Constraints on the star-formation rate of z ~ 3 LBGs with measured metallicity in the CANDELS GOODS-South field^⋆

¹ INAF – Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio, Italy
e-mail: marco.castellano@oa-roma.inaf.it
² Los Alamos National Laboratory, Los Alamos, NM, USA
³ Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh EH9 3HJ, UK
⁴ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore MD 21218, USA
⁵ UCO/Lick Observatory, Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA, USA
⁶ Department of Physics, University of California, Davis, CA 95616, USA
⁷ Department of Physics and Astronomy, Johns Hopkins University, Baltimore MD 21218, USA
⁸ Cavendish Laboratory, University of Cambridge, 19 J.J. Thomson Avenue, Cambridge CB3 0HE, UK
⁹ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK
¹⁰ Observatoire de Genève, Université de Genève, 51 Ch. des Maillettes, 1290 Versoix, Switzerland
¹¹ CNRS, IRAP, 14 Avenue E. Belin, 31400 Toulouse, France
¹² Astronomisches Institut, Ruhr-Universität Bochum, Universitätsstraße 150, 44780 Bochum, Germany
¹³ INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy

Received: 18 September 2013
Accepted: 4 March 2014

Abstract

Aims. We aim to constrain the assembly history of high-redshift galaxies and the reliability of UV-based estimates of their physical parameters from an accurate analysis of a unique sample of z ~ 3 Lyman-break galaxies (LBGs).

Methods. We analyse 14 LBGs at z ~ 2.8–3.8 constituting the only sample where both a spectroscopic measurement of their metallicity and deep IR observations (CANDELS+HUGS survey) are available. Fixing the metallicity of population synthesis models to the observed values, we determine best-fit physical parameters under different assumptions about the star-formation history (SFH) and also consider the effect of nebular emission. For comparison, we determine the UV slope of the objects, and use it to estimate their SFR_UV99 by correcting the UV luminosity.

Results. A comparison between star-formation rate (SFR) obtained through SED-fitting (SFR_fit) and the SFR_UV99 shows that the latter are underestimated by a factor of 2–10, regardless of the assumed SFH. Other SFR indicators (radio, far-IR, X-ray, recombination lines) coherently indicate SFRs a factor of 2–4 larger than SFR_UV99 and in closer agreement with SFR_fit. This discrepancy is due to the solar metallicity implied by the usual β − A₁₆₀₀ conversion factor. We propose a refined relation, appropriate for subsolar metallicity LBGs: A₁₆₀₀ = 5.32 + 1.99 ∗ β. This relation reconciles the dust-corrected UV with the SED-fitting and the other SFR indicators. We show that the fact that z ~ 3 galaxies have subsolar metallicity implies an upward revision by a factor of ~1.5–2 of the global SFRD, depending on the assumptions about the age of the stellar populations. We find very young best-fit ages (10–500 Myr) for all our objects. From a careful examination of the uncertainties in the fit and the amplitude of the Balmer break we conclude that there is little evidence of the presence of old stellar population in at least half of the LBGs in our sample, suggesting that these objects are probably caught during a huge star-formation burst, rather than being the result of a smooth evolution.