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Re: Plasma



If I understand correctly, what we have established so far
is (1) that photoionization of intergalactic media (IGM) was
important in the early universe when active galactic nuclei
were common, and (2) that photoionization is insignificant
in the IGM in the deep gravitational wells of rich clusters.

How important is photoionization in IGM that is not in
either of the two categories listed above?

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


-----Original Message-----
From: Leigh Palmer [mailto:palmer@SFU.CA]
Sent: Sunday, December 14, 2003 12:17 PM
To: PHYS-L@lists.nau.edu
Subject: Re: Plasma


My example of gravitational ionization was originally specifically
concerned with the x-ray emitting gas that has been observed in some
rich clusters of galaxies. This phenomenon has been detected in the
relatively nearby Coma cluster, so it is not an early universe
phenomenon. The gas in these clusters shares the same velocity
distribution as the galaxies observed in the cluster, meaning their
temperatures are typically in the keV range. The x-ray spectra exhibit
highly ionized iron lines as well.

Peebles's discussion relates to the much cooler part of the IGM and is
supported by observations of the IGM Lyman alpha forests. These lines
are seen in absorption in the visible spectra of very distant quasars.
They represent hydrogen atoms in the intervening IGM having redshifts
of the order of Z=3. At these early times quasars (now identified as
active galactic nuclei (AGN)) were abundant and were powerful sources
of ultraviolet radiation.

Gravitational ionization must have been the important process in the
initial formation of denser objects (stars, galaxies, clusters of
galaxies, etc.) from the cooling gas fireball after recombination,
300,000 years after the Big Bang. The question of which objects, stars
or galaxies, were formed first, is still unanswered, but before their
formation it is thought that no sources of ionizing radiation were
present in the universe. Of course there is ionizing radiation in the
tail of the cosmic microwave background radiation, but that is (was) of
insufficient intensity to account for the ionization that would be seen
as the denser objects formed under gravity.

Leigh