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Cluster Intergalactic Gas Heating & Cooling Problem

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Aug13-10, 02:32 PM
P: 272
This is an interesting subject. There have been discovered very large (up to around 100 kpc) hot (10^7 K) intergalactic gas clouds within galaxy clusters.

A simple calculation based on the density of the gas clouds in question and the emission rate of the hot gas shows the gas clouds should have cooled and formed stars/galaxies.

There are two anomalies. What is the source of energy to heat a very large gas cloud evenly. As noted in this paper AGN and gas in flow will heat the gas cloud unevenly. Also in the case of a gas flow hypothesis there is a mass problem as one needs too much infall gas to continually flow into the original gas cloud.

The second and related problem, is why does the gas cloud not cool.

Interesting also is the mass of the intergalactic cluster gas is approximately the same as the mass of the visible matter (stars) in the cluster's galaxies.

X-ray Spectroscopy of Cooling Clusters

Observations show that the X-ray emission from many clusters of galaxies is sharply peaked around the central brightest galaxy. The inferred radiative cooling time of the gas in that peak, where the temperature drops to the center, is much shorter than the age of the cluster, suggesting the existence of a cooling flow there (Fabian et al. 1994). X-ray spectroscopy over the past 5 yr shows that the temperature drop toward the center is limited to about a factor of three. Just when the gas should be cooling most rapidly it appears not to be cooling at all. This is sometimes known as the cooling flow problem. Careful observations show that gently distributed heat is required over a radius of up to 100 kpc to balance radiative cooling in these regions.
5.7 Definition of the Cooling Flow Problem

We now briefly discuss what we believe the cooling flow problem is, and how it might be resolved. Clearly, the problem is quite complex and it is difficult not to see the problem in historical terms. Peterson et al. (2003), for example, discussed a difference between the soft X-ray cooling-flow problem and the mass sink cooling-flow problem. The former refers to the recent discrepancy seen in the soft X-ray spectrum between what was predicted and what was observed. The latter refers to the difficulty in detecting any by-products in cooling clusters from the hypothesized cooling-flow plasma.

These definitions, however, might just categorize our ignorance of the solution to the problem. The major difficulty is that: 1) the cluster plasma loses energy by emitting the very X-rays we detect, 2) efficient and distributed heat sources 36 are difficult to construct, 3) the cluster plasma appears to cool most of the way, but 4) evidence for complete cooling is utterly lacking. The cooling-flow problem as we see it is to understand what happens in the middle of that process. After examining whether cooling flows are ruled out, we discuss many ideas that might alleviate the cooling-flow problem.

7 Heating
Some heating is always expected in the central regions of clusters. Examples are supernovae (Silk et al. 1986; Domainko et al. 2004), an active central nucleus (Bailey 1982; Tucker & Rosner 1983; Pedlar et al. 1990; Tabor & Binney 1993; Binney & Tabor 1995) and many more recent papers cited in Sec 7.2– 7.5), conduction (Takahara & Takahara 1979; Binney & Cowie 1981; Stewart et al. 1984; Friaca 1986; Bertschinger & Meiksin 1986; Rosner & Tucker 1989) and many more recent papers cited in Sec 7.1). A problem with heating the gas is that the cooling rate is proportional to the density squared whereas most heating processes are proportional to volume. This tends to make the gas unstable and means generally that the cooler denser gas will carry on cooling while hotter surrounding gas heats up. The gas appears to cool by about a factor of three and then stop cooling. A mechanism to do that is not obvious, since the gas does not appear to be piling up at the lower temperature. Indeed it seems that the gas temperature profile is ”frozen” and has been so for some Gyrs (Bauer et al. 2005).
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Aug13-10, 02:41 PM
P: 272
One proposed solution to the cluster intergalactic gas heating and cooling problem is AGN hot gas emission.

There are two problems with that solution. 1) The AGN hot gas will not evenly heat the intercluster gas and 2) there are no AGN in the local universe and there are clusters with very hot intergalactic gas in the local universe.

The role of black holes in galaxy formation and evolution

Clusters have typical entropy excesses of K ≈ 100 keV cm2 at 0.1rhalo (ref. 68; Fig. 3). These excesses weigh more heavily on smaller clusters, which have lower absolute entropies, but higher entropies relative to theoretical expectations. This problem is common to both cool-core and non-cool-core clusters, and affects a large fraction of the intracluster medium. The quasar winds invoked to quench star formation in the progenitors of giant ellipticals could solve this entropy problem by preheating the intergalactic gas destined to become the intracluster medium69–71, but they cannot solve the cooling-flow problem in the central regions of cool-core clusters. In these systems, which have K < 100 keV cm2, the cooling time is so short (<0.1 Gyr; Fig. 3) that one needs heating at least every 0.1 Gyr today to maintain these systems in their current state. This need for regular heating clashes with the scarcity of quasars in the low-redshift Universe.
Aug18-10, 05:09 PM
P: 4
The other main source of gas heating is supernovae. This is a feedback effect. When gas density increases, it cools (metal lines, radiative cooling) allowing stars to form. Collapse is then halted by SN blasting heated gas back into the cluster environment (but still bound to the halo), quenching the cold gas reserves required for star formation.

AGN are (I think) a somewhat more inefficient heating mechanism, but both modes of Intracluster Medium (ICM) heating are required to match model luminosity functions to those observed - SN for the faint end and AGN for the bright end. The cluster gas is in fact the dominant baryonic mass component in a cluster (I think Voit 2005 is a ood review here)

The X-ray emission of the cluster gas does not arise from the AGN itself, more the high election densities (thermal bremstrahlung) arising from large graviation potentials.

Aug18-10, 09:36 PM
P: 272
Cluster Intergalactic Gas Heating & Cooling Problem

Hello Dnam,

Thanks for the Voit 2005 reference. His review paper is a good summary of issues related to clusters.

Voit noted that the cluster gas mass is significantly greater than the mass of the stars in the cluster.

Fabian estimates the cluster gas to be six times the mass of all of the stars in the cluster galaxies.

As clusters seem to be closed boxes that would imply a significant portion of the galaxies' gas is ejected back into the intergalactic space.

Both Voit and Fabian invoke AGN heating as a possible explanation to heat the cluster gas. They note however AGN heating does not heat the gas evenly and cannot hence explain the observations. (AGN heated cluster gas will cool in one section and heat in another.)

There are other constraints on the quasar/AGN mechanism that is required to explain observations. I will start a separate thread later this fall. For example, AGN must also be cyclically active to explain the observations based on the classical BH model with an accretion disk to explain the evolution density of quasars per comoving region of space with redshift. (The explanation that a classical black hole is heating the cluster gas due to periodic in fall of gas in the black hole.)

They provide no explanation as to what could cyclically cause gas to fall into the BH in addition to problem that AGN will heat the gas unevenly.

Cooling and Clusters: When Is Heating Needed?

There are (at least) two unsolved problems concerning the current state of the thermal gas in clusters of galaxies. The first is identifying the source of the heating which offsets cooling in the centers of clusters with short cooling times (the “cooling flow” problem). The second is understanding the mechanism which boosts the entropy in cluster and group gas. Since both of these problems involve an unknown source of heating it is tempting to identify them with the same process, particular since AGN heating is observed to be operating at some level in a sample of well-observed “cooling flow” clusters. Here we show, using numerical simulations of cluster formation, that much of the gas ending up in clusters cools at high redshift and so the heating is also needed at high-redshift, well before the cluster forms. This indicates that the same process operating to solve the cooling flow problem may not also resolve the cluster entropy problem.

Tracing cosmic evolution with clusters of galaxies
Clusters of galaxies might have been called something different if they had first been discovered in a waveband other than visible light, because all of the stars in all of a cluster’s galaxies represent only a small fraction of a cluster’s overall mass. Clusters contain substantially more mass in the form of hot gas, observable with Xray and microwave instruments.

The most successful cosmological models to date envision structure formation as a hierarchical process in which gravity is constantly drawing lumps of matter together to form increasingly larger structures. Clusters of galaxies currently sit atop this hierarchy as the largest objects that have had time to collapse under the influence of their own gravity. Thus, their appearance on the cosmic scene is also relatively recent. Two features of clusters make them uniquely useful tracers of cosmic evolution. First, clusters are the biggest things whose masses we can reliably measure because they are the largest objects to have undergone gravitational relaxation and entered into virial equilibrium. Mass measurements of nearby clusters can therefore be used to determine the amount of structure in the universe on scales of 10^14-10^15 solar mass, and comparisons of the present-day cluster mass distribution with the mass distribution at earlier times can be used to measure the rate of structure formation, placing important constraints on cosmological models. Second, clusters are essentially “closed boxes” that retain all their gaseous matter, despite the enormous energy input associated with supernovae and active galactic nuclei, because the gravitational potential wells of clusters are so deep.

[B]^46 erg s−1. The emission is predominantly thermal bremsstrahlung from highly ionized hydrogen and helium in the intracluster medium (ICM) at temperatures T 10^7–1.5 10^8 K. Line emission, particularly from iron, is also present, showing that most of the gas has a mean metallicity of ca. 0.3 solar units. The total mass of the intracluster medium is about one-sixth of the total cluster mass, and the stars in all the member galaxies have about one-sixth of the mass of the hot gas. Most of the total mass of a cluster is due to dark matter.
Aug16-11, 04:21 AM
P: 1
It is very interesting topic. Interesting also is the mass of the intergalactic cluster gas is approximately the same as the mass of the visible matter (stars) in the cluster's galaxies.
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Aug16-11, 08:23 AM
P: 391
Quote Quote by central View Post
It is very interesting topic. Interesting also is the mass of the intergalactic cluster gas is approximately the same as the mass of the visible matter (stars) in the cluster's galaxies.
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Actually, the mass of the gas is approximately 5-10 times the mass of the stars in a cluster! Also, a large fraction of the stellar light coming from clusters (maybe 20%) comes from intracluster stars which are no longer part of a galaxy.
Feb17-12, 07:24 AM
P: 1
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