HerMES: Spectral Energy Distributions of Submillimeter Galaxies at z > 4

Abstract

We present a study of the infrared properties for a sample of seven spectroscopically confirmed submillimeter galaxies (SMGs) at z > 4.0. By combining ground-based near-infrared, Spitzer IRAC and MIPS, Herschel SPIRE, and ground-based submillimeter/millimeter photometry, we construct their spectral energy distributions (SEDs) and a composite model to fit the SEDs. The model includes a stellar emission component at λ_(rest) < 3.5 μm, a hot dust component peaking at λ_(rest) ~ 5 μm, and cold dust component which becomes significant for λ_(rest) > 50 μm. Six objects in the sample are detected at 250 and 350 μm. The dust temperatures for the sources in this sample are in the range of 40-80 K, and their L_(FIR) ~ 10^(13) L_☉ qualifies them as hyper-luminous infrared galaxies. The mean FIR-radio index for this sample is around 〈q〉 = 2.2 indicating no radio excess in their radio emission. Most sources in the sample have 24 μm detections corresponding to a rest-frame 4.5 μm luminosity of Log_(10)(L_(4.5)/L_☉) = 11 ~ 11.5. Their L_(4.5)/L_(FIR) ratios are very similar to those of starburst-dominated SMGs at z ~ 2. The L_(CO) – L_(FIR) relation for this sample is consistent with that determined for local ULIRGs and SMGs at z ~ 2. We conclude that SMGs at z > 4 are hotter and more luminous in the FIR but otherwise very similar to those at z ~ 2. None of these sources show any sign of the strong QSO phase being triggered.