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Register a new userEnvironment and the cosmic evolution of star formation
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Ravi K. Sheth
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We present a mark correlation analysis of the galaxies in the Sloan Digital Sky Survey using weights provided by MOPED. The large size of the sample permits statistically significant statements about how galaxies with different metallicities and star formation histories are spatially correlated. Massive objects formed a larger fraction of their stars at higher redshifts and over shorter timescales than did less massive objects (sometimes called down-sizing). We find that those galaxies which dominated the cosmic star formation at z~3 are predominantly in clusters today, whereas galaxies which dominate the star formation at z~0 inhabit substantially lower mass objects in less dense regions today. Hence, our results indicate that star formation and chemical enrichment occured first in the denser regions of the Universe, and moved to less dense regions at later times.
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The apparent correlation between the specific star formation rate (sSFR) and total stellar mass (M_star) of galaxies is a fundamental relationship indicating how they formed their stellar populations. To attempt to understand this relation, we hypothesize that the relation and its evolution is regulated by the increase in the stellar and gas mass surface density in galaxies with redshift, which is itself governed by the angular momentum of the accreted gas, the amount of available gas, and by self-regulation of star formation. With our model, we can reproduce the specific SFR-M_star relations at z~1-2 by assuming gas fractions and gas mass surface densities similar to those observed for z=1-2 galaxies. We further argue that it is the increasing angular momentum with cosmic time that causes a decrease in the surface density of accreted gas. The gas mass surface densities in galaxies are controlled by the centrifugal support (i.e., angular momentum), and the sSFR is predicted to increase as, sSFR(z)=(1+z)^3/t_H0, as observed (where t_H0 is the Hubble time and no free parameters are necessary). At z>~2, we argue that star formation is self-regulated by high pressures generated by the intense star formation itself. The star formation intensity must be high enough to either balance the hydrostatic pressure (a rather extreme assumption) or to generate high turbulent pressure in the molecular medium which maintains galaxies near the line of instability (i.e. Toomre Q~1). The most important factor is the increase in stellar and gas mass surface density with redshift, which allows distant galaxies to maintain high levels of sSFR. Without a strong feedback from massive stars, such galaxies would likely reach very high sSFR levels, have high star formation efficiencies, and because strong feedback drives outflows, ultimately have an excess of stellar baryons (abridged).
From two very simple axioms: (1) that AGN activity traces spheroid formation, and (2) that the cosmic star-formation history is dominated by spheroid formation at high redshift, we derive simple expressions for the star-formation histories of spheroids and discs, and their implied metal enrichment histories. Adopting a Baldry-Glazebrook initial mass function we use these relations and apply PEGASE.2 to predict the z=0 cosmic spectral energy distributions (CSEDs) of spheroids and discs. The model predictions compare favourably to the dust-corrected CSED recently reported by the Galaxy And Mass Assembly (GAMA) team from the FUV through to the K band. The model also provides a reasonable fit to the total stellar mass contained within spheroid and disc structures as recently reported by the Millennium Galaxy Catalogue team. Three interesting inferences can be made following our axioms: (1) there is a transition redshift at z ~ 1.7 at which point the Universe switches from what we refer to as hot mode evolution (i.e., spheroid formation/growth via mergers and/or collapse) to what we term cold mode evolution (i.e., disc formation/growth via gas infall and minor mergers); (2) there is little or no need for any pre-enrichment prior to the main phase of star-formation; (3) in the present Universe mass-loss is fairly evenly balanced with star-formation holding the integrated stellar mass density close to a constant value. The model provides a simple prediction of the energy output from spheroid and disc projenitors, the build-up of spheroid and disc mass, and the mean metallicity enrichment of the Universe.
We compare the cosmic evolution of star formation rates in galaxies with that of their neutral hydrogen densities. We highlight the need for neutral hydrogen to be continually replenished from a reservoir of ionized gas to maintain the observed star formation rates in galaxies. Hydrodynamic simulations indicate that the replenishment may occur naturally through gas infall, although measured rates of gas infall in nearby galaxies are insufficient to match consumption. We identify an alternative mechanism for this replenishment, associated with expanding supershells within galaxies. Pre-existing ionized gas can cool and recombine efficiently in the walls of supershells, molecular gas can form in situ in shell walls, and shells can compress pre-existing molecular clouds to trigger collapse and star formation. We show that this mechanism provides replenishment rates sufficient to maintain both the observed HI mass density and the inferred molecular gas mass density over the redshift range 0<z<5.
Combining the catalogue of galaxy morphologies in the COSMOS field and the sample of H$alpha$ emitters at redshifts $z=0.4$ and $z=0.84$ of the HiZELS survey, we selected $sim$ 220 star-forming bulgeless systems (Sersic index $n leq 1.5$) at both epochs. We present their star formation properties and we investigate their contribution to the star formation rate function (SFRF) and global star formation rate density (SFRD) at $z < 1$. For comparison, we also analyse H$alpha$ emitters with more structurally evolved morphologies that we split into two classes according to their Sersic index $n$: intermediate ($ 1.5 < n leq 3 $) and bulge-dominated ($n > 3$). At both redshifts the SFRF is dominated by the contribution of bulgeless galaxies and we show that they account for more than 60% of the cosmic SFRD at $z < 1$. The decrease of the SFRD with redshift is common to the three morphological types but it is stronger for bulge-dominated systems. Star-forming bulgeless systems are mostly located in regions of low to intermediate galaxy densities ($Sigma sim 1 - 4$ Mpc$^{-2}$) typical of field-like and filament-like environments and their specific star formation rates (sSFRs) do not appear to vary strongly with local galaxy density. Only few bulgeless galaxies in our sample have high (sSFR $>$ 10$^{-9}$ yr$^{-1}$) and these are mainly low-mass systems. Above $M_* sim 10^{10}$ M$_{odot}$ bulgeless are evolving at a normal rate (10$^{-9}$ yr$^{-1} <$ sSFR $<$10$^{-10}$ yr$^{-1}$) and in the absence of an external trigger (i.e. mergers/strong interactions) they might not be able to develop a central classical bulge.
We present a new exploration of the cosmic star-formation history and dust obscuration in massive galaxies at redshifts $0.5< z<6$. We utilize the deepest 450 and 850$mu$m imaging from SCUBA-2 CLS, covering 230arcmin$^2$ in the AEGIS, COSMOS and UDS fields, together with 100-250$mu$m imaging from Herschel. We demonstrate the capability of the T-PHOT deconfusion code to reach below the confusion limit, using multi-wavelength prior catalogues from CANDELS/3D-HST. By combining IR and UV data, we measure the relationship between total star-formation rate (SFR) and stellar mass up to $zsim5$, indicating that UV-derived dust corrections underestimate the SFR in massive galaxies. We investigate the relationship between obscuration and the UV slope (the IRX-$beta$ relation) in our sample, which is similar to that of low-redshift starburst galaxies, although it deviates at high stellar masses. Our data provide new measurements of the total SFR density (SFRD) in $M_ast>10^{10}M_odot$ galaxies at $0.5<z<6$. This is dominated by obscured star formation by a factor of $>10$. One third of this is accounted for by 450$mu$m-detected sources, while one fifth is attributed to UV-luminous sources (brighter than $L^ast_{UV}$), although even these are largely obscured. By extrapolating our results to include all stellar masses, we estimate a total SFRD that is in good agreement with previous results from IR and UV data at $zlesssim3$, and from UV-only data at $zsim5$. The cosmic star-formation history undergoes a transition at $zsim3-4$, as predominantly unobscured growth in the early Universe is overtaken by obscured star formation, driven by the build-up of the most massive galaxies during the peak of cosmic assembly.
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