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Register a new userThe Characteristic Star Formation Histories of Galaxies at Redshifts z~2-7
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A large sample of spectroscopically confirmed galaxies at 1.4<z<3.7, with complementary imaging in the near- and mid-IR from the ground and from Hubble and Spitzer, is used to infer the average star formation histories (SFHs) of typical galaxies from z~7 to 2. For a subset of 302 galaxies at 1.5<z<2.6, we perform a comparison of star formation rates (SFRs) determined from SED modeling (SFRs[SED]) and those calculated from deep Keck UV and Spitzer/MIPS 24 micron imaging (SFRs[IR+UV]). Exponentially declining SFHs yield SFRs[SED] that are 5-10x lower on average than SFRs[IR+UV], indicating that declining SFHs may not be accurate for typical galaxies at z>2. The SFRs of z~2-3 galaxies are directly proportional to their stellar masses M*, with unity slope---a result that is confirmed with Spitzer/IRAC stacks of 1179 UV-faint (R>25.5) galaxies---for M*>5e8 Msun and SFRs >2 Msun/yr. We interpret this result in the context of several systematic biases that can affect determinations of the SFR-M* relation. The average specific SFRs at z~2-3 are similar within a factor of two to those measured at z>4, implying an average SFH where SFRs increase with time. A consequence of these rising SFHs is that (a) a substantial fraction of UV-bright z~2-3 galaxies had faint sub-L* progenitors at z>4; and (b) gas masses must increase with time from z=7 to 2, over which time the net cold gas accretion rate---as inferred from the specific SFR and the Kennicutt-Schmidt relation---is ~2-3x larger than the SFR . However, if we evolve to higher redshift the SFHs and masses of the halos that are expected to host L* galaxies at z~2, we find that <10% of the baryons accreted onto typical halos at z>4 actually contribute to star formation at those epochs. These results highlight the relative inefficiency of star formation even at early cosmic times when galaxies were first assembling. [Abridged]
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The Spitzer Space Telescope has identified a population of ultra-luminous infrared galaxies (ULIRGs) at z ~ 2 that may play an important role in the evolution of massive galaxies. We measure the stellar masses of two populations of Spitzer-selected ULIRGs, both of which have extremely red R-[24] colors (dust-obscured galaxies, or DOGs) and compare our results with sub-millimeter selected galaxies (SMGs). One set of 39 DOGs has a local maximum in their mid-IR spectral energy distribution (SED) at rest-frame 1.6um associated with stellar emission (bump DOGs), while the other set of 51 DOGs has a power-law dominated mid-IR SED with spectral features typical of obscured AGN (power-law DOGs). We use stellar population synthesis models applied self-consistently to broad-band photometry in the rest-frame ultra-violet, optical, and near-infrared of each of these populations and test a variety of stellar population synthesis codes, star-formation histories (SFHs), and initial mass functions (IMFs). Assuming a simple stellar population SFH and a Chabrier IMF, we find that the median and inner quartile stellar masses of SMGs, bump DOGs and power-law DOGs are given by log(M_*/M_sun) = 10.42_-0.36^+0.42, 10.62_-0.32^+0.36, and 10.71_-0.34^+0.40, respectively. Implementing more complicated SFHs with multiple age components increases these mass estimates by up to 0.5 dex. Our stellar mass estimates are consistent with physical mechanisms for the origin of z~2 ULIRGs that result in high star-formation rates for a given stellar mass. Such mechanisms are usually driven by a major merger of two gas-rich systems, rather than smooth accretion of gas and small satellites.
[Abridged]We present a study based on a sample of 62 early-type galaxies (ETGs) at 0.9<z_spec<2 aimed at constraining their past star formation and mass assembly histories. The sample is composed of normal ETGs having effective radii comparable to the mean radius of local ones and of compact ETGs having effective radii from two to six times smaller. We do not find evidence of a dependence of the compactness of ETGs on their stellar mass. We find that the stellar mass of normal ETGs formed at z_form<3 while the stellar content of compact ETGs formed at 2<z_form<10 with a large fraction of them characterized by z_form>5. Earlier stars formed at z_form>5 are assembled in compact and more massive (M_*>10^11 M_sun) ETGs while stars later formed (z_form<3) or resulting from subsequent episodes of star formation are assembled both in compact and normal ETGs. Thus, the older the stellar population the higher the mass of the hosting galaxy but not vice versa. This suggests that the epoch of formation may play a role in the formation of massive ETGs rather than the mass itself. The possible general scheme in which normal <z>~1.5 ETGs are descendants of high-z compact spheroids enlarged through subsequent dry mergers is not compatible with the current models which predict a number of dry mergers two orders of magnitude lower than the one needed. Moreover, we do not find evidence supporting a dependence of the compactness of galaxies on their redshift of assembly. Finally, we propose a simple scheme of formation and assembly of the stellar mass of ETGs based on dissipative gas-rich merger which can qualitatively account for the co-existence of normal and compact ETGs observed at <z>~1.5 in spite of the same stellar mass, the lack of normal ETGs with high z_form and the absence of correlation between compactness, stellar mass and formation redshift.
We compare multi-wavelength SFR indicators out to z~3 in GOODS-South. Our analysis uniquely combines U-to-8um photometry from FIREWORKS, MIPS 24um and PACS 70, 100, and 160um photometry from the PEP survey, and Ha spectroscopy from the SINS survey. We describe a set of
We present first results of a study aimed to constrain the star formation rate and dust content of galaxies at z~2. We use a sample of BzK-selected star-forming galaxies, drawn from the COSMOS survey, to perform a stacking analysis of their 1.4 GHz radio continuum as a function of different stellar population properties, after removing AGN contaminants from the sample. Dust unbiased star formation rates are derived from radio fluxes assuming the local radio-IR correlation. The main results of this work are: i) specific star formation rates are constant over about 1 dex in stellar mass and up to the highest stellar mass probed; ii) the dust attenuation is a strong function of galaxy stellar mass with more massive galaxies being more obscured than lower mass objects; iii) a single value of the UV extinction applied to all galaxies would lead to grossly underestimate the SFR in massive galaxies; iv) correcting the observed UV luminosities for dust attenuation based on the Calzetti recipe provide results in very good agreement with the radio derived ones; v) the mean specific star formation rate of our sample steadily decreases by a factor of ~4 with decreasing redshift from z=2.3 to 1.4 and a factor of ~40 down the local Universe. These empirical SFRs would cause galaxies to dramatically overgrow in mass if maintained all the way to low redshifts, we suggest that this does not happen because star formation is progressively quenched, likely starting from the most massive galaxies.
A comparison is carried out among the star formation histories of early-type galaxies (ETG) in fossil groups, clusters and low density environments. Although they show similar evolutionary histories, a significant fraction of the fossils are younger than their counterparts, suggesting that fossils can be precursors of the isolated ETGs.
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