Fine-scale structure of the neutral ISM in M81

We present an analysis of the fine-scale 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 s⁻¹; 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.