The HI distribution in the outskirts of M33 with the ALFALFA survey
  Spiral galaxies appear to be dynamical systems whose disks are still forming at the current epoch and which continue to accrete mass. The presence of extraplanar gas in spirals indicates that galactic halos can contain at least part of the material needed to fuel the star formation activity in their disks. Here we present the analysis of the ALFALFA survey data in the region of M33 aimed at searching high velocity clouds around this Local Group galaxy. We find a varied population of HI clouds with masses ranging between 4 x 10^4 and few times 10^6 M_sun. We also detect an extended HI complex at anomalous velocities, whose extragalactic nature cannot be firmly established. We estimate that the total amount of neutral hydrogen mass associated to these clouds is around 10^7 M_sun. 
M. Grossi, C. Giovanardi, E. Corbelli