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CORES OR CUSPS IN ELLIPTICAL GALAXIES: LUMINOSITY
OR ENVIRONMENT?
ROELOF S. DE JONG, ROGER L. DAVIES, ROBERT F. MINCHIN,
JOHN R. LUCEY AND JAMES STEEL
Univ. of Durham, Dept. of Physics, South Road, Durham, UK
1. The emerging view of galaxy cores
Two classes of elliptical galaxies are now recognised (Kormendy & Bender
1996). Luminous ellipticals rotate slowly (Davies et al. 1983and tend to
have boxy isophotes. Ellipticals fainter than L \Lambda exhibit an increasing ten­
dency to be rotationally supported and to possess a stellar disk component.
This dichotomy led Bender, Burstein & Faber (1992) to suggest that the
physical variable that controls the ultimate nature of a forming galaxy is
the degree of gaseous dissipation that occurs in the final merger it experi­
ences. Low luminosity systems experience more dissipative mergers which
generate high rotation, disky end products. As bigger galaxies are formed,
the mergers become increasingly stellar, producing the classical slow ro­
tating ellipticals. They termed this the gas/stellar continuum. This global
dichotomy is also reflected in the bimodality of core morphologies of the
heterogeneous sample of local ellipticals observed with HST. The low lumi­
nosity disky galaxies have `hard' cores with a steep slope in the luminosity
profile at small radii, whereas the luminous galaxies have `soft' cores with
flat profiles at small radii (e.g. Faber et al. 1997).
2. Coma Cluster cores
In the gas/stellar continuum paradigm, one would expect that in the outer
parts of a cluster it is more likely that the last merger was gaseous than
in its central regions. Therefore, at fixed luminosity, there should be more
galaxies with 'hard' cores in the halo of the cluster than in the cluster
centre. To test this hypothesis, we used WFPC­2 on the HST to survey
a complete magnitude limited sample of ellipticals in the core of Coma,
together with a representative sample drawn from the cluster halo, to de­

2 ROELOF S. DE JONG ET AL.
Figure 1. The slope of the luminosity profile at 0.015 Re as function of absolute
V­magnitude of the galaxy. The Coma core sample is indicated by circles, the Coma
halo sample by squares and the Faber et al. (1997) sample by crosses. Upper limits are
indicated by arrows for some dusty galaxies and for galaxies with Re ! 3:5 00 .
termine the environmental r“ole in shaping cores. Fourteen out of the 46
galaxies show clear dust lanes and rings (evenly distributed among cluster
centre and halo galaxies), and we have not considered these in our analysis.
We used a non­parametric method similar to that used by Gebhardt et
al. (1996) to investigate the inner slope of the luminosity profile. Figure 1
shows this slope, \Gamma@ log(I)=@log(r) at 0.015R e
, as function of magnitude
for all samples. The distribution of the inner slope parameter of the Coma
centre and the Coma halo sample are very similar. The same holds true for
any other comparison of core parameters. Apparently, environment has no
influence on determining core morphology.
The Coma sample also shows no appreciable bi­modality in Fig. 1, con­
trary to the Faber et al. (1997) sample, which has a clear deficiency of
galaxies with \Gamma@ log(I)=@log(r) = 0:45 \Gamma 0:7. Our slope parameter is largely
resolution independent, so the fact that the Faber et al. sample is on aver­
age much nearer (but observed with the pre­refurbished HST) should not
effect our results. It seems unlikely that the Coma and Nuker samples are
drawn from populations with the same intrinsic core properties.
References
Bender R., Burstein D., Faber S. 1992, ApJ 399, 462
Davies R.L., Efstathiou G., Fall S.M., Illingworth G., Schechter P.L. 1983, ApJ 266, 41
Faber S., et al. 1997, astro­ph/961055
Gebhardt K.,et al. 1996, AJ 112, 105
Kormendy J., Bender R. 1996, ApJL 469, L119