The old and heavy bulge of M 31^*

I. Kinematics and stellar populations

¹ Max-Planck Institut für extraterrestrische Physik, Giessenbachstrasse, Postfach 1312, 85741 Garching, Germany e-mail: saglia@mpe.mpg.de
² Universitäts-Sternwarte München, Scheinerstrasse 1, 81679 München, Germany

Received: 1 July 2009
Accepted: 28 October 2009

Abstract

We present new optical long-slit data along six position angles of the bulge region of M 31. 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 provides 55% of the total light. We show that the velocity dispersions of McElroy (1983) are severely underestimated (by up to 50 km s^-1). As a consequence, previous dynamical models have underestimated the stellar mass of M 31's bulge by a factor of 2. As a further consequence, the light-weighted velocity dispersion of the galaxy grows to 166 km s^-1 and to 170 km s^-1 if rotation is also taken into account, thus reducing the discrepancy between the predicted and measured mass of the black hole at the center of M 31 from a factor of 3 to a factor of 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 α-element overabundance profiles. Except for the region in the inner arcsecs of the galaxy, the bulge of M 31 is uniformly old (12 Gyr, with many best-fit ages at the model grid limit of 15 Gyr), slightly α-elements overabundant ([ α/Fe] ≈ 0.2) and of solar metallicity, in agreement with studies of the resolved stellar components. The predicted , and Sloan color profiles match the dust-corrected observations reasonably well, within the known limitations of current simple stellar population models. The stellar populations have approximately radially constant mass-to-light ratios (M/L_R≈ 4-4.5  for a Kroupa IMF), which is in agreement with the 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 three times the solar value. Starting from 6 arcmin from the center along the major axis, the mean age drops to ≤8 Gyr with slight supersolar metallicity (+0.1 dex) and α-element overabundance (+0.2 dex) for a mass-to-light ratio ≤ 3 . Diagnostic diagrams based on the [OIII]/Hβ and [NI]/Hβ 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.