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Register a new userNature or nurture? Clues from the distribution of specific star formation rates in SDSS galaxies
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This work investigates the main mechanism(s) that regulate the specific star formation rate (SSFR) in nearby galaxies, cross-correlating two proxies of this quantity -- the equivalent width of the Ha line and the $(u-r)$ colour -- with other physical properties (mass, metallicity, environment, morphology, and the presence of close companions) in a sample of $sim82500$ galaxies extracted from the Sloan Digital Sky Survey (SDSS). The existence of a relatively tight `ageing sequence in the colour-equivalent width plane favours a scenario where the secular conversion of gas into stars (i.e. `nature) is the main physical driver of the instantaneous SSFR and the gradual transition from a `chemically primitive (metal-poor and intensely star-forming) state to a `chemically evolved (metal-rich and passively evolving) system. Nevertheless, environmental factors (i.e. `nurture) are also important. In the field, galaxies may be temporarily affected by discrete `quenching and `rejuvenation episodes, but such events show little statistical significance in a probabilistic sense, and we find no evidence that galaxy interactions are, on average, a dominant driver of star formation. Although visually classified mergers tend to display systematically higher EW(H$alpha$) and bluer $(u-r)$ colours for a given luminosity, most galaxies with high SSFR have uncertain morphologies, which could be due to either internal or external processes. Field galaxies of early and late morphological types are consistent with the gradual `ageing scenario, with no obvious signatures of a sudden decrease in their SSFR. In contrast, star formation is significantly reduced and sometimes completely quenched on a short time scale in dense environments, where many objects are found on a `quenched sequence in the colour-equivalent width plane.
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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.
We investigate the origin, the shape, the scatter, and the cosmic evolution in the observed relationship between specific angular momentum $j_star$ and the stellar mass $M_star$ in early-type (ETGs) and late-type galaxies (LTGs). Specifically, we exploit the observed star-formation efficiency and chemical abundance to infer the fraction $f_{rm inf}$ of baryons that infall toward the central regions of galaxies where star formation can occur. We find $f_{rm inf}approx 1$ for LTGs and $approx 0.4$ for ETGs with an uncertainty of about $0.25$ dex, consistent with a biased collapse. By comparing with the locally observed $j_star$ vs. $M_star$ relations for LTGs and ETGs we estimate the fraction $f_j$ of the initial specific angular momentum associated to the infalling gas that is retained in the stellar component: for LTGs we find $f_japprox 1.11^{+0.75}_{-0.44}$, in line with the classic disc formation picture; for ETGs we infer $f_japprox 0.64^{+0.20}_{-0.16}$, that can be traced back to a $z<1$ evolution via dry mergers. We also show that the observed scatter in the $j_{star}$ vs. $M_{star}$ relation for both galaxy types is mainly contributed by the intrinsic dispersion in the spin parameters of the host dark matter halo. The biased collapse plus mergers scenario implies that the specific angular momentum in the stellar components of ETG progenitors at $zsim 2$ is already close to the local values, in pleasing agreement with observations. All in all, we argue such a behavior to be imprinted by nature and not nurtured substantially by the environment.
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.
We perform the first spatially-resolved stellar population study of galaxies in the early universe (z = 3.5 - 6.5), utilizing the Hubble Space Telescope Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) imaging dataset over the GOODS-S field. We select a sample of 418 bright and extended galaxies at z = 3.5 - 6.5 from a parent sample of ~ 8000 photometric-redshift selected galaxies from Finkelstein et al. (2015). We first examine galaxies at 3.5< z < 4.0 using additional deep K-band survey data from the HAWK-I UDS and GOODS Survey (HUGS) which covers the 4000A break at these redshifts. We measure the stellar mass, star formation rate, and dust extinction for galaxy inner and outer regions via spatially-resolved spectral energy distribution fitting based on a Markov Chain Monte Carlo algorithm. By comparing specific star formation rates (sSFRs) between inner and outer parts of the galaxies we find that the majority of galaxies with the high central mass densities show evidence for a preferentially lower sSFR in their centers than in their outer regions, indicative of reduced sSFRs in their central regions. We also study galaxies at z ~ 5 and 6 (here limited to high spatial resolution in the rest-frame ultraviolet only), finding that they show sSFRs which are generally independent of radial distance from the center of the galaxies. This indicates that stars are formed uniformly at all radii in massive galaxies at z ~ 5 - 6, contrary to massive galaxies at z < 4.
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