HI kinematics and dynamics of Messier 31

TL;DR

This study provides a detailed analysis of M31's HI distribution, revealing complex kinematics, a warped disc, and new structures, while evaluating dark matter models and their limitations in explaining the galaxy's rotation curve.

Contribution

It offers the first high-resolution deep HI survey of M31, identifies new HI structures, and assesses dark matter halo models against observed rotation curves.

Findings

Rotation curve extends to 38 kpc with multiple features.

Discovery of new HI structures like spurs and an external arm.

Dark matter models fail to perfectly fit the rotation curve.

Abstract

[Abridged] We present a new deep 21-cm survey of the Andromeda galaxy, based on high resolution observations performed with the Synthesis Telescope and the 26-m antenna at DRAO. The HI distribution and kinematics of the disc are analyzed and basic dynamical properties are given. The rotation curve is measured out to 38 kpc, showing a nuclear peak, a dip around 4 kpc, two distinct flat parts and an increase in the outermost regions. Except for the innermost regions, the axisymmetry of the gas rotation is very good. A very strong warp of the HI disc is evidenced. The central regions appear less inclined than the average disc inclination, while the outer regions appear more inclined. Mass distribution models by LCDM NFW, Einasto or pseudo-isothermal dark matter halos with baryonic components are presented. They fail to reproduce the exact shape of the rotation curve. No significant differences are measured between the various shapes of halo. The dynamical mass of M31 enclosed within a radius of 38 kpc is (4.7 +/- 0.5) x 10^11 Msol. The dark matter component is almost 4 times more massive than the baryonic mass inside this radius. A total mass of 1.0 x 10^12 Msol is derived inside the virial radius. New HI structures are discovered in the datacube, like the detection of up to five HI components per spectrum, which is very rarely seen in other galaxies. The most remarkable new HI structures are thin HI spurs and an external arm in the disc outskirts. A relationship between these spurs and outer stellar clumps is evidenced. The external arm is 32 kpc long, lies on the far side of the galaxy and has no obvious counterpart on the other side of the galaxy. Its kinematics clearly differs from the outer adjacent disc. Both these HI perturbations could result from tidal interactions with galaxy companions.