Evidence for non-axisymmetry in M31 from wide-field kinematics of stars and gas

TL;DR

This study uses wide-field kinematic data of stars and gas in M31 to provide evidence for a non-axisymmetric structure, likely a bar, influencing the galaxy's dynamics and morphology.

Contribution

It offers the first comprehensive kinematic analysis of M31's bulge and disk, revealing features indicative of a bar through detailed stellar and gas velocity measurements.

Findings

Detection of velocity plateaus and h3-velocity correlation suggesting a bar

Irregular gas kinematics consistent with non-triaxial streaming motions

Gas morphology showing spiral pattern with lower inclination

Abstract

As the nearest large spiral galaxy, M31 provides a unique opportunity to learn about the structure and evolutionary history of this galaxy type in great detail. Among the many observing programs aimed at M31 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 M31'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 M31. We obtained $\approx$ 220 separate fields with the optical IFU spectrograph VIRUS-W, covering the whole bulge region of M31 and parts of the disk. We derive stellar line-of-sight velocity distributions from the stellar absorption lines, as well as velocity distributions and line fluxes of the emission lines H$\beta$, [OIII] and [NI]. Our data supersede any previous study in terms of spacial coverage and spectral resolution. 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.