The far-infrared/radio correlation as probed by Herschel

Abstract

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) 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-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.

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