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  • Evidence for non-axisymmetr...
    Opitsch, M.; Fabricius, M. H.; Saglia, R. P.; Bender, R.; Blaña, M.; Gerhard, O.

    Astronomy and astrophysics (Berlin), 3/2018, Volume: 611
    Journal Article

    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 H β , 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 h 3 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.