The VLA-COSMOS Perspective on the Infrared-Radio Relation. I. New Constraints on Selection Biases and the Non-Evolution of the Infrared/Radio Properties of Star-Forming and Active Galactic Nucleus Galaxies at Intermediate and High Redshift
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 deg^2 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 (q_(24) and q_(70)), 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.
© 2010 American Astronomical Society. Print publication: Issue 2 (2010 February); received 2009 September 30; accepted for publication 2010 January 8; published 2010 January 28. We gratefully acknowledge the contribution of the whole COSMOS collaboration and its more than 100 scientists around the globe. M.T.S. thanks Ute Lisenfeld, Chris Carilli, and George Helou for useful discussions, as well as Marco Scodeggio for comments on the manuscript. The German DFG supported this research with grant SCHI 536/3-2. G.Z. acknowledges partial support from an INAF contract (PRIN-2007/1.06.10.08) and an ASI grant (ASI/COFIS/WP3110 I/026/07/0). This work is partly based on observations made with the Spitzer Space Telescope, which is operated by NASA/JPL/Caltech. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.
Published - Sargent2010p7138Astrophys_J_Suppl_S.pdf