A joint analysis of BLAST 250500m and LABOCA 870m observations in the Extended Chandra Deep FieldSouth

We present a joint analysis of the overlapping BLAST 250, 350, 500μm, and Large APEX Bolometer Camera 870μm observations (from the LESS survey) of the Extended Chandra Deep Field South. Out to z ∼ 3, the BLAST filters sample near the peak wavelength of thermal far-infrared (FIR) emission from galaxies (rest-frame wavelengths ∼ 60–200μm), primarily produced by dust heated through absorption in star-forming clouds. However, identifying counterparts to individual BLAST peaks is very challenging, given the large beams (FWHM 36–60 arcsec). In contrast, the groundbased 870μm observations have a significantly smaller 19 arcsec FWHM beam, and are sensitive to higher redshifts (z ∼ 1–5, and potentially beyond) due to the more favourable negative K-correction. We use the LESS data, as well as deep Spitzer and VLA imaging, to identify 118 individual sources that produce significant emission in the BLAST bands. We characterize the temperatures and FIR luminosities for a subset of 69 sources which have well-measured submm SEDs and redshift measurements out to z ∼ 3. For flux-limited sub-samples in each BLAST band, and a dust emissivity index β = 2.0, we find a median temperature T = 30 K (all bands) as well as median redshifts: z = 1.1 (interquartile range 0.2–1.9) for S250 > 40 mJy; z = 1.3 (interquartile range 0.6–2.1) for S350 > 30 mJy; and z = 1.6 (interquartile range 1.3– 2.3) for S500 > 20 mJy. Taking into account the selection effects for our survey (a bias toward detecting lower-temperature galaxies), we find no evidence for evolution in the local FIR-temperature correlation out to z ∼ 2.5. Comparing with star-forming galaxy SED templates, about 8% of our sample appears to exhibit significant excesses in the radio and/or mid-IR, consistent with those sources harbouring an AGN. Since our statistical approach differs from most previous studies of submm galaxies, we describe the following techniques in two appendices: our ‘matched filter’ for identifying sources in the presence of point-source confusion; and our approach for identifying counterparts using likelihood ratios. This study is a direct precursor to future joint far-infrared/submm surveys, for which we outline a potential identification and SED measurement strategy.