Dust and gas in luminous proto-cluster galaxies at z = 4.05: the case for different cosmic dust evolution in normal and starburst galaxies

¹ CEA Saclay, DSM/Irfu/Service d’Astrophysique, Orme des Merisiers, 91191 Gif-sur-Yvette Cedex, France
e-mail: qhtan@pmo.ac.cn
² Purple Mountain Observatory & Key Laboratory for Radio Astronomy, Chinese Academy of Sciences, 210008 Nanjing, PR China
³ Graduate University of the Chinese Academy of Sciences, 19A Yuquan Road, Shijingshan District, 10049 Beijing, PR China
⁴ Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
⁵ Astronomy Center, Dept. of Physics & Astronomy, University of Sussex, Brighton BN1 9QH, UK
⁶ Department of Astronomy, Cornell University, 220 Space Sciences Building, Ithaca, NY 14853, USA
⁷ National Radio Astronomy Observatory, PO Box O, Socorro, NM 87801, USA
⁸ Max-Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁹ Universität Wien, Institut für Astrophysik, Türkenschanzstrasse 17, 1180 Vienna, Austria
¹⁰ National Optical Astronomical Observatory, 950 North Cherry Avenue, Tucson, AZ 85719, USA

Received: 30 March 2014
Accepted: 11 July 2014

Abstract

We measure the dust and gas content of the three submillimeter galaxies (SMGs) in the GN20 proto-cluster at z = 4.05 using new IRAM Plateau de Bure interferometer (PdBI) CO(4–3) and 1.2–3.3 mm continuum observations. All these three SMGs are heavily dust obscured, with UV-based star formation rate (SFR) estimates significantly smaller than the ones derived from the bolometric infrared (IR), consistent with the spatial offsets revealed by HST and CO imaging. Based also on evaluations of the specific SFR, CO-to-H₂ conversion factor and gas depletion timescale, we classify all the three galaxies as starbursts (SBs), although with a lower confidence for GN20.2b that might be a later stage merging event. We place our measurements in the context of the evolutionary properties of main sequence (MS) and SB galaxies. ULIRGs have 3–5 times larger L'_CO/M_dust and M_dust/M_⋆ ratios than z = 0 MS galaxies, but by z ~ 2 the difference appears to be blurred, probably due to differential metallicity evolution. SB galaxies appear to slowly evolve in their L'_CO/M_dust and M_dust/M_⋆ ratios all the way to z> 6 (consistent with rapid enrichment of SB events), while MS galaxies rapidly increase in M_dust/M_⋆ from z = 0 to 2 (due to gas fraction increase, compensated by a decrease of metallicities). While no IR/submm continuum detection is available for indisputably normal massive galaxies at z> 2.5, we show that if metallicity indeed decrease rapidly for these systems at z> 3 as claimed in the literature, we should expect a strong decrease of their M_dust/M_⋆, consistent with recent PdBI and ALMA upper limits. We conclude that the M_dust/M_⋆ ratio could be a powerful tool for distinguishing starbursts from normal galaxies at z> 4.