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 ( CR ) 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 . \sim ( 1 + z ) ^ { 4 } ) suggests that evolution in the FIR-radio correlation should occur with infrared ( IR ; 8 - 1000 ~ { } \mu m ) /radio ratios increasing with redshift .
This signature should be especially apparent once beyond z \sim 3 where the magnetic field of a normal star-forming galaxy must be \sim 50 \mu G to save the FIR-radio correlation .
At present , observations do not show such a trend with redshift ; z \sim 6 radio-quiet quasars appear to lie on the local FIR-radio correlation while a sample of z \sim 4.4 and z \sim 2.2 submillimeter galaxies ( SMGs ) exhibit ratios that are a factor of \sim 2.5 below the canonical value .
I also derive a 5 \sigma point-source sensitivity goal of \approx 20 nJy ( i.e . \sigma _ { RMS } \sim 4 nJy ) requiring that the SKA specified be A _ { eff } / T _ { sys } \approx 15000 m ^ { 2 } K ^ { -1 } ; achieving this sensitivity should enable the detection of galaxies forming stars at a rate of \gtrsim 25 ~ { } M _ { \odot } ~ { } { yr } ^ { -1 } , such as typical luminous infrared galaxies ( i.e . L _ { IR } \gtrsim 10 ^ { 11 } ~ { } L _ { \odot } ) , 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 \gtrsim 10 GHz may prove to be the best probe for characterizing the high- z star formation history of the Universe unbiased by dust .