Evidence for non-axisymmetry in M 31 from wide-field kinematics of stars and gas^{★,★★,★★★}

¹ Max Planck Institute for Extraterrestrial Physics, Giessenbachstr., 85748 Garching, Germany
e-mail: mxhf@mpe.mpg.de
² Universitäts-Sternwarte München, Scheinerstr. 1, 81679 Munich, Germany
³ Excellence Cluster Universe, Boltzmannstr. 2, 85748 Garching, Germany

Received: 10 February 2017
Accepted: 14 July 2017

Abstract

Aim. As the nearest large spiral galaxy, M 31 provides a unique opportunity to study the structure and evolutionary history of this galaxy type in great detail. Among the many observing programs aimed at M 31 are microlensing studies, which require good three-dimensional models of the stellar mass distribution. Possible non-axisymmetric structures like a bar need to be taken into account. Due to M 31’s high inclination, the bar is difficult to detect in photometry alone. Therefore, detailed kinematic measurements are needed to constrain the possible existence and position of a bar in M 31.

Methods. We obtained ≈220 separate fields with the optical integral-field unit spectrograph VIRUS-W, covering the whole bulge region of M 31 and parts of the disk. We derived stellar line-of-sight velocity distributions from the stellar absorption lines, as well as velocity distributions and line fluxes of the emission lines Hβ, [O III] and [N I]. Our data supersede any previous study in terms of spatial coverage and spectral resolution.

Results. We find several features that are indicative of a bar in the kinematics of the stars, we see intermediate plateaus in the velocity and the velocity dispersion, and correlation between the higher moment h3 and the velocity. The gas kinematics is highly irregular, but is consistent with non-triaxial streaming motions caused by a bar. The morphology of the gas shows a spiral pattern, with seemingly lower inclination than the stellar disk. We also look at the ionization mechanisms of the gas, which happens mostly through shocks and not through starbursts.