A Systematic Search for the Reddest Far-infrared and Submillimeter Galaxies: Revealing Dust-embedded Starbursts at High Redshifts

We present the results of our systematic search for the reddest far-infrared (FIR) and submillimeter (sub-mm) galaxies using the data from the Herschel Multi-tiered Extragalactic Survey (HerMES) and the SCUBA2 Cosmological Legacy Survey (S2CLS). The red FIR galaxies are "500 μm risers," whose spectral energy distributions increase with wavelength across the three FIR passbands of the Spectral and Photometric Imaging REceiver (SPIRE) of Herschel. Within 106.5 deg² of the HerMES fields, we have selected 629 500 μm risers. The red sub-mm galaxies are "SPIRE drop-outs," which are prominent detections in the S2CLS 850 μm data but are extremely weak or invisible in the SPIRE bands. Within the 2.98 deg² common area of HerMES and S2CLS, we have selected 95 such objects. These very red sources could be dusty starbursts at high redshifts (z ≳ 4–6) because the peak of their cold-dust emission heated by star formation is shifted to the reddest FIR/sub-mm bands. The surface density of 500 μm risers is ∼8.2 deg⁻² at the ≥20 mJy level in 500 μm, while that of SPIRE drop-outs is ∼19.3 deg⁻² at the ≥5 mJy level in 850 μm. Both types of objects could span a wide range of redshifts, however. Using deep radio data in these fields to further select the ones likely at the highest redshifts, we find that the surface density of z > 6 candidates is 5.5 deg⁻² among 500 μm risers and is 0.8–13.6 deg⁻² among SPIRE drop-outs. If this is correct, the dust-embedded star formation processes in such objects could contribute comparably as Lyman-break galaxies to the global SFR density at z > 6.