The far-infrared radio correlation as probed by Herschel [Letter]

We set out to determine the ratio, qIR, of rest-frame 8?1000-\ensuremath\mum flux, SIR, to monochromatic radio flux, S1.4 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, 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-\ensuremath\mum 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 \ensuremath\lambdaobs=24?1250 \ensuremath\mum, 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 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 GHz \texttt\char126 1022.7 WâHz-1, where active galactic nuclei (AGN) are starting to dominate the radio power density, and of weaker correlations with redshift and Td. From our 250-\ensuremath\mum-selected sample we identify a small number of far-IR-bright outliers, and see trends of qIR with L1.4 GHz, 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)\ensuremath\gamma