Formation of a giant Hi bridge between M31 and M33 from their tidal interaction

Recent observations have discovered a giant H i bridge that appears to connect between the outer halo regions of M31 and M33. We propose that this H i 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 H i gas from M33 to form H i streams around M31 which can be observed as a H i bridge if they are projected on to the sky. We show that the number fraction of models reproducing well the observed H i distribution of the bridge is only ∼0.01 per cent (i.e. ∼10 among ∼105 models), and thus suggest that the observed structure and kinematics of the H i bridge can give some constraints on the past orbits of M31 and M33. We suggest that the observed outer H i 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 H i gas originating from M33. We briefly discuss other possible scenarios for the formation of the H i bridge.