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Title:
A New Method to Separate Star-forming from AGN Galaxies at Intermediate Redshift: The Submillijansky Radio Population in the VLA-COSMOS Survey
Authors:
Smolčić, V.; Schinnerer, E.; Scodeggio, M.; Franzetti, P.; Aussel, H.; Bondi, M.; Brusa, M.; Carilli, C. L.; Capak, P.; Charlot, S.; Ciliegi, P.; Ilbert, O.; Ivezić, Ž.; Jahnke, K.; McCracken, H. J.; Obrić, M.; Salvato, M.; Sanders, D. B.; Scoville, N.; Trump, J. R.; Tremonti, C.; Tasca, L.; Walcher, C. J.; Zamorani, G.
Affiliation:
AA(Max-Planck-Institut-für-Astronomie, Königstuhl 17, Heidelberg, D-69117, Germany.; Fellow of the International Max Planck Research School for Astronomy and Cosmic Physics.; California Institute of Technology, MC 105-24, 1200 East California Boulevard, Pasadena, CA 91125.), AB(Max-Planck-Institut-für-Astronomie, Königstuhl 17, Heidelberg, D-69117, Germany.), AC(IASF Milano-INAF, Via Bassini 15, I-20133, Milan, Italy.), AD(IASF Milano-INAF, Via Bassini 15, I-20133, Milan, Italy.), AE(Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822.; Institut d'Astrophysique de Paris, UMR 7095, 98 bis Boulevard Arago, 75014 Paris, France.), AF(INAF-Istituto di Radioastronomia, via Gobetti 101, 40129 Bologna, Italy.), AG(Max-Planck-Institut-für-Extraterrestrische Physik, D-85478 Garching, Germany.), AH(National Radio Astronomy Observatory, P.O. Box 0, Socorro, NM 87801-0387.), AI(California Institute of Technology, MC 105-24, 1200 East California Boulevard, Pasadena, CA 91125.), AJ(Institut d'Astrophysique de Paris, UMR 7095, 98 bis Boulevard Arago, 75014 Paris, France.; Max-Planck-Institut-für-Astrophysik, Karl-Schwarzschild-Strasse 1, 85748 Garching, Germany.), AK(INAF-Osservatorio Astronomico di Bologna, via Ranzani 1, 40127, Bologna, Italy.), AL(Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822.), AM(Department of Astronomy, University of Washington, Box 351580, Seattle, WA 98195-1580.), AN(Max-Planck-Institut-für-Astronomie, Königstuhl 17, Heidelberg, D-69117, Germany.), AO(Institut d'Astrophysique de Paris, UMR 7095, 98 bis Boulevard Arago, 75014 Paris, France.), AP(Department of Astronomy, University of Washington, Box 351580, Seattle, WA 98195-1580.), AQ(California Institute of Technology, MC 105-24, 1200 East California Boulevard, Pasadena, CA 91125.), AR(Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822.), AS(California Institute of Technology, MC 105-24, 1200 East California Boulevard, Pasadena, CA 91125.; Visiting Astronomer, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI, 96822.), AT(Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721.), AU(Hubble Fellow, University of Arizona, Steward Observatory, 933 North Cherry Avenue, Tucson, AZ 85721.), AV(Laboratoire d'Astrophysique de Marseille, BP 8, Traverse du Siphon, 13376 Marseille Cedex 12, France.), AW(Institut d'Astrophysique de Paris, UMR 7095, 98 bis Boulevard Arago, 75014 Paris, France.), AX(INAF-Osservatorio Astronomico di Bologna, via Ranzani 1, 40127, Bologna, Italy.)
Publication:
The Astrophysical Journal Supplement Series, Volume 177, Issue 1, pp. 14-38. (ApJS Homepage)
Publication Date:
07/2008
Origin:
UCP
ApJ Keywords:
Cosmology: Observations, Radio Continuum: Galaxies, Surveys
DOI:
10.1086/588028
Bibliographic Code:
2008ApJS..177...14S

Abstract

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.
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