Our Dark Materials


John Baez has some information/speculation on the discovery of hard evidence of dark matter on This week's finds number 238. A couple of clusters of galaxies seems to have collided about 4 billion light-years away. The x-ray image above shows a smaller cluster moving to the right trailing a shock wave.


Baez's explanation:
Markevitch and company have been studying the "Bullet Cluster", a bunch of galaxies that has a small bullet-shaped sub-cluster zipping away from the center at 4,500 kilometers per second. Here's a picture of it from the above paper:


To help you understand this picture a bit: the official name of the Bullet Cluster is 1E0657-56. The "exposure" for this X-ray photograph taken by Chanda was apparently 0.5 million seconds - 140 hours! The distance scale shown, 0.5 megaparsecs, is about 1.6 million light years. The cluster itself has a redshift z = 0.3, meaning its light has wavelengths stretched by a factor of 1.3. Under currently popular ideas on cosmology, this means it's roughly 4 billion light years away.

Anyway, what are we seeing here?

You can see rapidly moving galaxy cluster with a shock wave trailing behind it. It seems to have hit another cluster at high speed. When this kind of thing happens, the gas in the clusters is what actually collides - the individual galaxies are too sparse to hit very often. And when the gas collides, it gets hot. In this case, it heated up to about 160 million degrees and started emitting X-rays like mad! The picture shows these X-rays. This may be hottest known galactic cluster.

That's fun. But that's not enough reason to call a press conference. The cool part is not the crashing of gas against gas. The cool part is that the dark matter in the clusters was unstopped - it kept right on going!

How do people know this? Simple. Folks can see the gravity of the dark matter bending the light from more distant galaxies! It's called "gravitational lensing". Here are the mass density contours, as seen by this effect.

To see the other picture you will have to go to John's site.

He is part of a cool new physics-math group blog, too called The n-Category Café.

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