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Register a new userUV Continuum Slope and Dust Obscuration from z~6 to z~2: The Star Formation Rate Density at High Redshift
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R.J. Bouwens
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We provide a systematic measurement of the rest-frame UV continuum slope beta over a wide range in redshift (z~2-6) and rest-frame UV luminosity (0.1-2L*) to improve estimates of the SFR density at high redshift. We utilize the deep optical and infrared data (ACS/NICMOS) over the CDF-S and HDF-N GOODS fields, as well as the UDF for our primary UBVi dropout sample. We correct the observed distributions for selection biases and photometric scatter. We find that the UV-continuum slope of the most luminous galaxies is substantially redder at z~2-4 than it is at z~5-6. Lower luminosity galaxies are also found to be bluer than higher luminosity galaxies at z~2.5 and z~4. We do not find a large number of galaxies with betas as red as -1 in our dropout selections at z~4, and particularly at z>~5, even though such sources could be readily selected from our data. This suggests that star-forming galaxies at z>~5 almost universally have very blue UV-continuum slopes, and that there are not likely to be a substantial number of dust-obscured galaxies at z>~5 that are missed in dropout searches. Using the same relation between UV-continuum slope and dust extinction as found to be appropriate at z~0 and z~2, we estimate the average dust extinction of galaxies as a function of redshift and UV luminosity in a consistent way. We find that the estimated dust extinction increases substantially with cosmic time for the most UV luminous galaxies, but remains small (<~2x) at all times for lower luminosity galaxies. Because these same lower luminosity galaxies dominate the luminosity density in the UV, the overall dust extinction correction remains modest at all redshifts. We include the contribution from ULIRGs in our SFR density estimates at z~2-6, but find that they contribute only ~20% of the total at z~2.5 and <~10% at z>~4.
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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.
We use 3035 Herschel-SPIRE 500$mu$m sources from 20.3 sq deg of sky in the HerMES Lockman, ES1 and XMM-LSS areas to estimate the star-formation rate density at z = 1-6. 500 mu sources are associated first with 350 and 250 mu sources, and then with Spitzer 24 mu sources from the SWIRE photometric redshift catalogue. The infrared and submillimetre data are fitted with a set of radiative-transfer templates corresponding to cirrus (quiescent) and starburst galaxies. Lensing candidates are removed via a set of colour-colour and colour-redshift constraints. Star-formation rates are found to extend from < 1 to 20,000 Mo/yr. Such high values were also seen in the all-sky IRAS Faint Source Survey. Star-formation rate functions are derived in a series of redshift bins from 0-6, combined with earlier far-infrared estimates, where available, and fitted with a Saunders et al (1990) functional form. The star-formation-rate density as a function of redshift is derived and compared with other estimates. There is reasonable agreement with both infrared and ultraviolet estimates for z < 3, but we find higher star-formation-rate densities than ultraviolet estimates at z = 3-6. Given the considerable uncertainties in the submillimetre estimates, we can not rule out the possibility that the ultraviolet estimates are correct. But the possibility that the ultraviolet estimates have seriously underestimated the contribution of dust-shrouded star-formation can also not be excluded.
Using new homogeneous LFs in the FUV and in the FIR Herschel/PEP and Herschel/HerMES, we study the evolution of the dust attenuation with redshift. With this information in hand, we are able to estimate the redshift evolution of the total (FUV + FIR) star formation rate density SFRD_TOT. By integrating SFRD_TOT, we follow the mass building and analyze the redshift evolution of the stellar mass density (SMD). This letter aims at providing a complete view of star formation from the local universe to z = 4 and, using assumptions on earlier star formation history, compares this evolution to what was known before in an attempt to draw a homogeneous picture of the global evolution of star formation in galaxies. The main conclusions of this letter are: 1) the dust attenuation A_FUV is found to increase from z = 0 to z sim 1.2 and then starts to decrease up to our last data point at z = 3.6; 2) the estimated SFRD confirms published results up to z = 2. At z > 2, we observe either a plateau or a small increase up to z = 3 and then a likely decrease up to z = 3.6; 3) the peak of A_FUV is delayed with respect to the plateau of SFRD_TOT and a likely origin might be found in the evolution of the bright ends of the FUV and FIR LFs; 4) using assumptions (namely exponential rise and linear rise with time) for the evolution of the star formation density from z = 3.6 to z_form = 10, we integrate SFRD_TOT and find a good agreement with the published SMDs.
We study the link between observed ultraviolet luminosity, stellar mass, and dust attenuation within rest-frame UV-selected samples at z~ 4, 3, and 1.5. We measure by stacking at 250, 350, and 500 um in the Herschel/SPIRE images from the HerMES program the average infrared luminosity as a function of stellar mass and UV luminosity. We find that dust attenuation is mostly correlated with stellar mass. There is also a secondary dependence with UV luminosity: at a given UV luminosity, dust attenuation increases with stellar mass, while at a given stellar mass it decreases with UV luminosity. We provide new empirical recipes to correct for dust attenuation given the observed UV luminosity and the stellar mass. Our results also enable us to put new constraints on the average relation between star formation rate and stellar mass at z~ 4, 3, and 1.5. The star formation rate-stellar mass relations are well described by power laws (SFR~ M^0.7), with the amplitudes being similar at z~4 and z~3, and decreasing by a factor of 4 at z~1.5 at a given stellar mass. We further investigate the evolution with redshift of the specific star formation rate. Our results are in the upper range of previous measurements, in particular at z~3, and are consistent with a plateau at 3<z<4. Current model predictions (either analytic, semi-analytic or hydrodynamic) are inconsistent with these values, as they yield lower predictions than the observations in the redshift range we explore. We use these results to discuss the star formation histories of galaxies in the framework of the Main Sequence of star-forming galaxies. Our results suggest that galaxies at high redshift (2.5<z<4) stay around 1 Gyr on the Main Sequence. With decreasing redshift, this time increases such that z=1 Main Sequence galaxies with 10^8<M_*/Msun<10^10 stay on the Main Sequence until z=0.
We present a new analysis of the dust obscuration in starburst galaxies at low and high redshift. This study is motivated by our unique sample of the most extreme UV-selected starburst galaxies in the nearby universe (z<0.3), found to be good analogs of high-redshift Lyman Break Galaxies (LBGs) in most of their physical properties. We find that the dust properties of the Lyman Break Analogs (LBAs) are consistent with the relation derived previously by Meurer et al. (M99) that is commonly used to dust-correct star formation rate measurements at a very wide range of redshifts. We directly compare our results with high redshift samples (LBGs, BzK, and sub-mm galaxies at z=2-3) having IR data either from Spitzer or Herschel. The attenuation in typical LBGs at z=2-3 and LBAs is very similar. Because LBAs are much better analogs to LBGs compared to previous local star-forming samples, including M99, the practice of dust-correcting the SFRs of high redshift galaxies based on the local calibration is now placed on a much more solid ground. We illustrate the importance of this result by showing how the locally calibrated relation between UV measurements and extinction is used to estimate the integrated, dust-corrected star formation rate density at z=2-6.
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