and Cosmology

6.2 Galaxies in Clusters and Groups
6.2.9 The Morphology–Density Relation
As mentioned several times before, the mixture of galaxy
types in clusters differs from that of isolated (field)
galaxies. Whereas about 70% of the field galaxies are
spirals, clusters are dominated by early-type galaxies, in
particular in their inner regions. Furthermore, the fraction
of spirals in a cluster depends on the distance to
the center and increases for larger r. Obviously, the local
density has an effect on the morphological mix of
galaxies.
More generally, one may ask whether the mixture of
the galaxy population depends on the local galaxy density.
While earlier studies of this effect were frequently
constrained to galaxies within and around clusters,
new extensive redshift surveys like the 2dFGRS and
the SDSS (see Sect. 8.1.2) allow us to systematically
investigate this question with very large and carefully
selected samples of galaxies. The morphological
classification of such large samples is performed by automated
software tools, which basically measure the
light concentration in the galaxies. A comparison of
galaxies classified this way with visual classifications
shows very good agreement.
Results from the Sloan Digital Sky Survey. As an example
of such an investigation, results from the Sloan
Digital Sky Survey are shown in Fig. 6.11. The galaxies
have been morphologically classified, based on SDSS
photometry, and separated into four classes, corresponding
to elliptical galaxies, S0 galaxies, and early (Sa) and
late (Sc) types of spiral. In this analysis, only galaxies
have been included for which the redshift was spectroscopically
measured. Therefore, the spatial galaxy
density can be estimated. However, one needs to take
into account the fact that the measured redshift is a superposition
of the cosmic expansion and the peculiar
velocity of a galaxy. The peculiar velocity may have
rather large values (∼ 1000 km/s), in particular in clusters
of galaxies. For this reason, for each galaxy in the
sample the surface number density of galaxies which
have a redshift within ±1000 km/s of the target galaxy
has been determined. The left panel in Fig. 6.11 shows
the fraction of the different galaxy classes as a function
of this local galaxy density. A very clear dependence, in
particular of the fraction of late-type spirals, on the local
density can be seen: in regions of higher galaxy density
Sc spirals contribute less than 10% of the galaxies,
whereas their fraction is about 30% in low-density re-
239
Fig. 6.11. The number fraction of galaxies of different morphologies
is plotted as a function of the local galaxy density
(left panel), and for galaxies in clusters as a function of the
distance from the cluster center, scaled by the corresponding
virial radius (right panel). Galaxies have been divided into
four different classes. “Early-types” contain mainly ellipticals,
“intermediates” are mainly S0 galaxies, “early and late
disks” are predominantly Sa and Sc spirals, respectively. In
both representations, a clear dependence of the galaxy mix on
the density or on the distance from the cluster center, respectively,
is visible. In the histograms at the top of each panel,
the number of galaxies in the various bins is plotted