Abstract
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−1 Mpc−3), and at least another 40% at 1 < z < 4 (average SFR 0.074 M☉ yr−1 Mpc−3). Our results confirm and quantify the "downsizing" scenario of galaxy formation. The most massive galaxies (M > 1012.0 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 1011.5 < M < 1012.0 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 < 1011.5 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 109.0 < M < 1011.0 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.
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