We present new optical long-slit data along 6 position angles of the bulge region of M31 .
We derive accurate stellar and gas kinematics reaching 5 arcmin from the center , where the disk light contribution is always less than 30 % , and out to 8 arcmin along the major axis , where the disk makes 55 % of the total light .
We show that the velocity dispersions of McElroy ( 1983 ) are severely underestimated ( by up to 50 km/s ) .
As a consequence , previous dynamical models have underestimated the stellar mass of M31 ’ s bulge by a factor 2 .
As a further consequence , the light-weighted velocity dispersion of the galaxy grows to 166 km/s and to 170 km/s if also rotation is taken into account , thus reducing the discrepancy between the predicted and measured mass of the black hole at the center of M31 from a factor 3 to a factor 2 .
The kinematic position angle varies with distance , pointing to triaxiality , but a quantitative conclusion can be reached only after simultaneous proper dynamical modeling of the bulge and disk components is performed .
We detect gas counterrotation near the bulge minor axis .
We measure eight emission-corrected Lick indices .
They are approximately constant on circles .
Using simple stellar population models we derive the age , metallicity and \alpha -element overabundance profiles .
Except for the region in the inner arcsecs of the galaxy , the bulge of M31 is uniformly old ( \geq 12 Gyr , with many best-fit ages at the model grid limit of 15 Gyr ) , slightly \alpha -elements overabundant ( [ \alpha / Fe ] \approx 0.2 ) and at solar metallicity , in agreement with studies of the resolved stellar components .
The predicted u-g , g-r and r-i Sloan color profiles match reasonably well the dust-corrected observations , within the known limitations of current simple stellar population models .
The stellar populations have approximately radially constant mass-to-light ratios ( M / L _ { R } \approx 4 - 4.5 M _ { \odot } / L _ { \odot } for a Kroupa IMF ) , in agreement with stellar dynamical estimates based on our new velocity dispersions .
In the inner arcsecs the luminosity-weighted age drops to 4-8 Gyr , while the metallicity increases to above 3 times the solar value .
Starting from 6 arcmin from the center along the major axis , the mean age drops to \leq 8 Gyr , with slight supersolar metallicity ( \approx + 0.1 dex ) and \alpha - element overabundance ( \approx + 0.2 dex ) , for a mass-to-light ratio M / L _ { R } \leq 3 M _ { \odot } / L _ { \odot } .
Diagnostic diagrams based on the [ OIII ] /H \beta and [ NI ] /H \beta emission line equivalent widths ( EWs ) ratios indicate that the gas is ionized by shocks outside 10 arcsec , but an AGN-like ionizing source could be present near the center .
We speculate that a gas-rich minor merger happened some 100 Myr ago , causing the observed minor axis gas counterrotation , the recent star formation event , and possibly some nuclear activity .