of light coming 
from between them is more than one would expect if the cD was a simple superposition of two 
galaxies that followed the “standard” King model in brightness (see also Hayes (1982)). On 
the basis of this data, we believe that the idea that the cD is a simple Superposition can also 
be ruled out. 
This leaves us with a collision as the best model left for A1775. The steepness of the colour 
gradients for both components, the boxy-shaped halo, the very poor fits by the deVaucouleurs 
r1I4 law to the brightness profiles, and the residual brightness from the model fitting are all 
evidence for an unusual interaction. It appears that both components are strongly affected by 
each other, and that there has been some severe modification of the normal brightness profile 
(assuming the r1I4 law is universal in all enviroments). Also of interest are the isophotal twists 
because while the outer isophotes of the NW component do point towards the SE component, 
the SE component’s isophotes do not seem to point to the NW (see Figure 1). In this case 
we believe we may be seeing the effects of the difference in mass between the two components. 
The SE component’s core is about twice as bright as the NW component’s core in all colours. 
If we assume a constant M/L ratio for both galaxies, then the SE component is about twice 
the mass of the NW component. Therefore, the NW component of the pair would manifest the 
effects of the collision first and to a greater degree that the more massive SE partner. One is 
inclined to point out that the high relative velocities between the two components (about 1700 
kms-’) is an argument against the galaxies being in the same cluster. However, both A98 and 
A2152 have cluster velocity dispersions which are roughly the same as difference in velocities 
of the two components in A1775 (Noonan 198l), so it seems reasonable to assume that the two 
components are in the same cluster. Therefore, we are probably seeing a high speed collision 
of two unequal mass ellipticals. 
Finally, we would like to thank George Jacoby at NOAO who took the CCD images. 
REFERENCES 
Chincarini, G., Rood, H.J., Sastry, G.N., Welch, G.A.; (1971) Ap. J., 168, 11 
devaucouleurs, G.; (1948) Ann. d’Ap., 11, 247 
Hayes, J.J.E.; (1982) unpublished M.Sc. thesis, Saint Mary’s University, Halifax 
Hintzen, P.; (1979) Pub. A.S. P., 91, 426 
Huchra, J.P.; (1982) - priwate communication 
Jenner, D.C.; (1974) Ap. J., 191, 55 
King, I.R.; (1966) Astron. J., 71, 64 
Miley, G. and Harris D.; (1977) Astr. Ap (Letters)., 61 ,  L23 
Noonan, T.W.; (1981) Ap. J. Supp. Ser., 45, 613 
O’Dea, C.P. and Owen, F.N.; (1985) Astron. J., 90, 927 
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The cD galaxy in Abell cluster 1775

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