Abstract
Abstract We present the first study of the far-infrared (FIR) properties of high-redshift, radio-selected ultraluminous infrared galaxies (ULIRGs) using deep observations obtained with the Spectral and Photometric Imaging Receiver (SPIRE) from the Herschel Multi-tiered Extragalactic Survey (HerMES). These galaxies span a large range of 850-μm fluxes from submillimetre-luminous ∼10 mJy sources (SCUBA galaxies) to ∼1.5 mJy from stacked SCUBA non-detections, thus likely representing a complete distribution of ULIRG spectral energy distributions (SEDs). From Keck spectroscopic surveys in the Lockman-North field we identified a sample of 31 submillimetre galaxies (SMGs) and 37 submillimetre-faint, optically faint radio galaxies (OFRGs), all with radio-inferred IR luminosities >1012 L⊙. These galaxies were cross-identified with SPIRE 250-, 350- and 500-μm catalogues based on fluxes extracted at 24-μm positions in the SWIRE survey, yielding a sample of more than half of the galaxies well detected in at least two of the SPIRE bandpasses. By fitting greybody dust models to the SPIRE photometry together with SCUBA 850-μm measurements (for OFRGs, only 850- upper limits), we infer dust temperatures and FIR luminosities. The OFRGs detected by SPIRE have median 〈Td〉= 41 ± 5 K and the SMGs have 〈Td〉= 34 ± 5 K, both in reasonable agreement with previous (pre-Herschel) estimates, reaffirming that the local FIR/radio correlation holds (at least for this subset of high-z ULIRGs) at high redshift (we measure 〈qIR〉= 2.43 ± 0.21 using SIR derived from greybody fit coupled with a power-law extrapolation to the 24 μm). Our observations first confirm that a substantial fraction of OFRGs exhibits large infrared luminosities corresponding to SFRs of ∼400 M⊙ yr−1. The SPIRE observations secondly confirm the higher dust temperatures for these OFRGs than similarly selected SMGs, consistent with early predictions of the submillimetre-faint radio populations. Our observations also clearly confirm the large infrared luminosities of most SMGs selected with S850 μm > 5 mJy and radio and strong 24-μm detections, corresponding to SFRs of ∼700 M⊙ yr−1.
Highlights
Submillimetre galaxies (SMGs – Smail, Ivison & Blain 1997) contribute significantly to the rapid build-up of stellar mass in the Universe at z ∼ 2
The final sample consists of 31 securely identified submillimetre galaxies (SMGs) and 37 optically faint radio galaxies (OFRGs) with IR luminosities >1012 L, and which are crossmatched with our Spectral and Photometric Imaging Receiver (SPIRE) 250, 350- and 500-μm multiwavelength catalogues based on sources extraction using prior detections at 24 μm. 19 of the SMGs and 21 of the OFRGs in our sample are detected in at least two of the SPIRE bandpasses and form the sample we focus on in this work
Our observations suggest that high-redshift ultraluminous infrared galaxies (ULIRGs) show a wide range of dust temperatures
Summary
Submillimetre (submm) galaxies (SMGs – Smail, Ivison & Blain 1997) contribute significantly to the rapid build-up of stellar mass in the Universe at z ∼ 2. Recent work (e.g. Chapman et al 2004b; Casey et al 2009; Magnelli et al 2010) has demonstrated that 850-μm-faint, high-redshift ULIRGs exist and may contribute significantly to the cosmic star formation rate density at its peak These optically faint radio galaxies (OFRGs) are defined as radio sources having inferred ULIRG luminosities, with starburst (SB) or hybrid SB–AGN spectral features in the UV, and having 2.5σ limits on their submm fluxes which are consistent with them being fainter than 5 mJy at 850 μm. The parent sample for this study consists of a sample of 376 radio sources with spectroscopic follow-up in the Lockman-East field For this initial foray into the SPIRE properties of luminous, high-z radio sources, we impose a >5σ 24-μm detection threshold (from the SWIRE catalogue – Lonsdale et al 2004) in order to extract the SPIRE fluxes at that position.
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