In clusters of galaxies, various physical processes play a role: Clusters are gravitationally dominated by dark matter but also contain vast quantities of hot gas ( Research Highlight May 2004). This hot gas cools by emitting X-ray radiation, decreasing its temperature and allowing more gas to flow to the center (so-called "cooling flows", Research Highlight November 2005). The galaxy at the center of a galaxy cluster sits at the center of the dark matter halo, where also the gas density is highest. Furthermore, these galaxies assembled via many mergers of mostly cluster galaxies. The goal of our investigation was to find out whether any of these processes cause BCGs to be notably different from normal galaxies.

For a sample of 625 galaxy groups and clusters from the Sloan Digital Sky Survey (SDSS; Research Highlight April 2002) we have selected the brightest galaxy in the densest, central part of the cluster (Fig. 2). From a catalog of 200000 SDSS galaxies, we chose for each BCG a control galaxy of similar stellar mass, redshift, and color (the color of a galaxy reflects the average age of its stars). Any systematic difference between the two samples can be attributed to the environment at the cluster center.

We find that BCGs are larger than non-BCGs, and have higher velocity dispersions (a measure of the speed of the stars that make up the galaxy). These findings imply a higher dark matter fraction in BCGs, which can be viewed as a direct consequence of their location at the center of a dark matter halo.

Since most BCGs are elliptical galaxies, we can test whether they follow the scaling relations which apply to other ellipticals. One of these is the Faber-Jackson relation, which relates the galaxy luminosity and velocity dispersion. While the non-BCG control sample lies on the same relation as elliptical galaxies in general, for BCGs the relation is significantly different. This is a confirmation of theoretical studies, which predict that the Faber-Jackson relation is determined by the eccentricity of merger orbits, i.e. the merger remnants of galaxy mergers which occur on highly elliptical orbits lie on a different Faber-Jackson relation than those taking place on more circular orbits. Since clusters are located at the intersections of the "cosmic web", the BCGs are expected to form from mergers along the filaments of the matter distribution, and thus mostly elliptical merger orbits.

Finally, we find that BCGs are more likely to host a radio-loud active galactic nucleus ("AGN"). This is particularly interesting, since it has been suggested that these AGNs stop the cooling flows in clusters ( Research Highlight November 2005).

Although the properties of BCGs are not exceedingly different from normal galaxies, we can trace the influences of three processes typical of the cluster center: the dark matter halo alters the structural properties, the merging history is reflected in different scaling relations, and the hot gas causes BCGs to be more likely to host radio-loud AGN.

Anja von der Linden

Publication

Anja von der Linden, Philip N. Best, Guinevere Kauffmann, Simon D.M. White,
"How special are Brightest Group and Cluster Galaxies?",
Submitted to MNRAS; astro-ph/0611196

Philip N. Best, Anja von der Linden, Guinevere Kauffmann, Timothy M. Heckman, Christian R. Kaiser,
"On the prevalence of radio-loud AGN in brightest cluster galaxies: implications for AGN heating of cooling flows",
Submitted to MNRAS; astro-ph/0611197