2. WHERE IS THE HOT GAS?
There is very little [if any] diffuse emission in normal spiral
galaxies. Gas appears to be associated mostly with:
- 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).
- 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. 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 H
+ [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.