A galaxy is a huge collection of mass drifting in the otherwise bland and mostly featureless void of intergalactic space. Gravity is a force where like attracts like – mass likes to attract more mass – so, as you’d expect, a galaxy tends to attract any stray gas, dust, and other intergalactic stuff which it can gravitationally sweep up on its travels. But you might not expect it to sweep up quite this much…
Some freshly released results from everyone’s favourite X-ray observatory found that the Milky Way (shown in the image above, alongside two of its satellite galaxies, the Large and Small Magellanic Clouds) is swathed in a huge halo of hot diffuse gas. A gas cloud so enormous that it makes the galaxy itself look tiny!
In fact, that vast sprawling gas halo stretches out for hundreds of thousands of light years in every direction, and may contain up to 60 billion solar masses of gas, superheated to millions of Kelvin. That’s immense. That big ball of gas stretches a significant fraction of the way to the Andromeda galaxy. While it may still be a lightweight compared with the estimated 1.2 trillion solar masses of the Milky Way galaxy, it’s still enough gas that if it were to all collapse and condense, it would form a rather impressive galaxy in its own right.
Of course, the mass is still an estimate. As with anything in physics, a few assumptions had to be made, but unless the composition of this gas is dramatically different to what’s expected, it’s likely a fairly good approximation of how much gas is out there. This could help us track down where those pesky missing baryons are hiding (yes, as well as dark matter, not all baryonic matter is accounted for either).
This is interesting for a whole host of reasons. For one, the intergalactic medium may be a less boring place, if vast gas clouds like this can exist elsewhere too (and there’s no reason to believe that they wouldn’t). It’s practically a galactic version of the Oort cloud. It also implies that high velocity intergalactic clouds like Smith’s Cloud aren’t just isolated patches of material, but clumps in a much larger cloud. And maybe, just maybe, a similar patch of intergalactic hydrogen gas like this may be able to resurrect a seemingly dead galaxy and give it a new burst of star formation…
(The idea of whole galaxies dying as their star formation shuts down always made me uneasy…)
Upper – NASA/CXC/M.Weiss; NASA/CXC/Ohio State/A Gupta et al
Lower – Andrew Z. Colvin/Wikimedia Commons