# Sensitive 21cm Observations of Neutral Hydrogen in the Local Group near M31 [GA]

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 total HI mass of all nine clouds is $1.4 \times 10^6$ M$_{\odot}$, 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. They may, however, be natural features of M31’s massive circumgalactic medium, perhaps triggered by the passage of a satellite, or related to the Local Group planes of satellites. 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.

S. Wolfe, F. Lockman and D. Pisano
Tue, 21 Jul 15
9/74

Comments: 28 pages, 10 figures, 5 tables, submitted to the Astrophysical Journal