Million Kelvin gas clouds? Really?

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Just read an article on the internet that a giant baryonic cloud surrounds the milky way with a temperature of 1 - 2.5 million Kelvin.

http://iopscience.iop.org/2041-8205/756/1/L8?fromSearchPage=true

Now my question is this: how on earth is it possible for such a cloud to stay that hot? Would it not cool off in *seconds* by radiation? I wouldn't expect a thin distributed could of particles to keep a temperature of million K for a millisecond let alone billions of years.

What am I missing?

A popsci reference to the article may be found here for those without access to the journal.
http://www.theregister.co.uk/2012/09/25/giant_gas_cloud_envelopes_milky_way/
 

Answers and Replies

  • #2
Simon Bridge
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Temperature is a measure of the mean speed of the particles - how fast would the baryons have to be travelling to stay bound at that kind of distance? How would they lose energy? (I mean - what what happens when something loses energy by "radiation"?) What would happen to their speed and mean "orbit" radius?
 
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Now my question is this: how on earth is it possible for such a cloud to stay that hot?
It's very thin gas and very thin gases take a very long time to cool

Would it not cool off in *seconds* by radiation? I wouldn't expect a thin distributed could of particles to keep a temperature of million K for a millisecond let alone billions of years.
What happens with thin gases is that they can only get rid of their energy through certain atomic line transitions so it's possible to heat them up to very high temperatures and keep them at those temperatures because they are restricted in the radiation that they can put out.

Now most gases that we are familiar with cool off very quickly because the atoms collide with each other and that redistributes energy. However, with very, very thin gas, it can be a very long time before one atom encounters another atom so they can't redistribute energy that way.

This is very common. The earth's ionosphere gets heated to some very high temperatures because it's so thin and the gas doesn't have a way of redistributing energy.

http://en.wikipedia.org/wiki/Ionosphere
 
  • #4
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Temperature is a measure of the mean speed of the particles - how fast would the baryons have to be travelling to stay bound at that kind of distance? How would they lose energy? (I mean - what what happens when something loses energy by "radiation"?) What would happen to their speed and mean "orbit" radius?
Ok, so a bunch of relativistic protons and neutrons sailing about without colliding is considered extremely hot by virtue of their kinetic energy. And they don't radiate since they're not being accelerated by one another.

Does this mean they have an emissivity of zero? I know it's a special case but I'm trying to understand if any of the laws of black body radiation apply. I don't think they do.

So then why don't we consider the rocks in the asteroid belt to be at billions of kelvin? Surely the kinetic energy of the rocks and dust there far exceeds that of these baryons surrounding the galaxy?
 
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Temperature is a measure of the mean speed of the particles
It's a bit more complicated than that. If you have a dense gas, then the gas quickly settle down to a special distribution of velocities. If you put two dense gases next to each other, the gas will settle down at this distribution. This is called "thermalization."

What happens with thick gases is that things thermalize very quickly.

Thin gases take a long time to thermalize.
 
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Ok, so a bunch of relativistic protons and neutrons sailing about without colliding is considered extremely hot by virtue of their kinetic energy.
It's not kinetic energy, but rather the distribution of particles. If you have a lot of interacting particles, they'll quickly settle down to a special distribution which gets associated with a particular temperature.

If you have a million protons, and you add in one proton that is moving at close to the speed of light, that proton will end up colliding and slowing down.

So then why don't we consider the rocks in the asteroid belt to be at billions of kelvin? Surely the kinetic energy of the rocks and dust there far exceeds that of these baryons surrounding the galaxy?
Temperature is something that we using to generalize for the behavior of atoms and molecules. If you have a rock with a lot of atoms and they are all moving in the same direction, then you can't use the concept of temperature. That rock burns up in the atmosphere and then the atoms are moving in random directions, then you can start using the concept of temperature.
 
  • #7
Simon Bridge
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What twofish-quant said :)
 
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Thumbs-up. Thanks.
 
  • #9
Dotini
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Last edited:
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Dotini
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  • #11
Chronos
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This phenomenon appear similar to the sun's coronal temperature.
 
  • #12
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This phenomenon appear similar to the sun's coronal temperature.
Hmm. Current belief is that the sun's coronal temperature is due to reconnection of flux tubes. The tubes become twisted and tangled and may release great energy when untangled. I suppose that could happen at a galactic level. The EM fields would be very diffuse but so large that the total energy is enormous.
 
  • #13
Dotini
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This phenomenon appear similar to the sun's coronal temperature.
To me, the phenomenon does also seem similar, in that electrons in the solar wind are accelerated away from the sun, pulling ions and neutrals with them. "Shocks and instabilities" in the plasma then enter the picture to produce the fabulous temperatures seen in the solar corona - possibly. Even though there have been astonishing recent discoveries in heliophysics, I don't think near enough is understood of the overall sun/heliosphere environment to provide a full explanation. To suddenly discover a major structure such as the warm-hot galactic plasma cloud is quite invigorating. I look forward toward more exciting discoveries from the global science community.

http://www.nasa.gov/mission_pages/sunearth/news/riding-plasma-waves.html
So, this suggests that shocks and the instabilities they create may play a larger role in transferring the energy from the plasma's bulk movement into heat, than previously thought. Wilson believes that the instabilities caused something called perpendicular ion heating – a process that increases the random kinetic energy of the positively-charged ions in a direction perpendicular to the background magnetic field. The waves also added energy to the negatively-charged electrons -- with the greatest effects observed not being heating, the random kinetic energy, but bulk acceleration in a direction parallel to the magnetic field.

"The same type of wave-particle interaction is thought to happen in solar flares, the heating of the sun's corona, and supernova blast waves," says Wilson. "All of these energizations have very similar properties. Now we have evidence that these Whistler-like fluctuations may be causing heating in all these places."


Respectfully submitted,
Steve
 
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The same article also talks about the absorption lines of two isotopes (of oxygen). Does the presence of these negatively impact on other measurements associated with (or based on, or relying on) absorption lines?

Regards,

Noel.
 
  • #15
Dotini
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In between the study of astrophysical phenomena such as distant clouds and jets, and the study of laboratory-scale plasmas, we have recent in-situ study of such phenomena in the solar and near-Earth environment. The following article suggests an explanation of anomalous heating in solar wind by studying turbulent eddies found by a fleet of four Cluster spacecraft.

http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=51231
The Sun ejects a continuous flow of electrically charged particles and magnetic fields in the form of the solar wind. One of the long-standing puzzles of solar physics is that the solar wind is hotter than it should be. However, a new study of data obtained by ESA's Cluster spacecraft may help to explain the mystery.

http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=51233 <---Interesting graphics found here.

Respectfully submitted,
Steve
 
  • #16
Ken G
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As has been said, the key is that very low density gas takes a very long time to cool, because collisions between particles is often required to make light. So if you have a heating process that acts independently on each particle, but the cooling requires collisions, then particles at low density can get heated without being cooled. This does indeed mean that the blackbody emission formula does not apply at all-- that applies at high density when light is constantly being created and destroyed.

Also, note that very hot, very low density gas is not at all the exception in our universe-- indeed, constant heating with rare cooling is the fate of the majority of all the baryonic matter in the universe-- most of the baryonic matter is at millions of Kelvin! That's because most of the baryons in the universe are in between the galaxies, not within the galaxies themselves, and their density is very low. It is not always clear what heats them, perhaps cosmic rays, but a common feature is that the heating can happen to an individual particle, whereas the cooling requires interactions between the particles, capable of making light. (A technical detail is that the energy input has to get thermalized, but that happens much faster than the cooling does-- indeed, any time we refer to a temperature we are assuming that thermalization has already occured, even at very low densities.)
 
  • #17
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A

Also, note that very hot, very low density gas is not at all the exception in our universe-- indeed, constant heating with rare cooling is the fate of the majority of all the baryonic matter in the universe-- most of the baryonic matter is at millions of Kelvin!
There is an article about a gas cloud at about 1 Kelvin, cooler than the CMB. It begins as a relativistic jet. As the jet diverges the gas cools. The particles aren't colliding with anything, so they are all going in a straight line at about the same speed and not interacting. So the temperature is very low. At least, that was the best explanation I could come up with.
 

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