The Star Formation History of Mass-Selected Galaxies in the Cosmos Field

Abstract

We explore the redshift evolution of the specific star formation rate (SSFR) for galaxies of different stellar mass by drawing on a deep 3.6 µm-selected sample of > 10 5 galaxies in the 2 deg 2 COSMOS field. The average star formation rate (SFR) for sub-sets of these galaxies is estimated with stacked 1.4 GHz radio continuum emission. We separately consider the total sample and a subset of galaxies that shows evidence for substantive recent star formation in the rest-frame optical spectral energy distributions. At redshifts 0.2 < z < 3 both populations show a strong and mass-independent decrease in their SSFR towards the present epoch. It is best described by a power-law (1 + z) n , where n ∼ 4.3 for all galaxies and n ∼ 3.5 for star forming (SF) sources. The decrease appears to have started at z > 2, at least for high-mass (M * 4 × 10 10 M ⊙ ) systems where our conclusions are most robust. Our data show that there is a tight correlation with power-law dependence, SSFR ∝ M * β , between SSFR and stellar mass at all epochs. The relation tends to flatten below M * ≈ 10 10 M ⊙ if quiescent galaxies are included; if they are excluded from the analysis a shallow index β SFG ≈ −0.4 fits the correlation. On average, higher mass objects always have lower SSFRs, also among SF galaxies. At z > 1.5 there is tentative evidence for an upper threshold in SSFR that an average galaxy cannot exceed, possibly due to gravitationally limited molecular gas accretion. It is suggested by a flattening of the SSFR-M * relation (also for SF sources), but affects massive (> 10 10 M ⊙ ) galaxies only at the highest redshifts. Since z = 1.5 there thus is no direct evidence that galaxies of higher mass experience a more rapid waning of their SSFR than lower mass SF systems. In this sense, the data rule out any strong 'downsizing' in the SSFR. We combine our results with recent measurements of the galaxy (stellar) mass function in order to determine the characteristic mass of a SF galaxy: we find that since z ∼ 3 the majority of all new stars were always formed in galaxies of M * = 10 10.6±0.4 M ⊙ . In this sense, too, there is no 'downsizing'. Finally, our analysis constitutes the most extensive SFR density determination with a single technique out to z = 3. Recent Herschel results are consistent with our results, but rely on far smaller samples.Note. -Median stacking-based average 1.4 GHz radio flux densities and derived average quantities for all our bins in mass and redshift for star forming systems within our mass-selected sample. For details see caption of Tab. 2. † Mass bin contains data below the limit of mass representativeness and yields an upper limit to the average SFR (see Sec. 2.6 for further details.) ⋆ First mass bin above the limit of representativeness (see Sec. 2.6) which contains a low fraction (< 15 %) of optically faint objects with m AB (i + ) ≥ 25.5 for which the photo-z accuracy is degraded (see Sec. 2.2 for further details). The average SFR measured in this bin might be slightly overestimated towards higher values (se...

X-shooter reveals powerful outflows in z ∼ 1.5 X-ray selected obscured quasi-stellar objects

“…• The main differences with respect to the previous samples at z∼ 2 is the lower starburstiness and radio luminosity of our targets: on average, our sources lie on or below the MS of star forming galaxies at z∼ 1.5 while both Harrison et al (2012b) and Förster Schreiber et al (2014) targets lie on average on the upper part or above the MS even when considering the redshift evolution of the sSFR (Whitaker et al 2012;Karim et al 2011).…”

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

“…Similarly, the curves for Speagle et al (2014) correspond to parameterizations from their Table 9: "All" and "Mixed" for the all-and star-forming galaxy samples shown here. We also note that the redshift bins of the Karim et al (2011) relations are different than those indicated at the top: 0.6 < z < 0.8, 1.0 < z < 1.2, 1.6 < z < 2.0, 2.0 < z < 2.5, and 2.5 < z < 3.0 respectively. z > 2.…”

“…Unfortunately however, imaging used to probe obscured star formation (typically far-IR and radio) rarely ever reach complementary depths. Thus, many studies over the past several years have turned to measuring SFRs from stacked data in order to compensate for this disparity (e.g., Dunne et al 2009;Rodighiero et al 2010;Karim et al 2011;Whitaker et al 2014;Schreiber et al 2015). However it is important to keep in mind that the interpretation of stacked results may be complicated by the fact that the intrinsic distribution of SFRs may not be unimodal or symmetric.…”

Galaxy formation in the Planck cosmology – I. Matching the observed evolution of star formation rates, colours and stellar masses

“…As in previous figures we show results for the new model of this paper as a solid red line, results for the model of as a dashed red line, and results for the model as a dotted red line. The observations are taken from COSMOS (Karim et al 2011), the Bouwens et al (2012) sample of Lyman-break galaxies, combined Herschel and HST H band-selected catalogues (Schreiber et al 2015) and the Behroozi et al (013a) compilation. The observed rate has a broad peak at relatively low redshift (z ∼ 2 to 3) and declines significantly by z = 0 but also to higher redshift.…”