# Density of the inter-cluster medium

#### Garth

Gold Member
SpaceTiger said:
Well, firstly, I don't think there's any reason that all of the gas in an overdensity should have to collapse into galaxies (note that there is still material falling in from outside of clusters).
That is interesting ST. How dense is the inter-galactic-cluster medium?
Garth

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#### Nereid

Staff Emeritus
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Garth said:
That is interesting ST. How dense is the inter-galactic-cluster medium?
This is a great question Garth ... and if I may add a rider: and how is such density measured/inferred?

If you don't mind, I'd like to keep this thread to just QSO absorption lines (and closely related topics) - May I split this off as the start of a new thread?

#### hellfire

Nereid said:
But first, for those not quite in the know, what is the "WHIM"?
WHIM stands for Warm-Hot Intergalactic Medium, a low density phase at 105 - 107 K, mainly located in the filaments and not part of any virialized system. About 30% - 40% of all baryons of the present universe (z < 2) were assumed to reside in this phase. It was postulated http://arxiv.org/astro-ph/0007217 [Broken] in the meanwhile.

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#### SpaceTiger

Staff Emeritus
Gold Member
Garth said:
That is interesting ST. How dense is the inter-galactic-cluster medium?
There are two main components to the intergalactic medium, the cold component (T<105 K) and the warm-hot component (105-107 K). The former is only a few times the critical density:

$$\rho_c=\frac{3H^2}{8\pi G}$$

corresponding to a density of about 10-5 cm-3. The warm-hot component is about a factor of ten more dense than this. Finally, intracluster gas has densities of order 10-3 cm-3 and temperatures of around 107 K.

You can determine these conditions from a lot of things, including fitting absorption lines in quasar spectra, looking at X-ray emission and absorption, and the Sunyaev-Zeldovich effect.

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#### Garth

Gold Member
SpaceTiger said:
There are two main components to the intergalactic medium, the cold component (T<105 K) and the warm-hot component (105-107 K). The former is only a few times the critical density:

$$\rho_c=\frac{3H^2}{8\pi G}$$

corresponding to a density of about 10-5 cm-3. The warm-hot component is about a factor of ten more dense than this. Finally, intracluster gas has densities of order 10-3 cm-3 and temperatures of around 107 K.

You can determine these conditions from a lot of things, including fitting absorption lines in quasar spectra, looking at X-ray emission and absorption, and the Sunyaev-Zeldovich effect.
These densities seem very high ST - with the critical density at around 10-29 cm-3?

Garth

#### SpaceTiger

Staff Emeritus
Gold Member
Garth said:
These densities seem very high ST - with the critical density at around 10-29 cm-3?
That's 10-29 g cm-3. I was quoting densities in terms of atoms per unit volume (instead of mass per unit volume).

#### Garth

Gold Member
Doh!!
Homer

Sometimes I just read too fast for my brain to catch up with my eyes.

ST what's that in real money? i.e. in terms of gms.cm-3 and as a component of Omega?

Garth

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#### SpaceTiger

Staff Emeritus
Gold Member
Garth said:
ST what's that in real money? i.e. in terms of gms.cm-3 and as a component of Omega?
The cold component corresponds to, as you said, about 10-29 g cm-3. The other two are a factor of 10 and 100 larger, respectively.

In terms of omega, it depends on the redshift you're referring to. I don't know the numbers off the top of my head, but I'll look it up later.

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#### Garth

Gold Member
Thank you Neried.

The first question is that of the inventory of the IGM and the contribution it makes to the total Omega density parameter of the universe; that is both of baryonic and non-baryonic dark matter.

We have cold gas clouds that leave the Lynman forest imprint on distant quasar spectra, WHIM (Warm/Hot Intergalactic Matter), primordial or otherwise Black Holes and any member of the exotic particle zoo that you care to dream up! Anything else I have forgotten? Ah yes! and a Dark Energy contribution; any way of measuring this?

Garth

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