Friday, 24 April 2009

The statistical nature of the brightest cluster galaxies

The brightest cluster galaxies (BCGs) lie at the center of clusters, and have very different properties than normal early-type galaxies.  They have flatter brightness profiles, with envelopes that extend well out into intra-cluster space, and have extremely large stellar masses.  Additionally, the luminosity function of cluster galaxies tends to show a bump at large luminosity, which corresponds to the BCGs.  Obviously, BCGs must form differently than other early-types.  But one can still ask the question, are BCG luminosities consistent with being drawn from the overall cluster population?

This issue has been visited several times in the past, and now most recently by Lin, Ostriker, & Miller using clusters detected in SDSS.  Their basic method is to create mock clusters by scrambling the galaxies among the observed clusters, and then to compare the luminosities of the brightest galaxies in the mock clusters to the luminosities of the actual BCGs.  The purple circles in the top panel of this figure show the observed BCG luminosities as a function of total cluster luminosity.  The squares show the BCG luminosities using a running mean, and the green crosses show the running mean computed from the mock clusters.  It appears that BCGs are more luminous than expected based on the mock clusters, a difference that becomes more apparent at high luminosities.

This conclusion is also apparent in the bottom panel, which shows the difference between the observed BCG luminosities and the the expected luminosity (filled purple symbols), compared to the difference observed in one realization of the mock cluster sample (open red).

The authors conclude that, in a flux limited sample of LRGs, BCGs will become more dominant at high redshifts.  So baryonic accoustic oscillation studies will have to take this into account.

No comments: