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

TL;DR

This study identifies and analyzes off-nuclear ultraluminous X-ray sources in galaxies up to redshift 0.3 within the COSMOS field, exploring their host galaxy properties and distribution.

Contribution

It provides the first detailed analysis of ULXs out to z~0.3, linking their occurrence to host galaxy star formation and morphology using multi-wavelength data.

Findings

ULXs are found in both late and early type galaxies.

The fraction of galaxies hosting ULXs decreases with increasing X-ray luminosity.

ULX host galaxies are located in regions with expected off-nuclear sources based on SFR and stellar mass.

Abstract

Using Chandra observations we have 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. Using the multi-wavelength coverage available in the COSMOS field, we study the properties of the host galaxies of these ULXs. In detail, we derived their star formation rate from H_alpha 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 divide the host galaxies in different morphological classes using the available ACS/HST imaging. We find that our ULXs candidates are located in regions of the SFR versus M$_star$ 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 L[0.5-2 keV]=3 x 10^39 erg s^-1 to L[0.5-2 keV]= 2 x 10^40 erg s^-1.