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There is very little [if any] diffuse emission in normal spiral galaxies. Gas appears to be associated mostly with:

  1. star formation. A figure from a talk by stricketalstricketal provides an excellent example and confirmation of this association. The X-ray images reveal more and more extended soft diffuse emission from hot gas as the line emission and star formation activity become more prominent (see Fig. 1).
  2. gravity. Large quantities of hot gas have been discovered in early type galaxies since Einstein observations, and are associated mostly with galaxies at the center of small groups. A long standing issue of whether the large scale gas should be associated with the galaxy or with the group is alive and unresolved as yet, but it is beyond the scope of this talk and it will be mostly [unjustly!] ignored here.

Figure 1

Figure 1. From stricketal (stricketal). Top panels: X-ray images in the 0.3-2.0 keV energy band from Chandra data for 4 galaxies from normal to actively forming stars. Lower panels: The same galaxies in the continuum-subtracted Halpha + [NII] emission. The arrow at the bottom indicates the increasing IRAS f60 / f100 ratio, which is used as a measure of the star formation (SF) intensity. The increasingly strong soft X-ray emission associated with increasing SF activity is evident.

Smaller quantities of gas have recently been found associated with spiral bulges (e.g. M31, NGC 1291) and low luminosity early type galaxies (e.g. NGC 4697), but these are still hard to quantify and characterize. If this is a property of the whole class, a new and interesting field will open up and bring new issues to study and resolve.

It is evident that the characteristics of the gas might not be the same in both cases above, considering the clearly different origins of the two phenomena. It is however likely that different systems will be dominated by only one of the two components, thereby allowing us a relatively clean study of both phenomena separately.

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