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Published June 20, 2009 | Published
Journal Article Open

Star Formation and Dust Obscuration at z ≈ 2: Galaxies at the Dawn of Downsizing


We present first results of a study aimed to constrain the star formation rate (SFR) and dust content of galaxies at z ≈ 2. We use a sample of BzK-selected star-forming galaxies, drawn from the Cosmic Evolution Survey, to perform a stacking analysis of their 1.4 GHz radio continuum as a function of different stellar population properties, after cleaning the sample from contamination by active galactic nuclei. Dust unbiased SFRs are derived from radio fluxes assuming the local radio-IR correlation. The main results of this work are: (1) specific star formation rate (SSFR)s are constant over about 1 dex in stellar mass and up to the highest stellar mass probed, (2) the dust attenuation is a strong function of galaxy stellar mass with more massive galaxies being more obscured than lower mass objects, (3) a single value of the UV extinction applied to all galaxies would lead to a gross underestimate of the SFR in massive galaxies, (4) correcting the observed UV luminosities for dust attenuation based on the Calzetti recipe provides results in very good agreement with the radio derived ones, (5) the mean SSFR 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.

Additional Information

© 2009 The American Astronomical Society. Received 2009 February 27; accepted 2009 May 8; published 2009 June 1. We thank the anonymous referee for constructive comments that improved the presentation of our results. M.P., V.S., and C.L.C. acknowledge partial support from the Max-Planck Forschungspreise 2005. E.D. and H.J.Mc.C. acknowledge support from the French grants ANR-07-BLAN-0228-03 and ANR- 08-JCJC-0008. A.R. acknowledges support from the ASI grant COFIS. F. C. acknowledges support from NASA-Chandra grant G07-8136A. This work is based in part on data products produced at TERAPIX. The HST COSMOS Treasury program was supported through NASA grant HST-GO-09822.We gratefully acknowledge the contributions of the entire COSMOS collaboration. Based on observations collected, within the COSMOS Legacy Survey, at the HST, Chandra, XMM, Keck, NRAO-VLA, Subaru, KPNO, CTIO, CFHT, and ESO observatories. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

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