The far - infrared / radio correlation as probed by Herschel ⋆

We set out to determine the ratio, qIR, of rest-frame 8–1000-μm flux, SIR, to monochromatic radio flux, S1.4GHz, for galaxies selected at far-infrared (-IR) and radio wavelengths, to search for signs that the ratio evolves with redshift, luminosity or dust temperature, Td, 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 λobs = 24− 1250μm, exploiting data from PACS and SPIRE (PEP; HerMES), as well as Spitzer, SCUBA, AzTEC and MAMBO. We also explore the properties of an LIR-matched sample, designed to reveal evolution of qIR with redshift, spanning log LIR = 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, qIR, at l1.4 ∼ 10 22.7 WHz, where active galactic nuclei (AGN) are starting to dominate the radio power density, and of weaker correlations with redshift and Td. From our 250-μm-selected sample we identify a small number of far-IR-bright outliers, and see trends of qIR with l1.4, LIR, Td and redshift, noting that some of these are inter-related. For our LIR-matched sample, there is no evidence that qIR changes significantly as we move back into the epoch of galaxy formation: we find qIR ∝ (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 ARCADE2, perhaps driven by <10-μJy radio activity amongst ordinary star-forming galaxies at z > 1.