The XMM-Newton Needles in the Haystack Survey: the local X-ray luminosity function of `normal' galaxies

Abstract

In this paper we estimate the local (z < 0.22) X-ray luminosity function of `normal' galaxies derived from the XMM-Newton Needles in the Haystack Survey. This is an on-going project that aims to identify X-ray-selected normal galaxies (i.e. non-AGN dominated) in the local Universe. We are using a total of 70 XMM-Newton fields covering an area of 11 deg² which overlap with the Sloan Digital Sky Survey Data Release 2. Normal galaxies are selected on the basis of their resolved optical light profile, their low X-ray-to-optical flux ratio [log(f_x/f_o) < - 2] and soft X-ray colours. We find a total of 28 candidate normal galaxies to the 0.5-8keV band flux limit of ~2 × 10^-15ergcm^-2s^-1. Optical spectra are available for most sources in our sample (82 per cent). These provide additional evidence that our sources are bona fide normal galaxies with X-ray emission coming from diffuse hot gas emission and/or X-ray binaries rather than a supermassive black hole. 16 of our galaxies have narrow emission lines or a late-type spectral energy distribution (SED) while the remaining 12 present only absorption lines or an early-type SED. Combining our XMM-Newton sample with 18 local (z < 0.22) galaxies from the Chandra Deep Field North and South surveys, we construct the local X-ray luminosity function of normal galaxies. This can be represented with a Schechter form with a break at L_*~ 3^+1.4_-1.0× 10⁴¹ergs^-1 and a slope of α~ 1.78 +/- 0.12. Using this luminosity function and assuming pure luminosity evolution of the form ~(1 +z)^3.3 we estimate a contribution to the X-ray background from normal galaxies of ~10-20 per cent (0.5-8keV). Finally, we derive, for the first time, the luminosity functions for early- and late-type systems separately.