![]() | Annu. Rev. Astron. Astrophys. 1988. 26:
509-560 Copyright © 1988 by Annual Reviews. All rights reserved |
6.2. Toward a Luminosity Function-Density Relation
It is easy in principle to calculate the local density in any
environment and to construct and intercompare
T(M)
for different
density bins. The problem is that the binning must, by necessity, be
very coarse because with finer binnings the galaxy numbers would be
too small to give any weight to the resulting
T(M).
The coarsest
binning is the traditional "field"-"cluster" separation emphasized
throughout this review. But the LFs for different types have usually
not been compared for the two environments, or at most only at the
bright end, where every LF is approximately the same (cf.
Sections 4, 5). The
treatment of a cluster as a mean environment is quite
unsatisfactory because the local (projected) density in a cluster can
vary by a factor of 100
(Dressler 1980);
indeed, the
morphology-density relation applies even to different regions within a
given cluster (which also proves beyond doubt either that virial
mixing has not occurred or that galaxies have changed morphological
type within a cluster!).
Godwin & Peach (1977)
were the first to look for differences of the
LF within a cluster. Constructing
(M) for the
Coma cluster separately
for an inner and an outer zone, they found a significant deficiency of
bright galaxies in the inner part (neglecting the two dominant cD
galaxies). The same phenomenon for the inner and outer regions of two
other clusters has recently been reported by
Lugger (1987).
Probably this is simply a refinement of the observation of
Oemler (1974) and
Dressler (1978)
that cD clusters on the whole have a deficit of bright
galaxies, perhaps to be interpreted as the result of merging and tidal
stripping
(Dressler 1984;
see also Section 4.1).
Dressler (1980) has compared the LFs of S + Irr and S0 galaxies in low-density and high-density regions of his 55 clusters and found no evidence for any difference. B. Binggeli (unpublished work) did the same for the Virgo cluster alone (which is feasible because of the large number of known members), separately for E, S0, dE, spiral, and Irr galaxies, and again found no clear indication that the type-specific cluster LFs depend on the local density. There is only a hint for E and S0 galaxies to be brighter in higher density regions.
The topic of the LF-morphology-density relation for field galaxies has been addressed by de Souza et al. (1985), Choloniewski & Panek (1987), and Einasto & Einasto (1987). All three studies are essentially based on the CfA redshift survey (Huchra et al. 1983). In the first two studies the local spatial densities have been estimated by a nearest-neighbor algorithm. No density dependence of the LFs was found for the main Hubble types. (A marginal effect was reported for S0s.) Instead of constructing full LFs, Einasto & Einasto (1987) have chosen to calculate the mean absolute magnitude of the three brightest galaxies for different morphological types and environments. Evidence was found that the brightest galaxies in groups and clusters are brighter than those in the field by up to 1 mag; this effect is claimed not to be caused by the Malmquist bias.