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

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<i>Context. <i/>Using Chandra observations we identified a sample of seven off-nuclear X-ray sources in the redshift range <i>z<i/> = 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 <i>L<i/>[0.5–7 keV] <i>><i/> 10<sup>39<sup/> erg s<sup>-1<sup/> and therefore can be classified as ultraluminous X-ray sources (ULXs). <i>Aims. <i/>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 H<i>α<i/> 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.<i>Methods. <i/>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 (<i>R<i/><sub>AB<sub/><i><<i/>22); 3) the redshift of the counterpart is lower than <i>z<i/> = 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.<i>Results. <i/>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 <i>≈<i/>0.5% to <i>≈<i/>0.2% going from = 3 × 10<sup>39<sup/> erg s<sup>-1<sup/> to = 2 × 10<sup>40<sup/> erg s<sup>-1<sup/>.