Formation of a giant HI bridge between M31 and M33 from their tidal   interaction
  Recent observations have discovered a giant HI bridge that appears to connect between the outer halo regions of M31 and M33. We propose that this HI bridge can be formed as a result of the past interaction between M31 and M33 based on test particle simulations with different orbits of M31 and M33 for the last ~ 9 Gyr. We show that strong tidal interaction between M31 and M33 about 4-8 Gyr ago can strip HI gas from M33 to form HI streams around M31 which can be observed as a HI bridge if they are projected onto the sky. We show that the number fraction of models reproducing well the observed HI distribution of the bridge is only ~0.01% (i.e., ~10 among ~10^5 models) and thus suggest that the observed structure and kinematics of the HI bridge can give some constraints on the past orbits of M31 and M33. We suggest that the observed outer HI warp in M33 could be fossil evidence for the past M31-M33 interaction. We also suggest that some of high velocity clouds (HVCs) recently found in M31 could be HI gas originating from M33. We briefly discuss other possible scenarios for the formation of the HI bridge. 
Kenji Bekki