Mild radial variations of the stellar IMF in the bulge of M31
  Using new, homogeneous, long-slit spectroscopy in the wavelength range from
~0.35 to ~1micron, we study radial gradients of optical and near-infrared (NIR)
IMF-sensitive features along the major axis of the bulge of M31, out to a
galacto-centric distance of ~200'' (~800pc). 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 (<0.5M_Sun) end slope (i.e. the fraction of
low-mass stars) of the stellar IMF, as a function of galacto-centric distance.
Outside a radial distance of ~10'', we infer an IMF similar to a Milky-Way-like
distribution, while at small galacto-centric 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 NaI8190), without requiring extremely
high Na abundance ratios. [Na/Fe] is ~0.4dex for most of the bulge, rising up
to ~0.6dex 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.
