<p>I second Pela's answer, apart from one addition. The CMB and the intracluster gas do interact with each other via the <a href="https://en.wikipedia.org/wiki/Compton_scattering#Inverse_Compton_scattering" rel="nofollow">inverse Compton effect</a>, whereby electrons in the gas cool by giving their energy to CMB photons, increasing their energies.</p>

<p>This process, when applied to hot gas in the intracluster medium, is also known as the <a href="https://en.wikipedia.org/wiki/Sunyaev%E2%80%93Zel%27dovich_effect" rel="nofollow">Sunyaev-Zel'dovich</a> effect. This causes the brightness of the CMB to be different when looking towards a galaxy cluster. </p>

<p>The importance of this effect can be found in these notes by <a href="http://ned.ipac.caltech.edu/level5/Birkinshaw/Birk4_1.html#4.1" rel="nofollow">Mark Birkinshaw</a>. Essentially, the S-Z effect when combined with X-ray images of the hot gas can provide vital information on the density structure and thermal structure of the hot gas. It also turns out that the comparison of the X-ray and S-Z properties of a cluster can yield a value for the Hubble parameter and provide other cosmological constraints (for example, see Planck results presented in <a href="http://arxiv.org/abs/1502.01597" rel="nofollow">Ade et al. 2015</a>).</p>

CC BY-SA 3.0
2015-07-24T21:31:46.427Z
Oops yes, thanks for correcting me! I'll edit my post.
pela
CC BY-SA 3.0
2015-07-24T21:26:05.043Z
2015-07-24T21:26:05.043Z
ProfRob
<p>The answer to your first question is "Yes, the temperature referred to is the 'normal' temperature, reflecting the average kinetic energy of the gas particles".</p>

<p>The answer to your second question is a bit more complex:</p>

<h1>Cooling function</h1>

<p>Gas cools by various processes, with an efficiency depending on the temperature, the density, and the composition of the gas. At "low" temperatures, most cooling happens because particles collide and their kinetic energy is used to excite or even ionize atoms. The atoms subsequently recombine or de-excite, emitting radiation which can carry energy away from the system.</p>

<p>Most of the gas in the Universe is hydrogen, which cools most efficiently around <span class="math-container">$10^4\,\mathrm{K}$</span>. At higher temperatures, helium becomes the major contributor. Additionally, various metals contribute at various temperatures depending on their density and ionization state. At very high temperatures (<span class="math-container">$\gtrsim10^7\,\mathrm{K}$</span>) where the gas is highly ionized, cooling mainly happens through bremsstrahlung, i.e. deceleration of charged particles by other charged particles.</p>

<p>The figure below (from <a href="http://staff.ustc.edu.cn/~wzhao7/c_index_files/main.files/HJMo.pdf" rel="nofollow noreferrer">Mo, Bosch &amp; White 2010</a>) shows the cooling function for various metallicities (with my own annotations):
<a href="https://i.sstatic.net/m1c85.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/m1c85.png" alt="Cooling function"></a></p>

<h1>Collapse of gasous halos</h1>

<p>So what makes the intracluster medium (ICM) different from the interstellar medium (ISM)? In the expanding Universe, overdensities try to contract. These overdensities can reach hydrostatic equilibrium only if radiative cooling is small. If the cooling timescale <span class="math-container">$t_\mathrm{cool}$</span> is much smaller than the free-fall timescale <span class="math-container">$t_\mathrm{ff}$</span>, it can collapse and form stars. From the virial theorem, which gives the relation between potential and kinetic energy of the system, you can calculate the relation between the two timescales. It's a bit too much math for this post, but not difficult. I recommend reading chapter 8.4 in Mo, Bosch &amp; White (2010). Their Fig. 8.6 shows a cooling diagram with the locus of <span class="math-container">$t_\mathrm{cool} = t_\mathrm{ff}$</span> in the density-temperature plane:</p>

<p><a href="https://i.sstatic.net/qt8Jq.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/qt8Jq.png" alt="Cooling diagram"></a></p>

<p>Above the locus, cooling is effective, and the cloud can collapse. The tilted dashed lines are lines of constant gas mass. Now you see that even for solar metallicity (the <span class="math-container">$Z = Z_\odot$</span> line), clouds of gas masses larger that <span class="math-container">$\sim10^{13}\,M_\odot$</span> are unable to cool. <strong>This is the fundamental difference between a galaxy and a cluster. They are both overdensities that resisted the expansion, but the gas in the cluster is not able to cool and form stars</strong>.</p>

<h1>Voids</h1>

<p>When people talk about "voids", they usually refer <em>not</em> to the ICM — which is the hot and dilute gas between the galaxies of a cluster — but to the immense regions of even more dilute gas which are… well, void of galaxies. Take a look at this image (from <a href="http://ganymede.nmsu.edu/tharriso/ast110/class25.html" rel="nofollow noreferrer">here</a>). Here, I marked clusters with green and voids with purple. I also marked some filaments with cyan.</p>

<p><a href="https://i.sstatic.net/t0TnP.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/t0TnP.jpg" alt="Cosmological simulation"></a></p>

<h1>The cosmic microwave background</h1>

<p>As for your last question, the ICM doesn't interact much with the CMB, so they're not in thermodynamic equilibrium, and there's no conflict. A small fraction (&lt;10%) of the CMB photons <em>does</em> interact. This doesn't change the state of the gas, but it tends to polarize a fraction of the CMB, and this we can use to study the ionization history of the Universe.</p>

<p>EDIT: I forgot about the <em>Sunyaev-Zel'dovich effect</em>, which is the interaction of hot electrons with CMB photons. Rob Jeffries discusses this in <a href="https://astronomy.stackexchange.com/a/11370/14094">his answer</a>.</p>

CC BY-SA 4.0
2015-07-24T20:29:47.487Z
2019-07-04T10:20:26.547Z
2019-07-04T10:20:26.547Z
pela
<p>I've read once in <a href="https://en.wikipedia.org/wiki/Intracluster_medium" rel="nofollow noreferrer">an article</a> that the temperature of the medium within galaxy clusters (the intracluster medium or ICM) is extremely high. </p>

<ol>
<li><p>What does that temperature really mean? Particle velocity? </p></li>
<li><p>Why is that so?</p></li>
<li><p>Is that medium the "Void"? </p></li>
<li><p>Is it in conflict with the CMB temperature (that's about 3 K)?</p></li>
</ol>

CC BY-SA 3.0
2015-07-24T15:48:42.257Z
Can you give a reference to the article. More specifically, can you define what you mean by "the medium between galaxy superclusters"?
ProfRob
CC BY-SA 3.0
2015-07-24T16:50:44.453Z
Sorry for unclear phrase. By "the medium between galaxy superclusters", I meant _Intracluster medium_ or ICM. 
There can be found a brief description [here](https://en.wikipedia.org/wiki/Intracluster_medium).
arash
CC BY-SA 3.0
2015-07-24T21:11:19.943Z
That is the gas between the galaxies in a cluster
ProfRob
<temperature><cosmic-microwave-background><galaxy-cluster>
Temperature of the intracluster medium (ICM)
arash
# Temperature of the intracluster medium (ICM)

(Asked by: username_0 on 2015-07-24)

I've read once in [an article](https://en.wikipedia.org/wiki/Intracluster_medium) that the temperature of the medium within galaxy clusters (the intracluster medium or ICM) is extremely high.

1. What does that temperature really mean? Particle velocity?
2. Why is that so?
3. Is that medium the "Void"?
4. Is it in conflict with the CMB temperature (that's about 3 K)?

**Tags:** temperature, cosmic-microwave-background, galaxy-cluster
> **Comments:**
> **username_1**: Can you give a reference to the article. More specifically, can you define what you mean by "the medium between galaxy superclusters"?  
> **username_0**: Sorry for unclear phrase. By "the medium between galaxy superclusters", I meant _Intracluster medium_ or ICM.  There can be found a brief description [here](https://en.wikipedia.org/wiki/Intracluster_medium).  
> **username_1**: That is the gas between the galaxies in a cluster  

---

## 2 answers

### Answer by username_1 on 2015-07-24. Score: 4 | Accepted answer

I second Pela's answer, apart from one addition. The CMB and the intracluster gas do interact with each other via the [inverse Compton effect](https://en.wikipedia.org/wiki/Compton_scattering#Inverse_Compton_scattering), whereby electrons in the gas cool by giving their energy to CMB photons, increasing their energies.

This process, when applied to hot gas in the intracluster medium, is also known as the [Sunyaev-Zel'dovich](https://en.wikipedia.org/wiki/Sunyaev%E2%80%93Zel%27dovich_effect) effect. This causes the brightness of the CMB to be different when looking towards a galaxy cluster.

The importance of this effect can be found in these notes by [Mark Birkinshaw](http://ned.ipac.caltech.edu/level5/Birkinshaw/Birk4_1.html#4.1). Essentially, the S-Z effect when combined with X-ray images of the hot gas can provide vital information on the density structure and thermal structure of the hot gas. It also turns out that the comparison of the X-ray and S-Z properties of a cluster can yield a value for the Hubble parameter and provide other cosmological constraints (for example, see Planck results presented in [Ade et al. 2015](http://arxiv.org/abs/1502.01597)).

> **Comments:**
> **username_2**: Oops yes, thanks for correcting me! I'll edit my post.  

---

### Answer by username_2 on 2015-07-24. Score: 6

The answer to your first question is "Yes, the temperature referred to is the 'normal' temperature, reflecting the average kinetic energy of the gas particles".

The answer to your second question is a bit more complex:

# Cooling function

Gas cools by various processes, with an efficiency depending on the temperature, the density, and the composition of the gas. At "low" temperatures, most cooling happens because particles collide and their kinetic energy is used to excite or even ionize atoms. The atoms subsequently recombine or de-excite, emitting radiation which can carry energy away from the system.

Most of the gas in the Universe is hydrogen, which cools most efficiently around $10^4\,\mathrm{K}$. At higher temperatures, helium becomes the major contributor. Additionally, various metals contribute at various temperatures depending on their density and ionization state. At very high temperatures ($\gtrsim10^7\,\mathrm{K}$) where the gas is highly ionized, cooling mainly happens through bremsstrahlung, i.e. deceleration of charged particles by other charged particles.

The figure below (from [Mo, Bosch & White 2010](http://staff.ustc.edu.cn/~wzhao7/c_index_files/main.files/HJMo.pdf)) shows the cooling function for various metallicities (with my own annotations):
[![Cooling function](https://i.sstatic.net/m1c85.png)](https://i.sstatic.net/m1c85.png)

# Collapse of gasous halos

So what makes the intracluster medium (ICM) different from the interstellar medium (ISM)? In the expanding Universe, overdensities try to contract. These overdensities can reach hydrostatic equilibrium only if radiative cooling is small. If the cooling timescale $t\_\mathrm{cool}$ is much smaller than the free-fall timescale $t\_\mathrm{ff}$, it can collapse and form stars. From the virial theorem, which gives the relation between potential and kinetic energy of the system, you can calculate the relation between the two timescales. It's a bit too much math for this post, but not difficult. I recommend reading chapter 8.4 in Mo, Bosch & White (2010). Their Fig. 8.6 shows a cooling diagram with the locus of $t\_\mathrm{cool} = t\_\mathrm{ff}$ in the density-temperature plane:

[![Cooling diagram](https://i.sstatic.net/qt8Jq.png)](https://i.sstatic.net/qt8Jq.png)

Above the locus, cooling is effective, and the cloud can collapse. The tilted dashed lines are lines of constant gas mass. Now you see that even for solar metallicity (the $Z = Z\_\odot$ line), clouds of gas masses larger that $\sim10^{13}\,M\_\odot$ are unable to cool. **This is the fundamental difference between a galaxy and a cluster. They are both overdensities that resisted the expansion, but the gas in the cluster is not able to cool and form stars**.

# Voids

When people talk about "voids", they usually refer *not* to the ICM — which is the hot and dilute gas between the galaxies of a cluster — but to the immense regions of even more dilute gas which are… well, void of galaxies. Take a look at this image (from [here](http://ganymede.nmsu.edu/tharriso/ast110/class25.html)). Here, I marked clusters with green and voids with purple. I also marked some filaments with cyan.

[![Cosmological simulation](https://i.sstatic.net/t0TnP.jpg)](https://i.sstatic.net/t0TnP.jpg)

# The cosmic microwave background

As for your last question, the ICM doesn't interact much with the CMB, so they're not in thermodynamic equilibrium, and there's no conflict. A small fraction (<10%) of the CMB photons *does* interact. This doesn't change the state of the gas, but it tends to polarize a fraction of the CMB, and this we can use to study the ionization history of the Universe.

EDIT: I forgot about the *Sunyaev-Zel'dovich effect*, which is the interaction of hot electrons with CMB photons. Rob Jeffries discusses this in [his answer](https://astronomy.stackexchange.com/a/11370/14094).


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