A substantial population of massive quiescent galaxies at z~4 from ZFOURGE

• Published in: arXiv.org 2014-02
• Main Authors: Straatman, Caroline M S, Labbé, Ivo, Spitler, Lee R, Allen, Rebecca, Altieri, Bruno, Brammer, Gabriel B, Dickinson, Mark, Pieter van Dokkum, Inami, Hanae, Glazebrook, Karl, Kacprzak, Glenn G, Kawinwanichakij, Lalit, Kelson, Daniel D, McCarthy, Patrick J, Mehrtens, Nicola, Monson, Andy, Murphy, David, Papovich, Casey, Persson, S Eric, Quadri, Ryan, Rees, Glen, Tomczak, Adam, Tran, Kim-Vy H, Tilvi, Vithal
• Format: Article
• Language: English
• Subjects: Astronomical models; Cosmic dust; Galactic evolution; Galaxies; Photometry; Red shift; Star & galaxy formation; Star formation; Stellar age; Stellar populations
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.1312.4952

We report the likely identification of a substantial population of massive M~10^11M_Sun 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. 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/4000AA breaks, relatively old stellar populations, large stellar masses and low SFRs, with a median specific SFR of 2.9+/-1.8 x 10^-11/yr. Ultradeep Herschel/PACS 100micron, 160micron and Spitzer/MIPS 24micron 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 SED fit, indicating independently that the average specific SFR is at least 10x smaller than 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 x 10^-5Mpc^-3 to a limit of log10M/M_Sun>10.6, which is 10x and 80x 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 0.8Gyr and stellar mass of 0.8x10^11M_Sun, the galaxies likely started forming stars before z =5, with SFRs well in excess of 100M_Sun/yr, 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.