2010ApJS..186..341S

VLA 1.4 GHz (σ∼ 0.012 mJy) and MIPS 24 and 70 µm (σ∼ 0.02 and 1.7 mJy, respectively) observations covering the 2 deg2 COSMOS field are combined with an extensive multiwavelength data set to study the evolution of the infrared (IR)-radio relation at intermediate and high redshift. With ∼4500 sources–of which ∼30% have spectroscopic redshifts–the current sample is significantly larger than previous ones used for the same purpose. Both monochromatic IR/radio flux ratios (q₂₄ and q₇₀), as well as the ratio of the total IR and the 1.4 GHz luminosity (q_TIR), are used as indicators for the IR/radio properties of star-forming galaxies and active galactic nuclei (AGNs). Using a sample jointly selected at IR and radio wavelengths in order to reduce selection biases, we provide firm support for previous findings that the IR-radio relation remains unchanged out to at least z∼ 1.4. Moreover, based on data from ∼150 objects we also find that the local relation likely still holds at z∈ [2.5, 5]. At redshift z< 1.4, we observe that radio-quiet AGNs populate the locus of the IR-radio relation in similar numbers as star-forming sources. In our analysis, we employ the methods of survival analysis in order to ensure a statistically sound treatment of flux limits arising from non-detections. We determine the observed shift in average IR/radio properties of IR- and radio-selected populations and show that it can reconcile apparently discrepant measurements presented in the literature. Finally, we also investigate variations of the IR/radio ratio with IR and radio luminosity and find that it hardly varies with IR luminosity but is a decreasing function of radio luminosity.