Re: Plasma

[Leigh Palmer <palmer@SFU.CA>]

Ludwik Kowalski wrote:

> On 11/25/03 Leigh wrote:
>
> > . . . What I am about to show is that gravity in clusters
> > of galaxies is easily strong enough to produce the very
> > hot plasmas that are observed in intergalactic space.
>
> How to understand such gravitational ionization?
> Does it happen because "clouds of particles" collide
> near the center of attraction? I am imagining a tiny
> object of a very large mass near the center of
> collision. Is this a correct mental image?

Ludwik, as always, you ask the best questions. I must apologize for
being somewhat obscure. I was really trying to evoke a very different
question, but your question demonstrates that I didn't emphasize
strongly enough that this is simply a (Newtonian) gravitational
problem. I'll give you my very different picture.

The atoms in cluster gas are in gravitational orbits within the
cluster. Their mean free paths in the plasma are comparable to or
longer than the mean spacing of the galaxies. The plasma is so tenuous
that gravitational interaction of its particles with the rest of the
cluster, which is always present for all particles, is feeding energy
into the atoms and ions of the system as fast as the rare encounters
("collisions") can radiate it away as bremsstrahlung and, far less
often, as recombination radiation. As the particles lose kinetic energy
through very brief, very infrequent electromagnetic interaction they
are reenergized by very long duration gravitational interaction with
all of the matter in the cluster.

There is no large mass near the center of the cluster; the intracluster
gas is itself a very large mass with some luminous density fluctuations
(galaxies) embedded in it.

Remember, the original problem faced by optical astronomers was that
the galaxies are moving too fast to be bound gravitationally solely by
the other galaxies in the cluster. There must be some "missing mass" to
account for the depth of the potential well to which they are bound.
The mass is no longer missing, nor is it any longer "dark matter",
because now we have x-ray eyes with which we can see it!

I realize that this view of clusters of galaxies will be a jarring
reversal for some of you. It was for me, I know, and I didn't
understand it myself until 1994. In the words of Burl Ives:

   As you go through life
   Make this your goal:
   Watch the doughnut,
   not the hole!

Oh, yes, what was the question I was hoping to evoke? Well, when I did
my calculation, I merely found the kinetic energy of an atom moving
with a velocity comparable to the velocities of the galaxies in the
cluster. I was hoping someone would ask "Why is that a reasonable thing
to do?"

Since the interaction that binds these systems of particles is
gravitational, the force on any given particle is proportional to the
particle's mass, and the consequential acceleration is inversely
proportional to the particle's mass. Who discovered that? Galileo!

Leigh