Well-sampled Far-infrared Spectral Energy Distributions of z ~ 2 Galaxies: Evidence for Scaled up Cool Galaxies

Abstract

We present an analysis of the far-infrared (FIR) spectral energy distributions (SEDs) of two massive K-selected galaxies at z= 2.122 and z= 2.024 detected at 24 μm, 70 μm, 160 μm by Spitzer, 250 μm, 350 μm, 500 μm by BLAST, and 870 μm by APEX. The large wavelength range of these observations and the availability of spectroscopic redshifts allow us to unambiguously identify the peak of the redshifted thermal emission from dust at ~300 μm. The SEDs of both galaxies are reasonably well fit by synthetic templates of local galaxies with L _IR ~ 10¹¹ L _sun-10¹² L _sun yet both galaxies have L _IR ~ 10¹³ L _sun. This suggests that these galaxies are not high-redshift analogs of the Hyper-LIRGs/ULIRGs used in local templates, but are instead "scaled up" versions of local ULIRGs/LIRGs. Several lines of evidence point to both galaxies hosting an active galactic nucleus (AGN); however, the relatively cool best-fit templates and the optical emission line ratios suggest that the AGN is not the dominant source heating the dust. For both galaxies, the star formation rate determined from the best-fit FIR SEDs (SFR(L _IR)) agrees with the SFR determined from the dust-corrected Hα luminosity (SFR(Hα)) to within a factor of ~2; however, when the SFR of these galaxies is estimated using only the observed 24 μm flux and the standard luminosity-dependent template method (SFR(24 μm)), it systematically overestimates the SFR by as much as a factor of six. A larger sample of 24 K-selected galaxies at z~ 2.3 drawn from the Kriek et al. GNIRS sample shows the same trend between SFR(24 μm) and SFR(Hα). Using that sample, we show that SFR(24 μm) and SFR(Hα) are in better agreement when SFR(24 μm) is estimated using the log average of local templates rather than selecting a single luminosity-dependent template, because this incorporates lower luminosity templates. The better agreement between SFRs from lower luminosity templates suggests that the FIR SEDs of the BLAST-detected galaxies may be typical for massive star-forming galaxies at z ~ 2, and that the majority are scaled up versions of lower luminosity local galaxies.