Molecular gas in the inner 0.7kpc-radius ring of M31

TL;DR

This study reveals complex molecular and ionised gas kinematics in M31's central region, linked to a past collision with M32, resulting in tilted rings and unusual gas motions, challenging the presence of a weak bar.

Contribution

It provides new insights into the gas dynamics and structure of M31's inner region, highlighting the impact of a past collision and clarifying the absence of a weak bar.

Findings

Detection of large line splittings up to 260 km/s in molecular gas.

Identification of tilted inner and ring-like gas structures.

Absence of evidence for a weak bar in the gas component.

Abstract

The study of the gas kinematic in the central 1.5kpc x 1.5kpc region of M31 has revealed several surprises. The starting point of this investigation was the detection at the IRAM-30m telescope of molecular gas with very large line splittings up to 260km/s within the beam (40 pc). In this region, which is known for its low gas content, we also detect an ionised gas outflow in the circumnuclear region (within 75pc from the centre) extending to the whole area in X-ray. Relying on atomic, ionised, and molecular gas, we account for most observables with a scenario that assumes that a few hundreds Myr ago, M31 underwent a frontal collision with M32, which triggered some star-formation activity in the centre, and this collision explains the special configuration of M31 with two rings observed at 0.7kpc and 10kpc. The inner disc (whose rotation is detected in HI and ionised gas ([NII])) has thus been tilted (inclination: 43deg, PA: 70deg) with respect to the main disc (inclination: 77deg, PA: 35deg). One of the CO velocity components is compatible with this inner disc, while the second one comes from a tilted ring-like material with 40deg inclination and PA=-35deg. The relic star formation estimated by previous works to have occurred more than 100Myr ago could have been triggered by the collision and could be linked to the outflow detected in the ionised gas. Last, we demonstrate that the amplitude of the line splittings detected in CO centred on the systemic velocity with a relatively high spatial resolution (40pc) cannot be accounted for by a possible weak bar that is roughly aligned along the minor axis. Although M31 has a triaxial bulge, there are no bar indicators in the gas component (photometry, no strong skewness of the isovelocities, etc.).