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We present a study to determine how star formation contributes to galaxy growth since z=1.5 over five decades in galaxy stellar mass. We investigate the specific star formation rate (SSFR; star formation rate [SFR] per unit galaxy stellar mass) as a function of galaxy stellar mass and redshift. A sample of 175 K-band selected galaxies from the MUnich Near-Infrared Cluster Survey spectroscopic dataset provide intermediate to high mass galaxies (mostly M* > 10^10 Msun) to z=1. The FORS Deep Field provides 168 low mass galaxies (mostly M* < 10^10 Msun) to z=1.5. We use a Sloan Digital Sky Survey galaxy sample to test the compatibility of our results with data drawn from a larger volume. We find that at all redshifts, the SSFR decreases with increasing galaxy stellar mass suggesting that star formation contributes more to the growth of low mass galaxies than to the growth of high mass galaxies, and that high mass galaxies formed the bulk of their stellar content before z=1. At each epoch we find a ridge in SSFR versus stellar mass that is parallel to lines of constant SFR and evolves independently of galaxy stellar mass to a particular turnover mass. Galaxies above this turnover mass show a sharp decrease in the SFR compared to the average at each epoch and the turnover mass increases with redshift. The SFR along the SSFR ridge decreases by roughly a factor of 10, from 10 Msun/yr at z=1.5 to 1 Msun/yr at z=0. High mass galaxies could sustain the observed rates of star formation over the 10 Gyr observed, but low mass galaxies likely undergo episodic starbursts.
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We explore the build-up of stellar mass in galaxies over a wide redshift range 0.4 < z < 5.0 by studying the evolution of the specific star formation rate (SSFR), defined as the star formation rate per unit stellar mass, as a function of stellar mass and age. Our work is based on a combined sample of ~ 9000 galaxies from the FORS Deep Field and the GOODS-S field, providing high statistical accuracy and relative insensitivity against cosmic variance. As at lower redshifts, we find that lower-mass galaxies show higher SSFRs than higher mass galaxies, although highly obscured galaxies remain undetected in our sample. Furthermore, the highest mass galaxies contain the oldest stellar populations at all redshifts, in principle agreement with the existence of evolved, massive galaxies at 1 < z < 3. It is remarkable, however, that this trend continues to very high redshifts of z ~ 4. We also show that with increasing redshift the SSFR for massive galaxies increases by a factor of ~ 10, reaching the era of their formation at z ~ 2 and beyond. These findings can be interpreted as evidence for an early epoch of star formation in the most massive galaxies, and ongoing star-formation activity in lower mass galaxies.
We investigate the location of an ultra-hard X-ray selected sample of AGN from the Swift Burst Alert Telescope (BAT) catalog with respect to the main sequence (MS) of star-forming galaxies using Herschel-based measurements of the star formation rate (SFR) and stellar mass (mstar) from Sloan Digital Sky Survey (SDSS) photometry where the AGN contribution has been carefully removed. We construct the MS with galaxies from the Herschel Reference Survey and Herschel Stripe 82 Survey using the exact same methods to measure the SFR and mstar{} as the Swift/BAT AGN. We find a large fraction of the Swift/BAT AGN lie below the MS indicating decreased specific SFR (sSFR) compared to non-AGN galaxies. The Swift/BAT AGN are then compared to a high-mass galaxy sample (COLD GASS), where we find a similarity between the AGN in COLD GASS and the Swift/BAT AGN. Both samples of AGN lie firmly between star-forming galaxies on the MS and quiescent galaxies far below the MS. However, we find no relationship between the X-ray luminosity and distance from the MS. While the morphological distribution of the BAT AGN is more similar to star-forming galaxies, the sSFR of each morphology is more similar to the COLD GASS AGN. The merger fraction in the BAT AGN is much higher than the COLD GASS AGN and star-forming galaxies and is related to distance from the MS. These results support a model in which bright AGN tend to be in high mass star-forming galaxies in the process of quenching which eventually starves the supermassive black hole itself.
The star formation rate (SFR) is a key parameter in the study of galaxy evolution. The accuracy of SFR measurements at z~2 has been questioned following a disagreement between observations and theoretical models. The latter predict SFRs at this redshift that are typically a factor 4 or more lower than the measurements. We present star-formation rates based on calorimetric measurements of the far-infrared (FIR) luminosities for massive 1.5<z<2.5, normal star-forming galaxies (SFGs), which do not depend on extinction corrections and/or extrapolations of spectral energy distributions. The measurements are based on observations in GOODS-N with the Photodetector Array Camera & Spectrometer (PACS) onboard Herschel, as part of the PACS Evolutionary Probe (PEP) project, that resolve for the first time individual SFGs at these redshifts at FIR wavelengths. We compare FIR-based SFRs to the more commonly used 24 micron and UV SFRs. We find that SFRs from 24 micron alone are higher by a factor of ~4-7.5 than the true SFRs. This overestimation depends on luminosity: gradually increasing for log L(24um)>12.2 L_sun. The SFGs and AGNs tend to exhibit the same 24 micron excess. The UV SFRs are in closer agreement with the FIR-based SFRs. Using a Calzetti UV extinction correction results in a mean excess of up to 0.3 dex and a scatter of 0.35 dex from the FIR SFRs. The previous UV SFRs are thus confirmed and the mean excess, while narrowing the gap, is insufficient to explain the discrepancy between the observed SFRs and simulation predictions.
The slope of the star formation rate/stellar mass relation (the SFR Main Sequence; ${rm SFR}-M_*$) is not quite unity: specific star formation rates $({rm SFR}/M_*)$ are weakly-but-significantly anti-correlated with $M_*$. Here we demonstrate that this trend may simply reflect the well-known increase in bulge mass-fractions -- portions of a galaxy not forming stars -- with $M_*$. Using a large set of bulge/disk decompositions and SFR estimates derived from the Sloan Digital Sky Survey, we show that re-normalizing SFR by disk stellar mass $({rm sSFR_{rm disk}equiv SFR}/M_{*,{rm disk}})$ reduces the $M_*$-dependence of SF efficiency by $sim0.25$ dex per dex, erasing it entirely in some subsamples. Quantitatively, we find $log {rm sSFR_{disk}}-log M_*$ to have a slope $beta_{rm disk}in[-0.20,0.00]pm0.02$ (depending on SFR estimator and Main Sequence definition) for star-forming galaxies with $M_*geq10^{10}M_{odot}$ and bulge mass-fractions $B/Tlesssim0.6$, generally consistent with a pure-disk control sample ($beta_{rm control}=-0.05pm0.04$). That $langle{rm SFR}/M_{*,{rm disk}}rangle$ is (largely) independent of host mass for star-forming disks has strong implications for aspects of galaxy evolution inferred from any ${rm SFR}-M_*$ relation, including: manifestations of mass quenching (bulge growth), factors shaping the star-forming stellar mass function (uniform $dlog M_*/dt$ for low-mass, disk-dominated galaxies), and diversity in star formation histories (dispersion in ${rm SFR}(M_*,t)$). Our results emphasize the need to treat galaxies as composite systems -- not integrated masses -- in observational and theoretical work.
Establishing the stellar masses (M*), and hence specific star-formation rates (sSFRs) of submillimetre galaxies (SMGs) is crucial for determining their role in the cosmic galaxy/star formation. However, there is as yet no consensus over the typical M* of SMGs. Specifically, even for the same set of SMGs, the reported average M* have ranged over an order of magnitude, from ~5x10^10 Mo to ~5x10^11 Mo. Here we study how different methods of analysis can lead to such widely varying results. We find that, contrary to recent claims in the literature, potential contamination of IRAC 3-8 um photometry from hot dust associated with an active nucleus is not the origin of the published discrepancies in derived M*. Instead, we expose in detail how inferred M* depends on assumptions made in the photometric fitting, and quantify the individual and cumulative effects of different choices of initial mass function, different brands of evolutionary synthesis models, and different forms of assumed star-formation history. We review current observational evidence for and against these alternatives as well as clues from the hydrodynamical simulations, and conclude that, for the most justifiable choices of these model inputs, the average M* of SMGs is ~2x10^11 Mo. We also confirm that this number is perfectly reasonable in the light of the latest measurements of their dynamical masses, and the evolving M* function of the overall galaxy population. M* of this order imply that the average sSFR of SMGs is comparable to that of other star-forming galaxies at z>2, at 2-3 Gyr^-1. This supports the view that, while rare outliers may be found at any M*, most SMGs simply form the top end of the main-sequence of star-forming galaxies at these redshifts. Conversely, this argues strongly against the viewpoint that SMGs are extreme pathological objects, of little relevance in the cosmic history of star-formation.
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