Sensitive 21cm Observations of Neutral Hydrogen in the Local Group near M31

TL;DR

This study used the Green Bank Telescope to detect and analyze nine discrete neutral hydrogen clouds near M31, revealing their properties, velocities, and potential relation to the galaxy's high-velocity cloud population, with implications for understanding local group dynamics.

Contribution

First detailed sensitive 21cm HI observations of discrete clouds around M31, characterizing their properties and kinematics, and assessing their relation to the galaxy's high-velocity cloud system.

Findings

Nine HI clouds detected with typical size of a few kpc and mass of 10^5 M_sun.

Total HI mass of clouds is approximately 1.4 million solar masses.

Clouds are unlikely to be gravitationally bound or part of the satellite system due to low HI to virial mass ratio.

Abstract

Very sensitive 21cm HI measurements have been made at several locations around the Local Group galaxy M31 using the Green Bank Telescope (GBT) at an angular resolution of 9.1', with a 5$\sigma$ detection level of $\rm{N_{HI} = 3.9 \times 10^{17}~cm^{-2}}$ for a 30 $\rm{km~s^{-1}}$ line. Most of the HI in a 12 square degree area almost equidistant between M31 and M33 is contained in nine discrete clouds that have a typical size of a few kpc and HI mass of $10^5$ M$_{\odot}$. Their velocities in the Local Group Standard of Rest lie between -100 and +40 $\rm{km~s^{-1}}$, comparable to the systemic velocities of M31 and M33. The clouds appear to be isolated kinematically and spatially from each other. The total HI mass of all nine clouds is $1.4 \times 10^6$ M$_{\odot}$ for an adopted distance of 800 kpc with perhaps another $0.2 \times 10^6$ M$_{\odot}$ in smaller clouds or more diffuse emission. The HI mass of each cloud is typically three orders of magnitude less than the dynamical (virial) mass needed to bind the cloud gravitationally. Although they have the size and HI mass of dwarf galaxies, the clouds are unlikely to be part of the satellite system of the Local Group as they lack stars. To the north of M31, sensitive HI measurements on a coarse grid find emission that may be associated with an extension of the M31 high-velocity cloud population to projected distances of $\sim 100$ kpc. An extension of the M31 high-velocity cloud population at a similar distance to the south, toward M33, is not observed.