Re: Plasma

[Leigh Palmer <palmer@SFU.CA>]

Dan Crowe 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.

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.

Brian Whatcott asked:

> 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?

I'm afraid that received factoid refers to the color temperature of
starlight. There is ultraviolet radiation in starlight of sufficient
energy to cause ionization. It is insufficiently intense and of too low
temperature to account for the ionization seen in the x-ray emitting
cluster gas.

Leigh