arXiv:1003.4271v1 [astro-ph.CO] 22 Mar 2010 Accepted to ApJL: March 22, 2010 Preprint typeset using L A T E X style emulateapj v. 11/10/09 NO EVOLUTION IN THE IR-RADIO RELATION FOR IR-LUMINOUS GALAXIES AT Z < 2 IN THE COSMOS FIELD † M. T. Sargent 1, ⋆ , E. Schinnerer 1 , E. Murphy 2 , C. L. Carilli 3 , G. Helou 4 , H. Aussel 5 , E. Le Floc’h 5 , D. T. Frayer 6 , O. Ilbert 7 , P. Oesch 8 , M. Salvato 9 , V. Smolˇ ci´ c 10 , J. Kartaltepe 11 , D. B. Sanders 12 Accepted to ApJL: March 22, 2010 ABSTRACT Previous observational studies of the infrared (IR)-radio relation out to high redshift employed any detectable star forming systems at a given redshift within the restricted area of cosmological survey fields. Consequently, the evolution inferred relies on a comparison between the average IR/radio properties of (i ) very IR-luminous high-z sources and (ii ) more heterogeneous low(er)-z samples that often lack the strongest IR emitters. In this report we consider populations of objects with comparable luminosities over the last 10Gyr by taking advantage of deep IR (esp. Spitzer 24 µm) and VLA 1.4 GHz observations of the COSMOS field. Consistent with recent model predictions, both Ultra Luminous Infrared Galaxies (ULIRGs) and galaxies on the bright end of the evolving IR luminosity function do not display any change in their average IR/radio ratios out to z ∼ 2 when corrected for bias. Uncorrected data suggested ∼0.3 dex of positive evolution. Subject headings: cosmology: observations — galaxies: active — galaxies: evolution — infrared: galaxies — radio continuum: galaxies — surveys 1. INTRODUCTION The IR/radio properties of galaxies at successively higher redshift have been probed in the past decade using either statistical samples from cosmological sur- vey fields (e.g. Appleton et al. 2004; Frayer et al. 2006; Sargent et al. 2010, hereafter: S10), the stacking tech- nique (e.g. Carilli et al. 2008; Ivison et al. 2010) or ded- icated samples of specific objects (e.g. sub-mm galax- ies (SMGs); Kov´ acs et al. 2006; Hainline et al. 2009; Micha lowski et al. 2009). Evolutionary studies, all based on samples poorly matched in terms of bolometric lu- minosity at low and high redshift, have provided con- flicting results, concluding that the local IR-radio rela- tion either does (e.g. Garrett 2002; Appleton et al. 2004; ⋆ E-mail: markmr@mpia.de 1 Max-Planck-Institut f¨ ur Astronomie, K¨ onigstuhl 17, D- 69117 Heidelberg, Germany 2 Spitzer Science Center, MC 314-6, California Institute of Technology, Pasadena, CA 91125 3 National Radio Astronomy Observatory, P.O. Box 0, So- corro, NM 87801-0387, USA 4 Infrared Processing and Analysis Center, MC 100-22, Cali- fornia Institute of Technology, Pasadena, CA 91125 5 AIM Unit´ e Mixte de Recherche CEA CNRS Universit´ e Paris VII UMR n158, France 6 National Radio Astronomy Observatory, P.O. Box 2, Green Bank, WV 24944, USA 7 Laboratoire d’Astrophysique de Marseille, Universit´ e de Provence, CNRS, 38 rue Fr´ ed´ eric Joliot-Curie, F-13388 Marseille Cedex 13, France 8 Department of Physics, ETH Zurich, CH-8093 Zurich, Switzerland 9 Max-Planck-Institut f¨ ur Plasmaphysik, Boltzmanstrasse, D- 85741 Garching, Germany 10 California Institute of Technology, MC 105-24, 1200 East California Boulevard, Pasadena, CA 91125, USA 11 National Optical Astronomy Observatory, 950 N. Cherry Ave, Tucson, AZ 85726, USA 12 Institute for Astronomy, 2680 Woodlawn Dr., University of Hawaii, Honolulu, Hawaii, 96822, USA † Partly based on observations collected at the European Or- ganisation for Astronomical Research in the Southern Hemi- sphere, Chile, ESO program ID 175.A-0839. Ibar et al. 2008; Garn et al. 2009; S10) or does not (e.g. Seymour et al. 2009; Ivison et al. 2010) hold out to high redshift. Recently, predictions have been made for the redshift evolution of the IR/radio properties of star-forming galaxies having different luminosities and geometries (e.g. compact starbursts and normal star-forming disks; Murphy 2009; Lacki & Thompson 2009). The current generation of IR and radio observatories can directly de- tect the brightest of these systems over a significant frac- tion of Hubble time, provided that sufficiently large cos- mological volumes are sampled. Here we make use of the VLA and Spitzer coverage of the 2 deg 2 COSMOS field to construct (cf. §2) a volume-limited sample of ULIRGs at z< 2 that allows a direct comparison of observations and theory. Our findings are presented in §3 and discussed in § 4. We adopt the WMAP-5 cosmology (Ω m = 0.258, Ω Λ +Ω m =1 and H 0 = 71.9 km s −1 Mpc −1 ; Dunkley et al. 2009). 2. DATA AND SAMPLE SELECTION 2.1. IR and Radio Measurements The 1.4 GHz map (Schinnerer et al. 2007) of the 2 deg 2 COSMOS field reaches an average sensitivity of ∼0.017 mJy/beam (FWHM = 2.5 ′′ ). Here we use the VLA-COSMOS ‘Joint’ Catalog (Schinnerer et al. 2010, subm.) containing ∼2900 sources detected with S/N ≥ 5. Spitzer/MIPS imaging by the S-COSMOS project (Sanders et al. 2007) achieves a resolution of 5.8 ′′ (18.6 ′′ ) and a 1 σ point source detection limit of ∼0.018 (1.7) mJy at 24 (70) µm (for details see LeFloc’h et al. 2009 and Frayer et al. 2009). The depth of the 24 µm ob- servations exceeds that at 70 µm and 1.4 GHz by roughly a factor of seven in terms of equivalent IR luminosity (cf. Fig. 1 in S10). At an equal detection significance level (3 σ), the 24 µm catalog consequently is roughly 20-fold larger than the 70 µm source list (∼50,000 vs. 2,700).