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Published July 2008 | Published
Journal Article Open

A new method to separate star-forming from AGN galaxies at intermediate redshift : the submillijansky radio population in the VLA-COSMOS survey


We explore the properties of the submillijansky radio population at 20 cm by applying a newly developed optical color-based method to separate star-forming (SF) from active galactic nucleus (AGN) galaxies at intermediate redshifts (z ≲ 1.3). Although optical rest-frame colors are used, our separation method is shown to be efficient and not biased against dusty starburst galaxies. This classification method has been calibrated and tested on a local radio-selected optical sample. Given accurate multiband photometry and redshifts, it carries the potential to be generally applicable to any galaxy sample where SF and AGN galaxies are the two dominant populations. In order to quantify the properties of the submillijansky radio population, we have analyzed ~2,400 radio sources, detected at 20 cm in the VLA-COSMOS survey; 90% of these have submillijansky flux densities. We classify the objects into (1) star candidates, (2) quasi-stellar objects, (3) AGN, (4) SF, and (5) high-redshift (z > 1.3) galaxies. We find, for the composition of the submillijansky radio population, that SF galaxies are not the dominant population at submillijansky flux levels, as previously often assumed, but that they make up an approximately constant fraction of 30%-40% in the flux density range of ~50 μJy to 0.7 mJy. In summary, based on the entire VLA-COSMOS radio population at 20 cm, we find that the radio population at these flux densities is a mixture of roughly 30%-40% of SF and 50%-60% of AGN galaxies, with a minor contribution (~10%) of QSOs.

Additional Information

© 2008 The American Astronomical Society. Print publication: Issue 1 (2008 July) Received 2007 September 27; accepted for publication 2008 February 28. The authors would like to thank M. Polletta for the spectral template library. V. S. would like to thank E. Bell, G. Fabbiano, G. Helou, and D. Frayer for insightful discussions, as well as M. Schartmann for his help regarding FORTAN compilers. C. C., E. S., and V. S. acknowledge support from NASA grant HSTGO-09822.31-A. C. C. would like to acknowledge support from the Max Planck Society and the Alexander von Humboldt Foundation through the Max-Planck-Forschungspreis 2005. K. J. acknowledges support by theGermanDFG under grant SCHI 536/3-1. C. J. W. is supported by the MAGPOP Marie Curie EU Research and Training Network. This work is based on observations with the National Radio Astronomy Observatory, which is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc., the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 05-26555, and XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA. This work is also based on data collected at the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan, the European Southern Observatory, Chile, Kitt Peak National Observatory, Cerro Tololo Inter-American Observatory, and the National Optical Astronomy Observatory, which are operated by the Association of Universities for Research in Astronomy, Inc., (AURA) under cooperative agreement with the National Science Foundation. It is based also on observations obtained with MegaPrime/MegaCam, a joint project of CFHT and CEA/DAPNIA, at the Canada-France-Hawaii Telescope (CFHT), which is operated by the National Research Council (NRC) of Canada, the Institut National des Science de l'Univers of the Centre National de la Recherche Scientifique (CNRS) of France, and the University of Hawaii. This work is based in part on data products produced at TERAPIX and the CanadianAstronomy Data Centre. Funding for the Sloan Digital Sky Survey (SDSS) and SDSS-II has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, the US Department of Energy, the National Aeronautics and Space Administration, the Japanese Monbukagakusho, the Max Planck Society, and the Higher Education Funding Council for England. The SDSS Web site is http://www.sdss.org/. The SDSS is managed by the Astrophysical Research Consortium (ARC) for the Participating Institutions. The Participating Institutions are the American Museum of Natural History, Astrophysical Institute Potsdam, University of Basel, University of Cambridge, Case Western Reserve University, the University of Chicago, Drexel University, Fermilab, the Institute forAdvanced Study, the Japan ParticipationGroup, the Johns Hopkins University, the Joint Institute for Nuclear Astrophysics, the Kavli Institute for Particle Astrophysics and Cosmology, the Korean Scientist Group, the Chinese Academy of Sciences (LAMOST), Los Alamos National Laboratory, theMax Planck Institute forAstronomy (MPIA), the Max Planck Institute for Astrophysics (MPA), New Mexico State University, Ohio State University, University of Pittsburgh, University of Portsmouth, Princeton University, the United States Naval Observatory, and the University of Washington.

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