Discrete clouds of neutral gas between the galaxies M31 and M33
  Spiral galaxies must acquire gas to maintain their observed level of star
formation beyond the next few billion years (Leroy et al. 2008). A source of
this material may be the gas that resides between galaxies, but our
understanding of the state and distribution of this gas is incomplete (Shull et
al. 2012). Radio observations (Braun & Thilker 2004) of the Local Group of
galaxies have revealed hydrogen gas extending from the disk of the galaxy M31
at least halfway to M33. This feature has been interpreted to be the neutral
component of a condensing intergalactic filament (Dav\'e et al. 2001) which
would be able to fuel star formation in M31 and M33, but simulations suggest
that such a feature could also result from an interaction between both galaxies
within the past few billion years (Bekki 2008). Here we report radio
observations showing that about 50 per cent percent of this gas is composed of
clouds, while the rest is distributed in an extended, diffuse component. The
clouds have velocities comparable to those of M31 and M33, and have properties
suggesting they are unrelated to other Local Group objects. We conclude that
the clouds are likely to be transient condensations of gas embedded in an
intergalactic filament and are therefore a potential source of fuel for future
star formation in M31 and M33.
