Extragalactic Astronomy and Cosmology by Schneider Peter

Author:Schneider, Peter
Language: eng
Format: epub
Publisher: Springer - A
Published: 2007-08-15T16:00:00+00:00

6.2.2 Luminosity Function of Cluster Galaxies

The luminosity function of galaxies in a cluster is defined as in Sect. 3.7 for the total galaxy population. In many clusters, the Schechter luminosity function (3.38) represents a very good fit to the data if the brightest galaxy is disregarded in each cluster (see Fig. 3.32 for the Virgo Cluster of galaxies). The slope α at the faint end is not easy to determine, since projection effects become increasingly important for fainter galaxies. The value of α seems to vary between clusters, but it is not entirely clear whether this result may also be affected by projection effects in different clusters of differing strength. Thus, no final conclusion has been reached as to whether the luminosity function has a steep increase at L L* or not, i.e., whether many more faint galaxies exist than luminous ~ L*-galaxies (compare the galaxy content in the Local Group, Sect. 6.1.1, where even in our close neighborhood it is difficult to obtain a complete census of the galaxy population). L* is very similar for many clusters, which is the reason why the distance estimate by apparent brightness of cluster members is quite reliable. However, a number of clusters exists with a clearly deviating value of L*.

Many clusters contain cD galaxies at their centers; these differ from large ellipticals in several respects. They have a very extended stellar envelope, whose size may exceed R ~ 100 kpc and whose surface brightness profile is much broader than that of a de Vaucouleurs profile (see Fig. 3.8). cD galaxies are found only in the centers of clusters or groups, thus only in regions of strongly enhanced galaxy density. Many cD galaxies have multiple cores, which is a rather rare phenomenon among the other cluster members.

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