H I Kinematics and Dynamics of Messier 31

Abstract

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 at 340 km s −1 , a dip at 202 km s −1 around 4 kpc, two distinct flat parts at 264 km s −1 and 230 km s −1 and an increase to 275 km s −1 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 of 74 • , while the outer regions appear more inclined. Mass distribution models by ΛCDM Navarro-Frenk-White, 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) × 10 11 M ⊙ . The dark matter component is almost 4 times more massive than the baryonic mass inside this radius. A total mass of 1.0 × 10 12 M ⊙ 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.

A wide-field H I mosaic of Messier 31

“…Secondly, the inclination of the outermost fitted radius is higher for our model compared to what Chemin et al (2009) find, while the position angle is slightly lower. We list below three main differences in the two fitting methods which can help explaining the variations in the resulting tilted ring model parameters.…”

“…As we mentioned in the Introduction, only recently a 21-cm survey made with a synthesis telescope has been used to model the warp and the rotation curve of M 31 simultaneously (Chemin et al 2009). The conclusion is similar to ours, namely that the warp of M 31 is such that the outer disk is at higher inclination and lower position angle than the inner disk.…”

“…However a close inspection reveals some differences. First, the average value of the disk inclination between 6 and 27 kpc that Chemin et al (2009) …”

“…Carignan et al 2006) analyze the extended rotation curve of M 31 using only HI data along the direction of the optical major axis, without considering the possibility of a warped disk. Only very recently Chemin et al (2009) use deep 21-cm survey of the M 31 based on high resolution synthesis observations to model the warp and the rotation curve simultaneously.…”

The Stellar Mass of M31 as inferred by the Andromeda Optical & Infrared Disk Survey

“…The DiskFit code (Spekkens & Sellwood 2007) allows us to correct the Hi velocity fields of Saglia et al (2010) and Chemin et al (2009) for non-circular motions in M31's central parts. The success of this mass model will be determined by the stability of a dynamical N-body realization.…”