The far-infrared/radio correlation as probed by Herschel

We set out to determine the ratio, q(IR), of rest-frame 8-1000-mu m flux, S-IR, to monochromatic radio flux, S 1.4 GHz, for galaxies selected at far-infrared (IR) and radio wavelengths, to search for signs that the ratio evolves with redshift, luminosity or dust temperature, T-d, and to identify any far-IR-bright outliers - useful laboratories for exploring why the far-IR/radio correlation (FIRRC) is generally so tight when the prevailing theory suggests variations are almost inevitable. We use flux-limited 250-mu m and 1.4-GHz samples, obtained using Herschel and the Very Large Array (VLA) in GOODS-North (-N). We determine bolometric IR output using ten bands spanning lambda(obs) = 24-1250 mu m, exploiting data from PACS and SPIRE (PEP; HerMES), as well as Spitzer, SCUBA, AzTEC and MAMBO. We also explore the properties of an L-IR-matched sample, designed to reveal evolution of q(IR) with redshift, spanning log L-IR = 11-12 L-circle dot and z = 0-2, by stacking into the radio and far-IR images. For 1.4-GHz-selected galaxies in GOODS-N, we see tentative evidence of a break in the flux ratio, q(IR), at L-1.4 GHz similar to 10(22.7) WHz(-1), where active galactic nuclei (AGN) are starting to dominate the radio power density, and of weaker correlations with redshift and T-d. From our 250-mu m-selected sample we identify a small number of far-IR-bright outliers, and see trends of q(IR) with L1.4 GHz, L-IR, T-d and redshift, noting that some of these are inter-related. For our L-IR-matched sample, there is no evidence that q(IR) changes significantly as we move back into the epoch of galaxy formation: we find q(IR) proportional to (1+ z)(gamma), where gamma = -0.04 +/- 0.03 at z = 0 -2; however, discounting the least reliable data at z < 0.5 we find gamma = -0.26 +/- 0.07, modest evolution which may be related to the radio background seen by ARCADE 2, perhaps driven by < 10-mu Jy radio activity amongst ordinary star-forming galaxies at z > 1.