The Milky Way’s hot halo

It’s easy to think about our Milky Way galaxy all by itself out in space, surrounded by a halo of globular star clusters, some small satellite galaxies, and that nothing else except for its neighbors millions of light-years away.

But the reality is likely quite different. Our home galaxy may be surrounded by an extended halo more than three times he diameter of our Milky Way, like this:

That’s our home galaxy at the center, and those two puffs to the lower left are the Large and Small Magellanic Clouds, which are two irregular-shaped satellite galaxies that orbit the Milky Way. But surrounding all three is a halo of hot gas that extends for hundreds of thousands of light years, and as hot as 1-2 million kelvin!

How they do that?

A team of astronomers used NASA’s Chandra X-Ray Observatory to observe eight bright X-Ray sources hundreds of millions of light-years away; much farther away than the extent of our own Local Group of galaxies. It turns out that the some of the X-Rays from these distant sources were absorbed by by ionized oxygen gas surrounding our galaxy.

The fact that the oxygen is ionized means that the gas itself must be very hot – between 1 million and 2.5 million kelvin. And the fact that this absorption is the same no matter which distant X-Ray source we look at means there is quite a lot of this gas surrounding the Milky Way. How much? Perhaps as much as 10 billion to 60 billion suns worth. That’s a lot of hot gas!

The hot halo (not to be confused with the warm halo)

Astronomers already understand there is a halo of cooler (anywhere from 100,000 to 1 million kelvin) gas surrounding the Milky Way. But these observations seem to imply there is a lot more hot gas making up a much larger halo, indeed.

Missing baryons?

Now to be clear, these results are not yet unconfirmed. It could be that no such halo exists, and the observations can be explained by some other phenomenon. That’s ok, that’s how science works. But if it turns out that the halo is real, it could help explain the Milky Way’s “missing baryon” problem.

Baryons are ordinary matter – things like protons, neutrons, elections – in other words, ordinary everyday matter. When we look at very distant galaxies, we’re looking back in time to see how they looked when the universe was one-sixth its current age. But when we look around in our own Milky Way and nearby galaxies, it turns out there is only about half as much of this baryonic matter visible.

This halo, if its real, would contain about enough mass to account for the “missing baryons.” They wouldn’t be missing at all, they’d just be in a very extended and diffuse cloud surrounding the galaxy!

And that’s a good thing, because the universe is mostly filled with weird and mysterious stuff like Dark Matter and Dark Energy as it is. Its nice to be able to account for some of it once in a while.