A SUBSTANTIAL POPULATION OF MASSIVE QUIESCENT GALAXIES AT z 4 FROM ZFOURGE
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We report the likely identification of a substantial population of massive M 1011 M galaxies at z 4 with suppressed star formation rates (SFRs), selected on rest-frame optical to near-IR colors from the FourStar Galaxy Evolution Survey (ZFOURGE). The observed spectral energy distributions show pronounced breaks, sampled by a set of near-IR medium-bandwidth filters, resulting in tightly constrained photometric redshifts. Fitting stellar population models suggests large Balmer/4000 breaks, relatively old stellar populations, large stellar masses, and low SFRs, with a median specific SFR of 2.9 1.8 10-11 yr-1. Ultradeep Herschel/PACS 100 m, 160 m and Spitzer/MIPS 24 m data reveal no dust-obscured SFR activity for 15/19(79%) galaxies. Two far-IR detected galaxies are obscured QSOs. Stacking the far-IR undetected galaxies yields no detection, consistent with the spectral energy distribution fit, indicating independently that the average specific SFR is at least 10 smaller than that of typical star-forming galaxies at z 4. Assuming all far-IR undetected galaxies are indeed quiescent, the volume density is 1.8 0.7 10 -5 Mpc-3 to a limit of log 10 M/M 10.6, which is 10 and 80 lower than at z = 2 and z = 0.1. They comprise a remarkably high fraction (35%) of z 4 massive galaxies, suggesting that suppression of star formation was efficient even at very high redshift. Given the average stellar age of 0.8 Gyr and stellar mass of 0.8 10 11 M, the galaxies likely started forming stars before z = 5, with SFRs well in excess of 100 M yr-1, far exceeding that of similarly abundant UV-bright galaxies at z 4. This suggests that most of the star formation in the progenitors of quiescent z 4 galaxies was obscured by dust. 2014. The American Astronomical Society. All rights reserved..
