2008ApJ...675..234P

Using a sample of ∼28,000 sources selected at 3.6-4.5 µm with Spitzer observations of the Hubble Deep Field North, the Chandra Deep Field South, and the Lockman Hole (surveyed area ∼664 arcmin²), we study the evolution of the stellar mass content of the universe at 0<z<4. We calculate stellar masses and photometric redshifts, based on ∼2000 templates built with stellar population and dust emission models fitting the ultraviolet to mid-infrared spectral energy distributions of galaxies with spectroscopic redshifts. We estimate stellar mass functions for different redshift intervals. We find that 50% of the local stellar mass density was assembled at 0<z<1 (average star formation rate [SFR] 0.048 M_☉/yr/Mpc3), and at least another 40% at 1<z<4 (average SFR 0.074 M_☉/yr/Mpc3). Our results confirm and quantify the ``downsizing'' scenario of galaxy formation. The most massive galaxies (M>10^12.0 M_☉) assembled the bulk of their stellar content rapidly (in 1-2 Gyr) beyond z∼3 in very intense star formation events (producing high specific SFRs). Galaxies with 10^11.5<M<10^12.0 M_☉ assembled half of their stellar mass before z∼1.5, and more than 90% of their mass was already in place at z∼0.6. Galaxies with M<10^11.5 M_☉ evolved more slowly (presenting smaller specific SFRs), assembling half of their stellar mass below z∼1. About 40% of the local stellar mass density of 10^9.0<M<10^11.0 M_☉ galaxies was assembled below z∼0.4, most probably through accretion of small satellites producing little star formation. The cosmic stellar mass density at z>2.5 is dominated by optically faint (R≳25) red galaxies (distant red galaxies or BzK sources), which account for ∼30% of the global population of galaxies, but contribute at least 60% of the cosmic stellar mass density. Bluer galaxies (e.g., Lyman break galaxies) are more numerous but less massive, contributing less than 50% of the global stellar mass density at high redshift.