Stellar masses of SDSS-III BOSS galaxies at z~0.5 and constraints to galaxy formation models

We calculate stellar masses for ~400,000 massive luminous galaxies at redshift ~0.2-0.7 using the first two years of data from the Baryon Oscillation Spectroscopic Survey (BOSS). Stellar masses are obtained by fitting model spectral energy distributions to u,g,r,i,z magnitudes. Accurate BOSS spectroscopic redshifts are used to constrain the fits. We find that the distribution of stellar masses in BOSS is narrow (Delta log M ~0.5 dex) and peaks at about log M/M_sun ~ 11.3 (for a Kroupa initial stellar mass function), and that the mass sampling is uniform over the redshift range 0.2 to 0.6, in agreement with the intended BOSS target selection. The galaxy masses probed by BOSS extend over ~ 10^{12} M_{sun}, providing unprecedented measurements of the high-mass end of the galaxy mass function. We find that the galaxy number density above ~ 2.5 10^{11} M_{sun} agrees with previous determinations within 2sigma, but there is a slight offset towards lower number densities in BOSS. This alleviates a tension between the z < 0.1 and the high-redshift mass function. We perform a comparison with semi-analytic galaxy formation models tailored to the BOSS target selection and volume, in order to contain incompleteness. The abundance of massive galaxies in the models compare well with the BOSS data. However, no evolution is detected from redshift ~ 0.6 to 0 in the data, whereas the abundance of massive galaxies in the models increases to redshift zero. BOSS data display colour-magnitude (mass) relations similar to those found in the local Universe, where the most massive galaxies are the reddest. On the other hand, the model colours do not display a dependence on stellar mass, span a narrower range and are typically bluer than the observations. We argue that the lack of a colour-mass relation in the models is mostly due to metallicity, which is too low in the models.