The far-infrared/radio correlation as probed by<i>Herschel</i>

We set out to determine the ratio, <i>q<i/><sub>IR<sub/>, of rest-frame 8–1000-<i>μ<i/>m flux, <i>S<i/><sub>IR<sub/>, to monochromatic radio flux, <i>S<i/><sub>1.4 GHz<sub/>, for galaxies selected at far-infrared (IR) and radio wavelengths, to search for signs that the ratio evolves with redshift, luminosity or dust temperature, <i>T<i/><sub>d<sub/>, 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-<i>μ<i/>m and 1.4-GHz samples, obtained using <i>Herschel<i/> and the Very Large Array (VLA) in GOODS-North (-N). We determine bolometric IR output using ten bands spanning λ<sub>obs<sub/> = 24–1250 <i>μ<i/>m, exploiting data from PACS and SPIRE (PEP; HerMES), as well as <i>Spitzer<i/>, SCUBA, AzTEC and MAMBO. We also explore the properties of an <i>L<i/><sub>IR<sub/>-matched sample, designed to reveal evolution of <i>q<i/><sub>IR<sub/> with redshift, spanning log <i>L<i/><sub>IR<sub/> = 11–12 and <i>z<i/> = 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, <i>q<i/><sub>IR<sub/>, at <i>L<i/><sub>1.4 GHz<sub/> ~ 10<sup>22.7<sup/> W Hz<sup>-1<sup/>, where active galactic nuclei (AGN) are starting to dominate the radio power density, and of weaker correlations with redshift and <i>T<i/><sub>d<sub/>. From our 250-<i>μ<i/>m-selected sample we identify a small number of far-IR-bright outliers, and see trends of <i>q<i/><sub>IR<sub/> with <i>L<i/><sub>1.4 GHz<sub/>, <i>L<i/><sub>IR<sub/>, <i>T<i/><sub>d<sub/> and redshift, noting that some of these are inter-related. For our <i>L<i/><sub>IR<sub/>-matched sample, there is no evidence that <i>q<i/><sub>IR<sub/> changes significantly as we move back into the epoch of galaxy formation: we find <i>q<i/><sub>IR<sub/> (1+z)<sup><i>γ<i/><sup/>, where <i>γ<i/> = -0.04<i>±<i/>0.03 at <i>z<i/> = 0–2; however, discounting the least reliable data at <i>z<i/> < 0.5 we find <i>γ<i/> = -0.26<i>±<i/>0.07, modest evolution which may be related to the radio background seen by ARCADE 2, perhaps driven by <10-<i>μ<i/>Jy radio activity amongst ordinary star-forming galaxies at <i>z<i/>>1.