2010MNRAS.402..245I

We investigate the correlation between far-infrared (FIR) and radio luminosities in distant galaxies, a lynchpin of modern astronomy. We use data from the Balloon-borne Large Aperture Submillimetre Telescope (BLAST), Spitzer, the Large Apex BOlometer CamerA (LABOCA), the Very Large Array and the Giant Metre-wave Radio Telescope (GMRT) in the Extended Chandra Deep Field South (ECDFS). For a catalogue of BLAST 250-µm-selected galaxies, we remeasure the 70-870-µm flux densities at the positions of their most likely 24-µm counterparts, which have a median [interquartile] redshift of 0.74 [0.25, 1.57]. From these, we determine the monochromatic flux density ratio, q₂₅₀(= log₁₀[S_250µm/S_1400MHz]), and the bolometric equivalent, q_IR. At z ~ 0.6, where our 250-µm filter probes rest-frame 160-µm emission, we find no evolution relative to q₁₆₀ for local galaxies. We also stack the FIR and submm images at the positions of 24-µm- and radio-selected galaxies. The difference between q_IR seen for 250-µm- and radio-selected galaxies suggests that star formation provides most of the IR luminosity in ≲100-µJy radio galaxies, but rather less for those in the mJy regime. For the 24-µm sample, the radio spectral index is constant across 0 < z < 3, but q_IR exhibits tentative evidence of a steady decline such that q_IR∝ (1 + z)^{–0.15±0.03} - significant evolution, spanning the epoch of galaxy formation, with major implications for techniques that rely on the FIR/radio correlation. We compare with model predictions and speculate that we may be seeing the increase in radio activity that gives rise to the radio background.