<p>Just to add to TidalWave's answer - something that is easier to simply imagine, the trivial "why".</p>

<p>What we call Temperature on thermodynamic level is Speed on atomic level. Saying the medium has a high temperature is equivalent to saying particles of that medium move awfully fast.</p>

<p>Well, they must be moving fast. They must be moving faster than the escape speed of the galaxies, or the ones ejected from galaxies wouldn't escape them and the primordial would get captured by the galaxies instead. Being so sparse they collide extremely rarely too - so whatever slowdowns resulting from collisions (expending energy e.g. as photons) simply almost never happens. In short, you're getting particles that were fast (hot) enough to get (and remain) there at all, and had no opportunity to cool down.</p>

CC BY-SA 3.0
2013-11-20T17:39:40.453Z
Do you happen to know, which fraction of warm-hot IGM is primordial?
Alexey Bobrick
CC BY-SA 3.0
2013-11-21T07:57:31.290Z
@AlexeyBobrick: Sorry, I don't.
SF.
CC BY-SA 3.0
2014-03-15T21:18:42.537Z
@AlexeyBobrick it is generally assumed that the WHIM is not primordial, but the result of shock heated gas in galaxies/clusters. The primordial phase is assumed to be colder ($\sim 10^4 K$).
chris
CC BY-SA 3.0
2014-03-18T14:21:59.690Z
@chris, very useful point, thank you again! Do you know if the primordial IGM cooling is mainly due to cosmological expansion or radiative cooling?
Alexey Bobrick
CC BY-SA 3.0
2014-03-18T18:25:48.573Z
@AlexeyBobrick I think (but I am not sure) it has been re-ionized (and re-heated?) by the mean flux from AGN (or possibly the stellar background). Certainly not the cosmological expansion which is much lower temperature at those redshifts.
chris
CC BY-SA 3.0
2014-03-19T16:33:24.977Z
@chris: Cosmological expansion should affect the temperatures of IGM similarly as it affects the CMB temperature, that's why I am mentioning it. But then IGM would be too cold comparing to 10^4 K. It indeed seems plausible that it was reheated when it was reionized. Thanks again for your comments!
Alexey Bobrick
CC BY-SA 3.0
2013-11-20T16:59:21.793Z
2014-02-27T08:11:45.203Z
2014-02-27T08:11:45.203Z
SF.
<p>They are hot in the sense of particle speed, but if you were out there, you will get freezed, since there is such a low density that any of these particles will likely impact you (and transfer you its energy, which is what you would notice as heat) while you will cold due to radiation.</p>

CC BY-SA 3.0
2013-11-25T08:03:14.253Z
Thank you! That is a nice, intuitive way of thinking about it.
Lays
CC BY-SA 3.0
2013-11-22T17:32:09.460Z
2013-11-22T17:32:09.460Z
Envite
<p>What that ESA (European Space Agency) page titled <a href="http://spaceinimages.esa.int/Images/2013/11/Hot_gas_sloshing_in_a_galactic_cauldron">Hot gas sloshing in a galactic cauldron</a> that you link to describes are called <a href="https://en.wikipedia.org/wiki/Warm%E2%80%93hot_intergalactic_medium">WHIM</a> (Warm–Hot Intergalactic Medium). They are not interstellar medium, but intergalactic medium gas. The difference in density is huge, with interstellar medium density at an average of $\rho ∼ 1\ ppcm$ (one proton per cubic centimeter), but the density of these WHIM being even a few orders of magnitude lower at $\rho ∼ 10^{−6}−10^{−5}\ ppcm$, or roughly 1 to 10 protons per cubic meter (<a href="http://www.nasa.gov/mission_pages/chandra/news/10-048.html">NASA's Chandra X-ray Observatory</a> quotes average density of 6 protons per cubic meter).</p>

<p>What is interesting about WHIM is that they are absolutely huge. We're talking of distances that extend across clusters of galaxies (so stretching multiple millions of light years), which means that even as tenuous as they are, account for a large portion of the baryonic matter of the Universe:</p>

<blockquote>
  <p>Such matter is predicted to account for a sizable fraction ($∼ 50\%$)
  of all the baryons in the local&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ($z &lt; 1$ <code>[redshift in the infrared
  spectrum]</code>) Universe, and it is thus considered the best candidate to
  host the baryons seen at high redshift and missing from the low
  redshift census.</p>
</blockquote>

<p>So now about their heat emissions, and why are they detected in the X-ray range in the first place (the ESA's article mentions the photograph featured there was taken by the ESA’s XMM-Newton X-ray observatory):</p>

<blockquote>
  <p>Electrons and baryons in the WHIM are shock-heated during their infall
  in the dark matter LSS <code>[Large–Scale Structures]</code> potential well, and settle in
  filamentary/sheet-like structures surrounding LSSs.</p>
</blockquote>

<p>I've added a few clarifications in quotes encapsulated in square brackets, but what this means is that parts of these WHIM interact with <a href="https://en.wikipedia.org/wiki/Active_galactic_nucleus">AGN</a> (Active Galactic Nucleus) as the galaxies pass by, and the X-ray emissions of AGN excite <a href="https://en.wikipedia.org/wiki/Baryon">Baryonic matter</a> to a temperature $T ∼ 10^5−10^7 K$.</p>

<p>Quote sources:</p>

<ul>
<li><a href="http://iopscience.iop.org/0004-637X/503/2/518/">The Cosmic Baryon Budget</a>, Fukugita et al. 1998</li>
<li><a href="http://adsabs.harvard.edu/cgi-bin/bib_query?2004ApJ...616..643F">The Cosmic Energy Inventory</a>, Fukugita &amp; Peebles 2004</li>
</ul>

<p>Additional reading:</p>

<ul>
<li><a href="http://www.nasa.gov/mission_pages/chandra/news/10-048.html">X-ray Discovery Points to Location of Missing Matter</a>, NASA</li>
<li><a href="http://www.skyandtelescope.com/news/93797364.html">Last “Missing” Normal Matter Is Found</a>, Sky &amp; Telescope</li>
<li><a href="http://arxiv.org/PS_cache/arxiv/pdf/1004/1004.5359v1.pdf">Studying the WHIM Content of the Galaxy Large–Scale Structures along the Line of Sight to H 2356-309</a> (PDF), L. Zappacosta et al., Harvard-Smithsonian Center for Astrophysics</li>
</ul>

CC BY-SA 3.0
2013-11-20T06:31:02.240Z
2013-11-20T06:45:20.137Z
2013-11-20T06:45:20.137Z
TildalWave
<p>On this <a href="http://spaceinimages.esa.int/Images/2013/11/Hot_gas_sloshing_in_a_galactic_cauldron">link</a>, it states the following: "large assemblies of galaxies that are permeated by even larger amounts of diffuse gas. With temperatures of 10 million degrees or more". </p>

<p>How are these diffused (ionized) gas able to become so hot when they are in large distances from one another and have very little density? </p>

<interstellar-medium>
Why is the interstellar medium so hot?
Lays
# Why is the interstellar medium so hot?

(Asked by: username_0 on 2013-11-20)

On this [link](http://spaceinimages.esa.int/Images/2013/11/Hot_gas_sloshing_in_a_galactic_cauldron), it states the following: "large assemblies of galaxies that are permeated by even larger amounts of diffuse gas. With temperatures of 10 million degrees or more".

How are these diffused (ionized) gas able to become so hot when they are in large distances from one another and have very little density?

**Tags:** interstellar-medium

---

## 3 answers

### Answer by username_5 on 2013-11-20. Score: 11 | Accepted answer

What that ESA (European Space Agency) page titled [Hot gas sloshing in a galactic cauldron](http://spaceinimages.esa.int/Images/2013/11/Hot_gas_sloshing_in_a_galactic_cauldron) that you link to describes are called [WHIM](https://en.wikipedia.org/wiki/Warm%E2%80%93hot_intergalactic_medium) (Warm–Hot Intergalactic Medium). They are not interstellar medium, but intergalactic medium gas. The difference in density is huge, with interstellar medium density at an average of $\rho ∼ 1\ ppcm$ (one proton per cubic centimeter), but the density of these WHIM being even a few orders of magnitude lower at $\rho ∼ 10^{−6}−10^{−5}\ ppcm$, or roughly 1 to 10 protons per cubic meter ([NASA's Chandra X-ray Observatory](http://www.nasa.gov/mission_pages/chandra/news/10-048.html) quotes average density of 6 protons per cubic meter).

What is interesting about WHIM is that they are absolutely huge. We're talking of distances that extend across clusters of galaxies (so stretching multiple millions of light years), which means that even as tenuous as they are, account for a large portion of the baryonic matter of the Universe:

> Such matter is predicted to account for a sizable fraction ($∼ 50\%$)
> of all the baryons in the local        ($z < 1$ `[redshift in the infrared
> spectrum]`) Universe, and it is thus considered the best candidate to
> host the baryons seen at high redshift and missing from the low
> redshift census.

So now about their heat emissions, and why are they detected in the X-ray range in the first place (the ESA's article mentions the photograph featured there was taken by the ESA’s XMM-Newton X-ray observatory):

> Electrons and baryons in the WHIM are shock-heated during their infall
> in the dark matter LSS `[Large–Scale Structures]` potential well, and settle in
> filamentary/sheet-like structures surrounding LSSs.

I've added a few clarifications in quotes encapsulated in square brackets, but what this means is that parts of these WHIM interact with [AGN](https://en.wikipedia.org/wiki/Active_galactic_nucleus) (Active Galactic Nucleus) as the galaxies pass by, and the X-ray emissions of AGN excite [Baryonic matter](https://en.wikipedia.org/wiki/Baryon) to a temperature $T ∼ 10^5−10^7 K$.

Quote sources:

* [The Cosmic Baryon Budget](http://iopscience.iop.org/0004-637X/503/2/518/), Fukugita et al. 1998
* [The Cosmic Energy Inventory](http://adsabs.harvard.edu/cgi-bin/bib_query?2004ApJ...616..643F), Fukugita & Peebles 2004

Additional reading:

* [X-ray Discovery Points to Location of Missing Matter](http://www.nasa.gov/mission_pages/chandra/news/10-048.html), NASA
* [Last “Missing” Normal Matter Is Found](http://www.skyandtelescope.com/news/93797364.html), Sky & Telescope
* [Studying the WHIM Content of the Galaxy Large–Scale Structures along the Line of Sight to H 2356-309](http://arxiv.org/PS_cache/arxiv/pdf/1004/1004.5359v1.pdf) (PDF), L. Zappacosta et al., Harvard-Smithsonian Center for Astrophysics


---

### Answer by username_1 on 2013-11-20. Score: 10

Just to add to TidalWave's answer - something that is easier to simply imagine, the trivial "why".

What we call Temperature on thermodynamic level is Speed on atomic level. Saying the medium has a high temperature is equivalent to saying particles of that medium move awfully fast.

Well, they must be moving fast. They must be moving faster than the escape speed of the galaxies, or the ones ejected from galaxies wouldn't escape them and the primordial would get captured by the galaxies instead. Being so sparse they collide extremely rarely too - so whatever slowdowns resulting from collisions (expending energy e.g. as photons) simply almost never happens. In short, you're getting particles that were fast (hot) enough to get (and remain) there at all, and had no opportunity to cool down.

> **Comments:**
> **username_2**: Do you happen to know, which fraction of warm-hot IGM is primordial?  
> **username_1**: @AlexeyBobrick: Sorry, I don't.  
> **username_3**: @AlexeyBobrick it is generally assumed that the WHIM is not primordial, but the result of shock heated gas in galaxies/clusters. The primordial phase is assumed to be colder ($\sim 10^4 K$).  
> **username_2**: @chris, very useful point, thank you again! Do you know if the primordial IGM cooling is mainly due to cosmological expansion or radiative cooling?  
> **username_3**: @AlexeyBobrick I think (but I am not sure) it has been re-ionized (and re-heated?) by the mean flux from AGN (or possibly the stellar background). Certainly not the cosmological expansion which is much lower temperature at those redshifts.  
> **username_2**: @chris: Cosmological expansion should affect the temperatures of IGM similarly as it affects the CMB temperature, that's why I am mentioning it. But then IGM would be too cold comparing to 10^4 K. It indeed seems plausible that it was reheated when it was reionized. Thanks again for your comments!  

---

### Answer by username_4 on 2013-11-22. Score: 3

They are hot in the sense of particle speed, but if you were out there, you will get freezed, since there is such a low density that any of these particles will likely impact you (and transfer you its energy, which is what you would notice as heat) while you will cold due to radiation.

> **Comments:**
> **username_0**: Thank you! That is a nice, intuitive way of thinking about it.  

---

