GALAXY STELLAR MASS FUNCTIONS FROM ZFOURGE/CANDELS: AN EXCESS OF LOW-MASS GALAXIES SINCEz= 2 AND THE RAPID BUILDUP OF QUIESCENT GALAXIES

Abstract

Using observations from the FourStar Galaxy Evolution Survey (ZFOURGE), we obtain the deepest measurements to date of the galaxy stellar mass function at 0.2 < z < 3. ZFOURGE provides wellconstrained photometric redshifts made possible through deep medium-bandwidth imaging at 1-2µm. We combine this with HST imaging from the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS), allowing for the efficient selection of both blue and red galaxies down to stellar masses ∼ 10 9.5 M ⊙ at z ∼ 2.5. The total surveyed area is 316 arcmin 2 distributed over three independent fields. We supplement these data with the wider and shallower NEWFIRM Medium-Band Survey (NMBS) to provide stronger constraints at high masses. Several studies at z ≤ 1.5 have revealed a steepening of the slope at the low-mass end of the stellar mass function (SMF), leading to an upturn at masses < 10 10 M ⊙ that is not well-described by a standard single-Schechter function. We find evidence that this feature extends to at least z ∼ 2, and that it can be found in both the star-forming and quiescent populations individually. The characteristic mass (M * ) and slope at the lowest masses (α) of a double-Schechter function fit to the SMF stay roughly constant at Log(M/M ⊙ ) ∼ 10.65 and ∼ −1.5 respectively. The SMF of star-forming galaxies has evolved primarily in normalization, while the change in shape is relatively minor. Our data allow us for the first time to observe a rapid buildup at the low-mass end of the quiescent SMF. Since z = 2.5, the total stellar mass density of quiescent galaxies (down to 10 9 M ⊙ ) has increased by a factor of ∼12 whereas the mass density of star-forming galaxies only increases by a factor of ∼2.2. * This paper includes data gathered with the 6.5 meter Magellan Telescopes located at Las Campanas Observatory, Chile.

THE SFR–M* RELATION AND EMPIRICAL STAR FORMATION HISTORIES FROM ZFOURGE AT 0.5 < z < 4*

“…Nevertheless, our low-mass slope of γ ∼ 1 is consistent with the relative constancy of the low-mass slope α∼−1.5 of the SMF (Tomczak et al 2014); if γ deviated significantly from unity then α would be expected to evolve strongly with redshift (Peng et al 2010;Weinmann et al 2012;Leja et al 2015). Figure 8 shows that (s 0 , M 0 , γ) follow similar trends with redshift between the star-forming and total samples indicating that the presence and evolution of the turnover mass is intrinsic to the star-forming population and is not simply due to a growing quiescent population diluting the star formation rates at the high mass end.…”

“…Figure 8 shows that (s 0 , M 0 , γ) follow similar trends with redshift between the star-forming and total samples indicating that the presence and evolution of the turnover mass is intrinsic to the star-forming population and is not simply due to a growing quiescent population diluting the star formation rates at the high mass end. We also point out that the apparent similarity in the fitted parameters does not suggest a redshiftindependent quiescent fraction; in fact, the difference between the SFR-M * relations between the star-forming and the total sample are roughly consistent with the evolution of the quiescent fraction as derived from the Tomczak et al (2014) mass functions, and assuming negligible star formation in the quiescent galaxies.…”

“…Given that ZFOURGE covers less area (≈400 arcmin 2 versus ≈900 arcmin 2 for 3D-HST) we acknowledge that cosmic variance could explain the differences mentioned in the previous paragraph. Furthermore, Tomczak et al (2014) quantify cosmic variance among the three ZFOURGE fields in their measurement of the SMF, finding it to range from 8% to 25% over roughly the same mass and redshift ranges as used in this work.…”

“…Leja et al (2015) performed such an analysis using SMFs from Tomczak et al (2014) and SFR-M * relations from Whitaker et al (2012) finding that the inferred growth from star formation greatly overpredicts the observed number densities of galaxies, even on short cosmic timescales (<1 Gyr). Those authors suggest that the SFR-M * relation must have a steeper slope (α  0.9) at masses below 10 10.5 M e at z < 2.5, which is consistent with measurements from this work and recent literature (Whitaker et al 2014;Lee et al 2015;Schreiber et al 2015).…”

The Galaxy UV Luminosity Function Before the Epoch of Reionization

“…We find good agreement with observations (Pérez- González et al 2008;Stark et al 2013;Tomczak et al 2014;Grazian et al 2015;Song et al 2015) taking into account the scatter in observations at z < 2, and the uncertain but potentially significant contribution of massive quiescent galaxies at low redshift. (2015); Song et al (2015).…”