HI Holes in the Interstellar Medium of Messier 31

Abstract

Observations in recent years of our Galaxy in HI and in UV and soft X-rays have prompted various theoretical models which describe the structure of the interstellar medium (IM). It has been realized that supernovae and stellar winds from massive stars play a crucial role in determining the energy balance of the IM. Cox and Smith (1974) conclude that there might exist an extensive network of low-density gas at coronal temperatures. In a more detailed study McKee and Ostriker (1977) describe the energy balance between three phases of the IM, the hot (~10⁶ K), warm (~10⁴ K), and cool (~10² K) phase, which are in pressure equilibrium. A crucial parameter in their model is which fraction of the IM is occupied by each of these phases. Observationally there is controversy about these filling factors and it is not yet clear if, for example, the filling factor for the hot phase is as large as 70–80%, as suggested by McKee and Ostriker, the basic problem being that we are restricted in our view of our Galaxy by our position in the disk.