Mining for normal galaxies in the first XMM-Newton Serendipitous Source Catalog

This paper uses the first XMM-Newton Serendipitous Source Catalog compiled by the XMM-Newton Science Centre to identify low-z X-ray selected normal galaxy candidates. Our sample covers a total area of ~6deg² to the 0.5-2keV limit ~10^-15ergs^-1cm^-2. A total of 23 sources are selected on the basis of low X-ray to optical flux ratio logf_X/f_opt < -2, soft X-ray spectral properties and optical spectra, when available, consistent with stellar formation rather than active galactic nucleus (AGN) processes. This sample is combined with similarly selected systems from the Needles in the Haystack Survey to provide a total of 46 unique (z<~ 0.2) X-ray detected normal galaxies, the largest low-z sample yet available. This is first used to constrain the normal galaxy logN-logS at bright fluxes (10^-15-10^-13ergs^-1cm^-2). We estimate a slope of -1.46 +/- 0.13 for the cumulative number counts consistent with the Euclidean prediction. We further combine our sample with 23 local (z<~ 0.2) galaxies from the Chandra Deep Field-North and -South surveys to construct the local X-ray luminosity function of normal galaxies. A Schechter form provides a good fit to the data with a break at logL_*= 41.02^+0.14_-0.12ergs^-1 and a slope of α=-1.76 +/- 0.10. Finally, for the sample of 46 systems, we explore the association between X-ray luminosity and host galaxy properties, such as star formation rate (SFR) and stellar mass. We find that the L_X of the emission-line systems correlates with Hα luminosity and 1.4-GHz radio power, both providing an estimate of the current SFR. In the case of early-type galaxies with absorption-line optical spectra, we use the K band as an approximation of stellar mass and find a correlation of the form L_X~L^1.5_K. This is flatter than the L_X-L_B relation for local ellipticals. This may be due to either L_K providing a better approximation of galaxy mass or selection effects biasing our sample against very luminous early-type galaxies, L_X > 10⁴²ergs^-1.