Clouds of hot gas

If you are standing in the waterfall, how can you tell whether the sky is foggy? There's too much spray in your eyes and mist floating around you to see clearly. But if you see sharp-edged shadows, the sky is probably clear.

Gupta et al used the Chandra X-Ray Observatory data to do something just as difficult. Here in our Solar System we're in not quite the heart but certainly the swirl of a large galaxy, surrounded by hot gas and cold dust and magnetic fields every which way. Look outward, and see hot gas wherever you look.

They did something clever. They were looking at absorption lines of oxygen in X-rays emitted from distant galaxies. Chandra is accurate enough about direction that they can look only at the kinds of galaxies that will be bright in X-rays but not too wild and wooly, and be sure they're not picking up too much stuff from galaxies nearby. The X-rays are emitted with a broad spectrum. Some of the X-rays are absorbed by oxygen in distinctive wavelengths: some absorbed at the distant galaxy and some absorbed nearby.

Here's the cool trick. Galaxies are moving away from each other, so the absorbing gas is moving too. They picked galaxies far enough away that the speed at which we are moving apart is great enough to make a large red-shift in the light spectrum. Therefore the gaps in the X-ray spectrum due to absorption by oxygen in the distant galaxies are red-shifted to longer wavelengths. The gaps due to absorption by local (our galaxy's halo) oxygen are where they ought to be.

The researchers can tell the difference, and conclude that there's a lot of local (and quite hot! 1,000,000 degrees K) oxygen in a kind of cloud around our galaxy.

If the proportions of hydrogen and oxygen are like those elsewhere in the galaxy (ie. lots of hydrogen and only a little bit of oxygen), then the cloud around our galaxy has to include an awful lot of hydrogen we can't see with this method. In fact, there should be as much mass in the cloud as there is in the galaxy itself!

If this is correct, then one annoying problem with models of big bang evolution may be solved: the models predict about twice as much baryonic (ordinary) matter as we seem to see in the glowing galaxies.

If the cloud is that large and that hot, it should have some interesting effects on galactic magnetic fields. I'm not sure how easy it will be to see the hydrogen.

I look forward to hearing more.