Large-scale Hydrodynamical Shocks as the Smoking Gun Evidence for a Bar in M31

TL;DR

This study provides evidence for a rotating bar in M31 through the detection of large-scale hydrodynamical shocks in gas kinematics, supported by simulations that match observed velocity jumps.

Contribution

It presents the first clear observational evidence of bar-induced shocks in M31, linking gas kinematics to the galaxy's dynamical structure.

Findings

Sharp velocity jumps (>100 km/s) identified in [OIII] and HI data.

Simulations reproduce observed shocks, supporting the bar hypothesis.

Shock features are mainly on the leading sides of the bar, consistent with bar-driven inflow.

Abstract

The formation and evolutionary history of M31 are closely related to its dynamical structures, which remain unclear due to its high inclination. Gas kinematics could provide crucial evidence for the existence of a rotating bar in M31. Using the position-velocity diagram of [OIII] and HI, we are able to identify clear sharp velocity jump (shock) features with a typical amplitude over 100 km/s in the central region of M31 (4.6 kpc X 2.3 kpc, or 20 arcmin X 10 arcmin). We also simulate gas morphology and kinematics in barred M31 potentials and find that the bar-induced shocks can produce velocity jumps similar to those in [OIII]. The identified shock features in both [OIII] and HI are broadly consistent, and they are found mainly on the leading sides of the bar/bulge, following a hallmark pattern expected from the bar-driven gas inflow. Shock features on the far side of the disk are clearer than those on the near side, possibly due to limited data coverage on the near side, as well as obscuration by the warped gas and dust layers. Further hydrodynamical simulations with more sophisticated physics are desired to fully understand the observed gas features and to better constrain the parameters of the bar in M31.