The Neutral Hydrogen Bridge between M31 and M33
  The Green Bank Telescope has been used to search for 21cm HI emission over a
large area between the galaxies M31 and M33 in an attempt to confirm at 9.1
arcmin angular resolution the detection by Braun and Thilker (2004) of a very
extensive neutral gas "bridge" between the two systems at the level NHI
approximately 10^{17} cm^{-2}. We detect HI emission at several locations up to
120 kpc in projected distance from M31, at least half the distance to M33, with
velocities similar to that of the galaxies, confirming the essence of the Braun
and Thilker discovery. The HI does not appear to be associated with the
extraplanar high-velocity clouds of either galaxy. In two places we measure NHI
> 3 x 10^{18} cm^{-2}, indicative of concentrations of HI with ~10^5 solar
masses on scales <2 kpc, but over most of the field we have only 5sigma upper
limits of NHI <= 1.4 x 10^{18} cm^{-2}. In very deep measurements in two
directions HI lines were detected at a few 10^{17} cm^{-2}. The absence of
emission at another location to a 5sigma limit NHI <= 1.5 x 10^{17} cm^{-2}
suggests that the HI bridge is either patchy or confined to within ~125 kpc of
M31. The measurements also cover two of M31's dwarf galaxies, And II and And
XV, but in neither case is there evidence for associated HI at the 5sigma level
of 1.4 x 10^4 solar masses of HI for And II, and 9.3 x 10^3 solar masses for
And XV.
