The far-infrared/radio correlation as probed by Herschel

We set out to determine the ratio, q_IR, of rest-frame 8-1000-μm 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-μm 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 λ_obs = 24-1250 μm, 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_⊙ 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} ∼ 10^22.7 W Hz^-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-μm-selected sample we identify a small number of far-IR-bright outliers, and see trends of q_IR with L_{1.4 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 (1+z)^γ, where γ = -0.04±0.03 at z = 0-2; however, discounting the least reliable data at z < 0.5 we find γ = -0.26±0.07, modest evolution which may be related to the radio background seen by ARCADE 2, perhaps driven by <10-μJy radio activity amongst ordinary star-forming galaxies at z>1.