This Week's Finds in Mathematical Physics (Week 238)

Some updates on the Bullet Cluster / dark matter saga.
You can now see a picture of the Bullet Cluster he

http://math.ucr.edu/home/baez/week238.html

I've also updated a bunch of information and fixed some mistakes
in the initial version of "week238", thanks in part to Matt Owers.

Markevitch and company have been studying the "Bullet Cluster", a
a bunch of galaxies that has a small bullet-shaped subcluster zipping
away from the center at 4,500 kilometers per second.

It seems that one of the rapidly moving galaxies in this subcluster
has hit a bystander galaxy - I'm not sure, but a high-speed collision
of galaxies occurred.

In fact, the whole subcluster hit another subcluster!

So, the picture is a picture of colliding bunches of galaxies.
The individual galaxies in these bunches mainly shoot right past
each other - but the intergalactic gas in one bunch is hitting
the gas in the other, and getting so hot it emits X-rays.

So, dark matter is seeming more and more real. In fact, last year
folks found evidence for "ghost galaxies" made mainly of dark matter
and cold hydrogen, with very few stars:

6) PPARC, New evidence for a dark matter galaxy,
http://www.interactions.org/cms/?pid=1023641

Apparently the consensus is now that this ghost, VIRGOHI 21,
is hydrogen stripped off from a galaxy by the "wind" it felt
as it fell into the Virgo Cluster. This effect is called
"ram pressure stripping" - the gas of a galaxy can be stripped
off if the galaxy is moving rapidly through a cluster, due
to interaction with the gas in the cluster.