The Munich Near-Infrared Cluster Survey: Number Density Evolution of Massive Field Galaxies to z~1.2 as Derived from the K-Band-selected Survey

We derive the number density evolution of massive field galaxies in the redshift range 0.4<z<1.2 using the K-band-selected field galaxy sample from the Munich Near-IR Cluster Survey. We rely on spectroscopically calibrated photometric redshifts to determine distances and absolute magnitudes in the rest-frame K band. To assign mass-to-light ratios, we use an approach that maximizes the stellar mass for any K-band luminosity at any redshift. We take the mass-to-light ratio, M/L_K, of a simple stellar population that is as old as the universe at the galaxy's redshift as a likely upper limit. This is the most extreme case of pure luminosity evolution, and in a more realistic model M/L_K will probably decrease faster with redshift because of increased star formation. We compute the number density of galaxies more massive than 2×10¹⁰, 5×10¹⁰, and 1×10¹¹ h^-2 M_solar, finding that the integrated stellar mass function is roughly constant for the lowest mass limit and that it decreases with redshift by a factor of ~3 and by a factor of ~6 for the two higher mass limits, respectively. This finding is in qualitative agreement with models of hierarchical galaxy formation, which predict that the number density of ~M^* objects is fairly constant while it decreases faster for more massive systems over the redshift range that our data set probes.