Re: Plasma

[Leigh Palmer <palmer@SFU.CA>]

Daniel Crowe writes:

> In intergalactic HII regions, the hydrogen atoms are ionized
> by ultraviolet radiation, not gravitational interactions.
> The ionizing UV is usually emitted in nearby galaxies.

The majority of the radiation seen from intragalactic cluster gas is
bremsstrahlung in the x-ray region. Andy Fabian at Cambridge told me
that the high temperature (much higher than could be sustained by
ultraviolet (uv) radiation) of this gas is what one should expect for
gravitational interactions, and I did a calculation at that time (1995)
that convinced me he was right. There are some lines from highly
ionized iron seen in these spectra that confirm the temperatures
inferred from the x-rays. (They also demonstrate that at least some of
this gas has been processed through supernovae.)

I think galaxies are too distant from and too weakly emitting in the uv
to provide significant heating to the intragalactic cluster gas. It is
the case, of course, that HII regions in *galactic clusters* are
ionized by starlight uv. These HII regions are luminous and so can be
seen in quite distant external galaxies. The unfortunately named
galactic clusters are, of course, clusters of stars within one galaxy,
typically accreting and evolving within a dense region of the
interstellar medium. The Orion nebula is an example of such a region in
our own Galaxy.

I tried to find a suitable reference to the gravitational story in
Carroll and Ostlie, and by Google, but I didn't look very hard. I came
up with, inter alia,
http://nedwww.ipac.caltech.edu/level5/Fabian3/Fabian2.html.
If someone finds a better reference please share it with the group.

Leigh