Ultraluminous X-ray sources out to z ~ 0.3 in the COSMOS field

¹ ESO, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany e-mail: vmainier@eso.org
² Dipartimento di Astronomia, Università degli Studi di Bologna, via Ranzani 1, 40127 Bologna, Italy
³ INAF - Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁴ Excellence Cluster Universe, TUM, Boltzmannstr. 2, 85748 Garching bei München, Germany
⁵ Max-Planck-Institute für Extraterrestrische Physik, Postfach 1312, 85741, Garching bei München, Germany
⁶ Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, USA
⁷ ASI Science Data Center, via Galileo Galilei, 00044 Frascati Italy
⁸ Institute of Astronomy, Swiss Federal Institute of Technology (ETH Hönggerberg), 8093, Zürich, Switzerland
⁹ Spitzer Science Center, Pasadena, CA, 91125, USA
¹⁰ INAF - Osservatorio astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone, Italy
¹¹ Space Telescope Science Institute, Baltimore, Maryland 21218, USA
¹² Laboratoire d'Astrophysique de Toulouse-Tarbes, Université de Toulouse, CNRS, 14 avenue Edouard Belin, 31400 Toulouse, France
¹³ Laboratoire d'Astrophysique de Marseille, Marseille, France
¹⁴ INAF - Osservatorio astronomico di Padova, Vicolo Dell'Osservatorio 5, 35122 Padova, Italy
¹⁵ INAF - IASF Milano, Milan, Italy
¹⁶ INAF Osservatorio Astronomico di Brera, Milan, Italy
¹⁷ Dipartimento di Astronomia, Universita di Padova, Padova, Italy
¹⁸ California Institute of Technology, MC 105-24, 1200 East California Boulevard, Pasadena, CA 91125, USA
¹⁹ Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA
²⁰ Research Center for Space and Cosmic Evolution, Ehime University, Bunkyo-cho 2-5, Matsuyama 790-8577, Japan

Received: 20 May 2009
Accepted: 22 February 2010

Abstract

Context. Using Chandra observations we identified a sample of seven off-nuclear X-ray sources in the redshift range z = 0.072–0.283, located within optically bright galaxies in the COSMOS Survey. All of them, if associated with their closest bright galaxy, would have L[0.5–7 keV] > 10³⁹ erg s^-1 and therefore can be classified as ultraluminous X-ray sources (ULXs).

Aims. Using the multi-wavelength coverage available in the COSMOS field, we studied the properties of the host galaxies of these ULXs. In detail, we derived their star formation rate from Hα measurements and their stellar masses using SED fitting techniques with the aim to compute the probability to have an off-nuclear source based on the host galaxy properties. We divided the host galaxies in different morphological classes with the available ACS/HST imaging.

Methods. We selected off-nuclear candidates with the following criteria: 1) the distance between the X-ray and the optical centroid has to be larger than 0.9´´, larger than 1.8 times the radius of the Chandra positional error circle and smaller than the Petrosian radius of the host galaxy; 2) the optical counterpart is a bright galaxy (R_AB<22); 3) the redshift of the counterpart is lower than z = 0.3; 4) the source has been observed in at least one Chandra pointing at an off-axis angle smaller than 5´; 5) the X-ray positional error is smaller than 0.8´´. We verified each candidate super-imposing the X-ray contours on the optical/IR images. We expect less than one misidentified AGN due to astrometric errors and on average 1.3 serendipitous background source matches.

Results. We find that our ULXs candidates are located in regions of the SFR versus plane where one or more off-nuclear detectable sources are expected. From a morphological analysis of the ACS imaging and the use of rest-frame colours, we find that our ULXs are hosted both in late and early type galaxies. Finally, we find that the fraction of galaxies hosting a ULX ranges from ≈0.5% to ≈0.2% going from = 3 × 10³⁹ erg s^-1 to = 2 × 10⁴⁰ erg s^-1.