Friday, 17 April 2009

Early Assembly of the Most Massive Galaxies

In this Nature paper, Collins et al. compare the stellar masses of brightest cluster galaxies (BCGs) at z=1-1.5 to BCGs at z~0 and to the predictions from semi-analytic models.  This figure shows the observed masses (red circles; the red cross shows the mean value) compared to the mean mass for a low-redshift sample (dashed line).  It also shows the masses of BCGs in the semi-analytic models (grey diamonds are the masses of individual galaxies, and the black circles are the mean).  It seems that the high-redshift BCGs have already accumulated essentially all of their mass, with very little of the additional growth that is predicted by the models. 

Interestingly, the authors state that two of the BCGs may be undergoing major mergers, which will make them even more massive.

One worry I have is that this analysis relies on matching the high-redshift cluster sample to an appropriate local sample.  The authors don't discuss this matching in detail.  If the local sample was selected to have the same total cluster mass (which I think is the case), then all that the authors have shown is that the ratio of BCG stellar mass to total cluster mass doesn't evolve with redshift.  But, as the authors note (although in a different context), previous studies have already arrived at this conclusion out to z~0.8.

There are other uncertainties in this analysis.  The authors use a crude method to estimate the stellar masses (they seem to use only one or two observed bands).  Additionally, it is well-known that the total mass of extended low-redshift galaxies is difficult to measure (the authors don't address this issue).

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