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Re: Plasma



I'm glad that someone else is following up on this one.
I have a conceptual conflict:
1) On a starry, night, looking to the Milky Way as
representative of the medium, that does indeed seem
an unpromising environment for energetic interaction.
2) I have the received factoid, that starlight represents
a temperature of several thousand degrees. While
probability of interactions may be low, they would
seem to have the requisite energy?

Brian W

At 05:03 PM 12/5/2003, you wrote:
I understand Leigh's response to three of the four
questions I asked, but I still do not understand why
photoionization is not a significant process.

In his discussion of ionization of intergalactic media
(IGM), Peebles (1993) states:

"The transitions to be considered are the radiative ones,

H + Y <-> p + e,

and electron collisions,

H + e -> p + e + e."

Only the recombination channel of the radiative transition
is common to both Leigh's analysis and Peebles (1993).

Peebles (1993) further states:

"If the temperature were well below 100 eV,
the dominant source of ionization could be
electromagnetic radiation."

Is Leigh's analysis limited to IGM in the deep gravity
wells of rich clusters? Is photoionization important
in most IGM, but not in IGM in rich clusters?

Daniel Crowe
Oklahoma School of Science and Mathematics
Ardmore Regional Center
dcrowe@sotc.org

Reference:

Peebles, P.J.E. (1993) Principles of Physical Cosmology.
Princeton, NJ: Princeton University Press, pp. 554-559.


-----Original Message-----
=46rom: Leigh Palmer [mailto:palmer@SFU.CA]
Sent: Thursday, December 04, 2003 4:52 PM
To: PHYS-L@lists.nau.edu
Subject: Re: Plasma

Dan Crowe asks:

> Leigh has presented very interesting arguments,
> but I have a few questions and comments.
>
> 1. Why is the ionization channel Y + H -> p + e
> ignored? Photons of appropriate energies are
> emitted (1) during recombination and (2) in
> nearby galaxies.

More distant galaxies can be seen though nearby clusters (e.g. the Co=
ma
cluster). The cluster gas is optically thin; almost all photons are
transmitted, so the process is not important, and each photon only ge=
ts
one trip through. The particles, on the other hand, stay in the
cluster's gravitational well for billions of years even if they under=
go
few interactions in a trip through.

I think Dan originally suggested that ultraviolet radiation from the
galaxies in the cluster might be responsible for ionizations. The fac=
t
is that galaxies are just not bright enough to matter. To get an idea
of how bright galaxies are, go outside on a dark night and look at th=
e
Milky Way. The galaxies in a cluster have comparable surface
brightness, but they do not cover as much of the sky when viewed from=
a
point in the gas. You might also try looking at the nearby galaxy M31
in Andromeda.

<snip>

Leigh


Brian Whatcott Altus OK Eureka!