The XMM–Newton Needles in the Haystack Survey: the local X‐ray luminosity function of ‘normal’ galaxies

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 deg 2 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 0.5–8 keV band flux limit of ≈2 × 10 −15  erg cm −2  s −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 × 10 41  erg s −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 (0.5–8 keV). Finally, we derive, for the first time, the luminosity functions for early‐ and late‐type systems separately.