A SUBSTANTIAL POPULATION OF MASSIVE QUIESCENT GALAXIES AT z ∼ 4 FROM ZFOURGE

Abstract

We report the likely identification of a substantial population of massive M ∼ 10 11 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 Å 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 μm, 160 μm and Spitzer/MIPS 24 μm 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.

The galaxy stellar mass function at 3.5 ≤z≤ 7.5 in the CANDELS/UDS, GOODS-South, and HUDF fields

“…that are faint in the UV rest-frame but are bright in the IR bands, indicating that they are either highly obscured by dust or old/evolved objects, and thus they have large stellar masses. While the existence of passively evolved galaxies at z = 4 has also been shown recently by Straatman et al (2014) using the ZFOURGE data set and deep Herschel observations, in agreement with early results by Fontana et al (2009) in the GOODS-South field, we show here that they do contribute significantly to the high-mass end of the GSMF, certainly at z 4.…”

The Galaxy UV Luminosity Function Before the Epoch of Reionization

“…Galaxy ages are challenging to constrain observationally, due to the degeneracy between dust extinction, age and metallicity in spectral energy distribution (SED) fitting, which are especially severe at z ∼ > 4 where HST covers only the restframe UV wavelengths. Our modeling results are consistent with the picture emerging from multiple recent studies that combine HST and Spitzer/IRAC data and find that the majority of z ∼ > 4 galaxies to have old stellar populations (> 100 Myr) and relatively low specific SFR (∼ 100 M yr −1 ) (Oesch et al 2013a;Straatman et al 2014; see also González et al 2011 for earlier studies reaching the same conclusion but without accounting for nebular emission lines). However, other independent studies based mostly on overlapping datasets reached the different conclusion that z ∼ > 4 galaxies have a high chance of being young systems (ages < 50 Myr) with high specific SFRs (de Barros et al 2014;Finkelstein et al 2015a), suggesting that improved observational constraints are needed to evaluate the fidelity of stellar ages predicted by our model.…”

Massive quiescent galaxies at z &gt; 3 in the Millennium simulation populated by a semi-analytic galaxy formation model

“…This value is much lower than that of the observed density claimed by Straatman et al (2014), ∼ 1 × 10 −6 Mpc −3 . However, we argue below that the number density of the ZF analogues has large uncertainties both in the observation and theoretical models.…”

“…As the quite rare objects, the clustering of the massive QGs is indeed strong at z > 3, and therefore a much larger survey volume than ZFOURGE is needed to securely obtain the accurate observational number density. Secondly, SFR of the QGs given by Straatman et al (2014) may be underestimated; for instance, a part of ZF may be star forming and obscured by dust. Simpson et al (2017) argued that the strong Balmer features shown in ZF are not a unique signature of a poststarburst galaxy and are indeed frequently observed in infrared-luminous galaxies, and understanding high-redshift obscured starbursts will only be possible with multiwavelength and high-resolution observations.…”

“…Daddi et al 2005;Labbé et al 2005;Kriek et al 2008aKriek et al ,b, 2009Fontana et al 2009;Nayyeri et al 2014;Straatman et al 2014;Belli, Newman & Ellis 2015;Kriek et al 2016;Strazzullo et al 2016;Kado-Fong et al 2017). In particular, Glazebrook et al (2017) recently reported the spectroscopic confirmation of a high-redshift massive QG ZF-COSMOS-20115 (hereafter ZF) at z = 3.717 with a stellar mass M ∼ 1.7 × 10 11 M , extremely low star formation rate E-mail: rongyuastrophysics@gmail.com SFR < 4 M yr −1 and short star formation time-scale <250 Myr.…”