The central region of M 31 observed with XMM-Newton I. Group properties and diffuse emission

We present the results of a study based on an XMM-Newton Performance Verification observation of the central 30' of the nearby spiral galaxy M 31. In the 34-ks European Photon Imaging Camera (EPIC) exposure, we detect 116 sources down to a limiting luminosity of 6 10(35) erg s(-1) (0.3-12 keV, d = 760 kpc). The luminosity distribution of the sources detected with XMM-Newton flattens at luminosities below similar to2.5 10(37) erg s(-1). We make use of hardness ratios for the detected sources in order to distinguish between classes of objects such as super-soft sources and intrinsically hard or highly absorbed sources. We demonstrate that the spectrum of the unresolved emission in the bulge of M 31 contains a soft excess which can be fitted with a similar to0.35-keV optically-thin thermal-plasma component; clearly distinct from the composite point-source spectrum. We suggest that this may represent diffuse gas in the centre of M 31, and we illustrate its extent in a wavelet-deconvolved image.