Characterizing elusive, faint dusty star-forming galaxies: a lensed, optically undetected ALMA galaxy at z ~ 3.3

¹ INAF–Osservatorio Astronomico di Roma, via di Frascati 33, 00078 Monte Porzio Catone, Italy
e-mail: paola.santini@oa-roma.inaf.it
² Cavendish Laboratory, University of Cambridge, 19 J. J. Thomson Ave., Cambridge CB3 0HE, UK
³ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁴ Department of Physics, University of California, Santa Barbara, CA, 93106-9530, USA
⁵ Department of Physics and Astronomy, UCLA, Los Angeles, CA, 90095-1547, USA
⁶ INAF–Istituto di Radioastronomia, via P. Gobetti 101, 40129 Bologna, Italy
⁷ Max Planck Inst. f. Extraterrestrische Physik, Giessenbachstrasse, 85748 Garching bei Muenchen, Germany
e-mail: ste_atreb@yahoo.it
⁸ Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh, EH9 3HJ, UK
⁹ INAF–Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
¹⁰ Laboratoire AIM, CEA/DSM-CNRS-Université Paris Diderot, IRFU/Service d’Astrophysique, Bât. 709, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France
¹¹ INAF IAPS, via Fosso del Cavaliere 100, 00133 Roma, Italy
¹² Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
¹³ Dipartimento di Astronomia, Università di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
¹⁴ Leiden Observatory, Leiden University, 2300 RA Leiden, The Netherlands

Received: 19 July 2016
Accepted: 24 October 2016

Abstract

We present the serendipitous ALMA detection of a faint submillimeter galaxy (SMG) lensed by a foreground z ~ 1 galaxy. By optimizing the source detection to deblend the system, we accurately build the full spectral energy distribution of the distant galaxy from the I814 band to radio wavelengths. It is extremely red, with a I−K colour larger than 2.5. We estimate a photometric redshift of 3.28 and determine the physical parameters. The distant galaxy turns out to be magnified by the foreground lens by a factor of ~1.5, which implies an intrinsic K_s-band magnitude of ~24.5, a submillimeter flux at 870 μm of ~2.5 mJy and a SFR of ~150−300 M_⊙/ yr, depending on the adopted tracer. These values place our source towards the faint end of the distribution of observed SMGs, and in particular among the still few faint SMGs with a fully characterized spectral energy distribution, which allows us not only to accurately estimate its redshift, but also to measure its stellar mass and other physical properties. The galaxy studied in this work is a representative of the population of faint SMGs, of which only few objects are known to date, that are undetected in optical and therefore are not typically accounted for when measuring the cosmic star formation history (SFH). This faint galaxy population thus likely represents an important and missing piece in our understanding of the cosmic SFH. Its observation and characterization is of major importance to achieve a solid picture of galaxy evolution.