Star formation and mass assembly in high redshift galaxies

Abstract

Aims. The goal of this work is to infer the star formation properties and the mass assembly process of high redshift (0.3 ≤ z < 2.5) galaxies from their IR emission using the 24 μm band of MIPS-Spitzer. Methods. We used an updated version of the GOODS-MUSIC catalog, which has multiwavelength coverage from 0.3 to 24 μm and either spectroscopic or accurate photometric redshifts. We describe how the catalog has been extended by the addition of mid-IR fluxes derived from the MIPS 24 μm image. We compared two different estimators of the star formation rate (SFR hereafter). One is the total infrared emission derived from 24 μm, estimated using both synthetic and empirical IR templates. The other one is a multiwavelength fit to the full galaxy SED, which automatically accounts for dust reddening and age-star formation activity degeneracies. For both estimates, we computed the SFR density and the specific SFR. Results. We show that the two SFR indicators are roughly consistent, once the uncertainties involved are taken into account. However, they show a systematic trend, IR-based estimates exceeding the fit-based ones as the star formation rate increases. With this new catalog, we show that: a) at z > 0.3, the star formation rate is correlated well with stellar mass, and this relationship seems to steepen with redshift if one relies on IR-based estimates of the SFR; b) the contribution to the global SFRD by massive galaxies increases with redshift up to 2.5, more rapidly than for galaxies of lower mass, but appears to flatten at higher z; c) despite this increase, the most important contributors to the SFRD at any z are galaxies of about, or immediately lower than, the characteristic stellar mass; d) at z 2, massive galaxies are actively star-forming, with a median SFR 300 M yr −1 . During this epoch, our targeted galaxies assemble a substantial part of their final stellar mass; e) the specific SFR (SSFR) shows a clear bimodal distribution. Conclusions. The analysis of the SFR density and the SSFR seems to support the downsizing scenario, according to which high mass galaxies have formed their stars earlier and more rapidly than their low mass counterparts. A comparison with renditions of theoretical simulations of galaxy formation and evolution indicates that these models follow the global increase in the SSFR with redshift and predict the existence of quiescent galaxies even at z > 1.5. However, the average SSFR is systematically underpredicted by all models considered.

Black hole growth and starburst activity atz= 0.6–4 in the Chandra Deep Field South

“…In order to quantify how (and if) the use of catalogs selected in different bands leads to significantly different X-ray to optical associations, we decided to perform again the identification of the Chandra sources, using a new version of the multicolor GOODS-MUSIC sample (GOODS MUlticolor Southern Infrared Catalog; Grazian et al 2006;De Santis et al 2007), extracted from the public data of the GOODS-South survey (Giavalisco et al 2004), and updated as described in Santini et al (2009). The 15-bands multi-wavelength coverage ranges from 0.35 to 24 μm, as a result of the combination of images coming from different instruments (2.2ESO, VIMOS/VLT, ACS/HST, ISAAC/VLT, IRAC/Spitzer and MIPS/Spitzer).…”

“…Such a catalog covers an area of 143.2 arcmin 2 and contains 15 208 sources. After culling Galactic stars, it contains 14 999, extragalactic objects selected in the z band, in the Ks band and at 4.5 μm (see Santini et al 2009, for further details).…”

“…We parametrize the star formation histories with a variety of exponentially declining laws (with timescales ranging from 0.1 to 15 Gyr), metallicities (from Z = 0.02 Z to Z = 2.5 Z ) and dust extinctions (0 < E(B − V) < 1.1, with a Calzetti or Small Magellanic extinction curve). These SFR estimates have been compared with the ones derived from the observed IR (MIPS) emission, following the approach presented by Papovich et al (2007): despite the majority of galaxies are assigned a consistent SFR (with overall median ratio around unity, see details in Santini et al 2009), in the following, we will use the SFR from the SED fitting for all the obscured AGN in our sample, in order to minimize the possible AGN contribution in the computation of SFRs from MIPS data in AGN samples (see Daddi et al 2007a).…”

“…Statistical errors for both masses and SFR have been calculated from the 1σ convidence level in each parameter estimate by scanning the χ 2 levels, allowing the redshift to change within the errors in case of objects with photometric redshifts 4 (see Fontana et al 2006;Santini et al 2009). …”

“…The masses and SFR estimates are, however, limited by the systematics related to model degeneracy and, in particular, the ones associated to the adopted IMF, the SSP, the metallicity and the star formation history (see Santini et al 2009; and the discussion in Ilbert et al 2009). The largest, systematic, uncertainty on the estimate of the stellar mass is the assumption on the IMF.…”