Letter to the Editor

The dust content of high-z submillimeter galaxies revealed by Herschel ^*,**

¹ INAF – Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monte Porzio Catone, Italy e-mail: paola.santini@oa-roma.inaf.it
² Max-Planck-Institut für Extraterrestrische Physik (MPE), Postfach 1312, 85741 Garching, Germany
³ INAF – Osservatorio Astronomico di Trieste, via Tiepolo 11, 34131 Trieste, Italy
⁴ Science Centre; European Space Astronomy Centre
⁵ ESO, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
⁶ Laboratoire AIM, CEA/DSM-CNRS-Université Paris Diderot, IRFU/Service d'Astrophysique, Bât.709, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France
⁷ Instituto de Astrofísica de Canarias, 38205 La Laguna, Spain
⁸ Departamento de Astrofísica, Universidad de La Laguna, Spain
⁹ Department of Astronomy, 610 Space Sciences Building, Cornell University, Ithaca, NY 14853, USA
¹⁰ Jeremiah Horrocks Institute for Astrophysics and Supercomputing, University of Central Lancashire, Preston PR1 2HE, UK
¹¹ Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
¹² INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
¹³ INAF – IFSI, Via Fosso del Cavaliere 100, 00133 Roma, Italy
¹⁴ Dipartimento di Astronomia, Università di Trieste, via G. B. Tiepolo 11, 34143 Trieste, Italy
¹⁵ Dipartimento di Astronomia, Università di Padova, Vicolo dell'Osservatorio 3, 35122 Padova, Italy

Received: 7 April 2010
Accepted: 11 May 2010

Abstract

We use deep observations taken with the Photodetector Array Camera and Spectrometer (PACS), on board the Herschel satellite as part of the PACS evolutionary probe (PEP) guaranteed project along with submm ground-based observations to measure the dust mass of a sample of high-z submillimeter galaxies (SMGs). We investigate their dust content relative to their stellar and gas masses, and compare them with local star-forming galaxies. High-z SMGs are dust rich, i.e. they have higher dust-to-stellar mass ratios compared to local spiral galaxies (by a factor of 30) and also compared to local ultraluminous infrared galaxies (ULIRGs, by a factor of 6). This indicates that the large masses of gas typically hosted in SMGs have already been highly enriched with metals and dust. Indeed, for those SMGs whose gas mass is measured, we infer dust-to-gas ratios similar or higher than local spirals and ULIRGs. However, similarly to other strongly star-forming galaxies in the local Universe and at high-z, SMGs are characterized by gas metalicities lower (by a factor of a few) than local spirals, as inferred from their optical nebular lines, which are generally ascribed to infall of metal-poor gas. This is in contrast with the large dust content inferred from the far-IR and submm data. In short, the metalicity inferred from the dust mass is much higher (by more than an order of magnitude) than that inferred from the optical nebular lines. We discuss the possible explanations of this discrepancy and the possible implications for the investigation of the metalicity evolution at high-z.