Letter to the Editor

Far-infrared properties of submillimeter and optically faint radio galaxies^*

¹ Max-Planck-Institut für Extraterrestrische Physik (MPE), Postfach 1312, 85741 Garching, Germany e-mail: Magnelli@mpe.mpg.de
² Laboratoire AIM, CEA/DSM-CNRS-Université Paris Diderot, IRFU/Service d'Astrophysique, Bât.709, Saclay, 91191 Gif-sur-Yvette Cedex, France
³ Herschel Science Centre; European Space Astronomy Centre
⁴ ESO, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
⁵ INAF-Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
⁶ Instituto de Astrofísica de Canarias, 38205 La Laguna, Spain
⁷ Departamento de Astrofísica, Universidad de La Laguna, Spain
⁸ Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁹ INAF-Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
¹⁰ INAF-Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monte Porzio Catone, Italy
¹¹ Dipartimento di Astronomia, Università di Padova, Vicolo dell'Osservatorio 3, 35122 Padova, Italy

Received: 31 March 2010
Accepted: 22 April 2010

Abstract

We use deep observations obtained with the Photodetector Array Camera and Spectrometer (PACS) onboard the Herschel Space Observatory to study the far-infrared (FIR) properties of submillimeter and optically faint radio galaxies (SMGs and OFRGs). From literature we compiled a sample of 35 securely identified SMGs and nine OFRGs located in the GOODS-N and the A2218 fields. This sample is cross-matched with our PACS 100 μm and 160 μm multi-wavelength catalogs based on sources-extraction using prior detections at 24 μm. About half of the galaxies in our sample are detected in at least the PACS 160 μm bandpass. The dust temperatures and the infrared luminosities of our galaxies are derived by fitting their PACS and SCUBA 850 μm (only the upper limits for the OFRGs) flux densities with a single modified (β = 1.5) black body function. The median dust temperature of our SMG sample is T_dust = 36±8 K while for our OFRG sample it is T_dust = 47±3 K. For both samples, median dust temperatures derived from Herschel data agree well with previous estimates. In particular, Chapman et al. (2005, ApJ, 622, 772) found a dust temperature of T_dust = 36±7 K for a large sample of SMGs assuming the validity of the FIR/radio correlation (i.e., q= log₁₀(L_FIR[W]/L_{1.4 GHz}[W Hz^-1] /3.75×10¹²)). The agreement between our studies confirms that the local FIR/radio correlation effectively holds at high redshift even though we find q = 2.17±0.19, a slightly lower value than that observed in local systems. The median infrared luminosities of SMGs and OFRGs are 4.6×10¹²  and 2.6×10¹² , respectively. We note that for both samples the infrared luminosity estimates from the radio part of the spectral energy distribution (SED) are accurate, while estimates from the mid-IR are considerably (~×3) more uncertain. Our observations confirm the remarkably high luminosities of SMGs and thus imply median star-formation rates of 960  yr^-1 for SMGs with S(850 μm)>5 mJy and 460  yr^-1 for SMGs with S(850 μm)>2 mJy, assuming a Chabrier IMF and no dominant AGN contribution to the far-infrared luminosity.