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

Abstract

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 [O III] and H i, we are able to identify clear sharp velocity jump (shock) features with a typical amplitude over 100 km s−1 in the central region of M31 (4.6 kpc × 2.3 kpc, or 20 ′ × 10 ′ ). 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 [O III]. The identified shock features in both [O III] and H i 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 to 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.