Milky Way Is Surrounded By Halo of Hot Gas 121
New submitter kelk1 writes "If the size and mass of this gas halo is confirmed, it also could be an explanation for what is known as the 'missing baryon' problem for the galaxy [...] a census of the baryons present in stars and gas in our galaxy and nearby galaxies shows at least half the baryons are unaccounted for [...] Although there are uncertainties, the work by Gupta and colleagues provides the best evidence yet that the galaxy's missing baryons have been hiding in a halo of million-kelvin gas that envelopes the galaxy."
Re:1,000,000 K ?!? (Score:5, Informative)
Depends on how dense it is. If you immerse yourself in water at 100C (boiling point for you imperial scumdogs :) you won't last long at all, but in dry air at 100C you can survive for substantially longer. If the gas was so sparse that you might only hit a molecule every few seconds or so then the temperature might not matter so much. The article hints that the density is low "The estimated density of this halo is so low that similar halos around other galaxies would have escaped detection." but that doesn't really help in absolute terms.
(or maybe you're making a joke... i don't get the reference in the first line you posted)
Re:How does something so un-dense... (Score:5, Informative)
retain it's 1,000,000K for 14,000,000 years?
First, that's 14,000,000,000, not 14 million.
The key is how undense it is. When a physicist talks about "temperature" in this context it's just short-hand for "average velocity"... it doesn't necessarily imply thermal equilibrium, even. So 1e6K means a high average velocity. Now, if it were a dense gas there might be collisions that would do things like excite electrons into higher states, which would then decay by emitting photons (light), and so the gas would lose thermal-kinetic energy over time.
In a sufficiently diffuse gas, loss processes like this are very slow because the chances of collision are very slow, so it can stay "hot" (that is, have a high average velocity) for a long, long time.
Re:How does something so un-dense... (Score:5, Informative)
Temperature (in Kelvin) is actually more useful in astrophysics and thermodynamics of plasmas. It wraps up a bunch of messy real world constants into one number, and also neatly describes the behavior of the volume of gas as a whole, rather than forcing the analyst to perform a lot of messy integrating and averaging of distributions of actual velocities in three dimensions.
Think about it this way. No one is really interested in how fast a specific particle is moving. They're more interested in how the Thermal Energy of the gas couples with other systems.
A galactic halo would be coupled very, very, (very^18) poorly with other systems, at least conductively. And probably even worse convectively, given the scales involved. Radiatively, I don't know near enough about the behavior of these particles to talk about why, but if it's stayed that hot for the life of the universe, effectivelt, then apparently its either not coupled to another system, coupled far more strongly to itself than anything else, or somehow not stimulated to emit blackbody radiation... or all three of the above.
Re:Wait (Score:5, Informative)
Hold on a second... so they just discovered the Galaxy is surrounded by gas that's the same temperature as the surface of the sun, and is 300,000 lightyears across... possibly extending far into other galaxies... I'm going to take a wild stab here and say that, if that's true it probably pervades the entire universe... Isn't this the biggest scientific discovery in the past decade? What effect does this have on Dark Matter, Dark Energy, etc... etc...
It has been known for a long time that the intergalactic medium is hot enough to be ionized [ua.edu]. That part is not news. The thing that's news is that the hot gas makes it possible to account for the baryons in the Milky Way halo, which were previously undetected.
Re:How does something so un-dense... (Score:3, Informative)
If the collision rate weren't high enough to excite electrons into higher states, it wouldn't be radiating X-rays, which is how Chandra detects the gas.
Chandra isn't seeing X-ray emissions from the gas, it's seeing X-rays being absorbed by the gas. Specifically, observing 8 X-ray sources hundreds of millions of light-years beyond the gas, it was discovered that some of the X-rays from those sources were being absorbed, and it was possible to deduce the temperature of the absorbing gas.
Re:Warm Gas (Score:4, Informative)
It's relative warmth in the 100,000 K and up club it's rather difficult to keep track because once you've boiled away Rhenium, there's not much meaning in additional units of heat.
https://en.wikipedia.org/wiki/Rhenium [wikipedia.org] :)
The more you know...