Bagetakos, IoannisBrinks, EliasWalter, Fabiande Blok, Willem2015-12-07August 14-0521863465http://hdl.handle.net/1885/21272We present an analysis of the finescale structure of the neutral ISM as traced via the 21-cm line of atomic hydrogen (HI) in the nearby galaxy M 81. The data show a stunning amount of detail in the form of 330 expanding shells and holes in the neutral ISM of M 81. A comparison with similar structures found in two other spirals and two dwarf galaxies (M 31, M 33, IC 2574 and Holmberg II) reveals that the ISM in M 81 shares a lot of similarities with the two spirals, whereas the structure of its ISM is different to that in dwarf galaxies. The sizes of the HI holes in M 81 range from 80 pc (close to the resolution limit) to 600 pc; the expansion velocities can reach 20 km s1; estimated ages are 2.5 to 35 Myrs and the energies involved range from 1050 to 3.5 x 1052 ergs. The amount of neutral gas involved is of order 104 to 106 solar masses.Keywords: Galaxies: individual (M 81); Galaxies: ISM; ISM: structure; Radio lines: ISM200710.1017/S17439213070018222015-12-07