Mild radial variations of the stellar IMF in the bulge of M31

Using new, homogeneous, long-slit spectroscopy in the wavelength range from similar to 0.35 to m, we study radial gradients of optical and near-infrared (NIR) initial mass function (IMF)-sensitive features along the major axis of the bulge of M31, out to a galactocentric distance of similar to 200 arcsec (similar to 800 pc). Based on state-of-the-art stellar population synthesis models with varying Na abundance ratio, we fit a number of spectral indices, from different chemical species (including TiO's, Ca, and Na indices), to constrain the low-mass (less than or similar to 0.5 M-circle dot) end slope (i.e. the fraction of low-mass stars) of the stellar IMF, as a function of galactocentric distance. Outside a radial distance of similar to 10 arcsec, we infer an IMF similar to a Milky Way-like distribution, while at small galactocentric distances, an IMF radial gradient is detected, with a mildly bottom-heavy IMF in the few inner arcsec. We are able to fit Na features (both NaD and ), without requiring extremely high Na abundance ratios. is similar to 0.4 dex for most of the bulge, rising up to similar to 0.6 dex in the innermost radial bins. Our results imply an overall, luminosity-weighted, IMF and mass-to-light ratio for the M31 bulge, consistent with those for a Milky Way-like distribution, in contrast to results obtained, in general, for most massive early-type galaxies.