| Abstract: | We set out to determine the ratio, q(IR), of rest-frame 8-1000um flux, S(IR),
to monochromatic radio flux, S(1.4GHz), for galaxies selected at far-IR and
radio wavelengths, to search for signs that the ratio evolves with redshift,
luminosity or dust temperature, and to identify any far-IR-bright outliers -
useful laboratories for exploring why the far-IR/radio correlation is generally
so tight when the prevailing theory suggests variations are almost inevitable.
We use flux-limited 250-um and 1.4-GHz samples, obtained in GOODS-N using
Herschel (HerMES; PEP) and the VLA. We determine bolometric IR output using ten
bands spanning 24-1250um, exploiting data from PACS and SPIRE, 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 z, spanning
log L(IR) = 11-12 L(sun) and z=0-2, by stacking into the radio and far-IR
images. For 1.4-GHz-selected galaxies, we see tentative evidence of a break in
the flux ratio, q(IR), at L(1.4GHz) ~ 10^22.7 W/Hz, where AGN are starting to
dominate the radio power density, and of weaker correlations with z and T(d).
From our 250-um-selected sample we identify a small number of far-IR-bright
outliers, and see trends of q(IR) with L(1.4GHz), L(IR), T(d) and z, 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) goes as (1+z)^gamma, where gamma = -0.04 +/-
0.03 at z=0-2; however, discounting the least reliable data at z<0.5 we find
gamma = -0.26 +/- 0.07, modest evolution which may be related to the radio
background seen by ARCADE2, perhaps driven by <10uJy radio activity amongst
ordinary star-forming galaxies at z>1. |
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