BLAST: the far-infrared/radio correlation in distant galaxies

Abstract

We investigate the correlation between far-infrared (FIR) and radio\nluminosities in distant galaxies, a lynchpin of modern astronomy. We use data\nfrom the Balloon-borne Large Aperture Submillimetre Telescope (BLAST), Spitzer,\nthe Large Apex BOlometer CamerA (LABOCA), the Very Large Array (VLA) and the\nGiant Metre-wave Radio Telescope (GMRT) in the Extended Chandra Deep Field\nSouth (ECDFS). For a catalogue of BLAST 250-micron-selected galaxies, we\nre-measure the 70--870-micron flux densities at the positions of their most\nlikely 24-micron counterparts, which have a median [interquartile] redshift of\n0.74 [0.25, 1.57]. From these, we determine the monochromatic flux density\nratio, q_250 = log_10 (S_250micron / S_1400MHz), and the bolometric equivalent,\nq_IR. At z ~= 0.6, where our 250-micron filter probes rest-frame 160-micron\nemission, we find no evolution relative to q_160 for local galaxies. We also\nstack the FIR and submm images at the positions of 24-micron- and\nradio-selected galaxies. The difference between q_IR seen for 250-micron- and\nradio-selected galaxies suggests star formation provides most of the IR\nluminosity in ~< 100-uJy radio galaxies, but rather less for those in the mJy\nregime. For the 24-micron sample, the radio spectral index is constant across 0\n< z < 3, but q_IR exhibits tentative evidence of a steady decline such that\nq_IR is proportional to (1+z)^(-0.15 +/- 0.03) - significant evolution,\nspanning the epoch of galaxy formation, with major implications for techniques\nthat rely on the FIR/radio correlation. We compare with model predictions and\nspeculate that we may be seeing the increase in radio activity that gives rise\nto the radio background.\n