Probing the Extended Gaseous Regions of M31 with Quasar Absorption Lines
  We present HST-COS spectra of ten quasars located behind M31, selected to
investigate the properties of gas associated with its extended disk and high
velocity clouds (HVCs). The sightlines have impact parameters ranging between
b= 13 kpc and 112 kpc. No absorption is detected in the four sightlines beyond
b=57 kpc. Of the six remaining sightlines, all of which lie at b<32 kpc and
within the N(HI)= 2E18 cm^{-2} boundary of the HI disk of M31, we detect
low-ionization absorption at M31 velocities along four of them (three of which
include MgII absorption). We also detect MgII absorption from an HVC. We find
that along sightlines where both are detected, the velocity location of the
low-ion gas tracks the peak in 21 cm emission. High-ionization absorption is
detected along the three inner sightlines, but not along the three outer
sightlines, for which CIV data exist. As inferred from 21 cm emission line
maps, only one sightline may have a damped Ly-alpha system. This sightline has
b= 17.5 kpc, and we detect both low- and high-ion absorption lines associated
with it.
  The impact parameters through M31 are similar to the impact parameters of
galaxies identified with MgII absorbers at redshifts 0.1<z<1.0 in a 2011 study
by Rao et al. However, the M31 MgII2796 rest equivalent width values are
significantly smaller. In comparison, moderate-to-strong MgII absorption from
Milky Way gas is detected along all ten sightlines. Thus, this study indicates
that M31 does not present itself as an absorbing galaxy which is typical of
higher-redshift galaxies inferred to give rise to moderate-strength quasar
absorption lines. M31 also appears not to possess an extensive large gaseous
cross section, at least not along the direction of its major axis. (Abridged.)
