The Far-Infrared-Radio Correlation at High Redshifts: Physical Considerations and Prospects for the Square Kilometer Array

Abstract

I present a predictive analysis for the behavior of the far-infrared (FIR)-radio correlation as a function of redshift in light of the deep radio continuum surveys which may become possible using the Square Kilometer Array (SKA). To keep a fixed ratio between the FIR and predominantly non-thermal radio continuum emission of a normal star-forming galaxy, whose cosmic-ray electrons typically lose most of their energy to synchrotron radiation and inverse Compton (IC) scattering, requires a nearly constant ratio between galaxy magnetic field and radiation field energy densities. While the additional term of IC losses off of the cosmic microwave background (CMB) is negligible in the local universe, the rapid increase in the strength of the CMB energy density (i.e., ~(1 + z)^4) suggests that evolution in the FIR-radio correlation should occur with infrared (IR; 8-1000 μm)/radio ratios increasing with redshift. This signature should be especially apparent once beyond z ~ 3 where the magnetic field of a normal star-forming galaxy must be ~50 μG to save the FIR-radio correlation. At present, observations do not show such a trend with redshift; z ~ 6 radio-quiet quasars appear to lie on the local FIR-radio correlation while a sample of z ~ 4.4 and z ~ 2.2 submillimeter galaxies exhibit ratios that are a factor of ~2.5 below the canonical value. I also derive a 5σ point-source sensitivity goal of ≈20 nJy (i.e., σ_(rms) ~ 4 nJy) requiring that the SKA specified sensitivity be A_ (eff)/T_(sys) ≈ 15,000 m^2 K^(–1); achieving this sensitivity should enable the detection of galaxies forming stars at a rate of ≳25 M_☉ yr–1, such as typical luminous infrared galaxies (i.e., L_(IR) ≳ 10^(11) L_☉), at all redshifts if present. By taking advantage of the fact that the non-thermal component of a galaxy's radio continuum emission will be quickly suppressed by IC losses off of the CMB, leaving only the thermal (free-free) component, I argue that deep radio continuum surveys at frequencies ≳10 GHz may prove to be the best probe for characterizing the high-z star formation history of the universe unbiased by dust.

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