Friday, 5 March 2010

The Merger-Driven Evolution of Massive Galaxies

Robaina et al. estimate the contribution of major mergers to the growth of the red sequence at M>10^11 Msun from z~1 to z~0.  To do this, they estimate the merger rate of all galaxies (i.e. both red and blue) more massive then 5x10^10 Msun using the fraction of galaxies in close pairs in COSMOS and COMBO-17, and make the assumption that the remnants of all such mergers lie on the red sequence (or that they move onto the RS very quickly).  The data points in this figure show the observed evolution in the number density of M>10^11 Msun RS galaxies, and the curve shows the predicted evolution due to their major merger estimates.

The agreement is impressively good.  But given the uncertainties (in the number density evolution, which is uncertain in part because of the uncertainty in the M/L evolution, as well as the uncertainties involved in getting a merger rate from a correlation function), it doesn't seem like you can draw very strong conclusions.  Additionally, as the authors mention, they somewhat underestimate the growth due to mergers because they would not count a merger between e.g. an 6x10^10 Msun galaxy and an 4x10^10 Msun galaxy.

The authors also find that a present day M>10^11 Msun galaxy has undergone 0.5 major mergers between M>5x10^10 Msun galaxies since z=0.6, and 0.7 such mergers since z=1.2.

Anyway, this is a nice work, and points to the importance in major mergers at the massive end.  I am somewhat curious about what the growth due to more minor mergers is, especially given that minor mergers are supposed to be what drives the size evolution of the RS galaxies.

1 comment:

why said...