MOIRCS DEEP SURVEY. IV. EVOLUTION OF GALAXY STELLAR MASS FUNCTION BACK TO z ∼ 3

We use very deep near-infrared (NIR) imaging data obtained in MOIRCS Deep Survey (MODS) to investigate the evolution of the galaxy stellar mass function back to z similar to 3. The MODS data reach J = 24.2, H = 23.1, and K = 23.1 (5 sigma, Vega magnitude) over 10(3) arcmin(2) (wide) and J = 25.1, H = 23.7, and K = 24.1 over 28 arcmin(2) (deep) in the GOODS-North region. The wide and very deep NIR data allow us to measure the number density of galaxies down to low stellar mass (10(9)-10(10) M-circle dot) even at high redshift with high statistical accuracy. The normalization of the mass function decreases with redshift, and the integrated stellar mass density becomes similar to 8%-18% of the local value at z similar to 2 and similar to 4%-9% at z similar to 3, which are consistent with results of previous studies in general fields. Furthermore, we found that the low-mass slope becomes steeper with redshift from alpha similar to -1.3 at z similar to 1 to alpha similar to -1.6 at z similar to 3 and that the evolution of the number density of low-mass (10(9)-10(10) M-circle dot) galaxies is weaker than that of M* (similar to 10(11) M-circle dot) galaxies. This indicates that the contribution of low-mass galaxies to the total stellar mass density has been significant at high redshift. The steepening of the low-mass slope with redshift is an opposite trend expected from the stellar mass dependence of the specific star formation rate reported in previous studies. The present result suggests that the hierarchical merging process overwhelmed the effect of the stellar mass growth by star formation and was very important for the stellar mass assembly of these galaxies at 1 less than or similar to z less than or similar to 3.