Galaxy clusters

Figure 2.15 Luminous arc located near the galaxy cluster 2242-02. (Reproduced from photos by courtesy of NASA)...

Recent computer simulations suggest the existence of vast filamentary networks of ionized gas, or plasma—a cosmic cobweb that now links galaxies and galaxy clusters. These warm cobwebs may be difficult to detect with current satellites. [For more information, see Glanz, J. (1998) Cosmic web captures lost matter. Science. June 26, 280(5372) 2049-50.]... 

Today, convincing proofs have been obtained that galaxy clusters on the largest scale are grouped along threads or surfaces, rather than in clusters. 

X-ray observations detect deep gravitational potentials. The X-ray emission from galaxy clusters is optically thin thermal bremsstrahlung. The luminosity of a cluster is given as ... 

Abstract We discuss some of the astrophysical techniques to detect the presence and the nature of Dark Matter in large scale structures. We focus here on the cases of galaxies and galaxy clusters. 

Keywords Cosmology, Dark Matter, Galaxies, Galaxy Clusters. 

Other estimates of the amount and of the distribution of DM in the universe come from the study of large scale structures at more recent epochs in the cosmic evolution. The reason why cosmic structures contain a record of the DM distribution in the universe is due to the fact that the evolution of the parent density perturbations was dominated by their DM content from early times on (see Peacock in these Proceedings). Thus the study of galaxies and galaxy clusters - the largest gravitationally bound structures in the universe whose potential wells are dominated by DM - provide information on both the amount of DM and on its density distribution. 

Numerical simulations indicate that virialized DM halos on the scales of galaxies and galaxy clusters show a cuspy density profile p(r) = pog(r) where... 

Figure 5.2. A simple model which shows the basic astrophysical mechanisms underlying the search for the nature of DM particles in large-scale structures (galaxies and galaxy clusters).

Figure 5.3. Left. The gamma-ray emission from XX annihilation in a rich, Coma-like, nearby galaxy cluster is shown Mx = 70 — 500 GeV (from top down). The integral flux is compared to the sensitivity of ongoing and planned gamma-ray experiments, as labelled. Right. The diffuse synchrotron emission spectrum of secondary electrons produced in XX annihilation is shown to fit the Coma radio-halo spectrum the green area represent the prediction of a model in which the x annihilates predominantly into fermions, while the blue area represent the gauge-boson dominated x annihilation (from Colafrancesco Mele 2001).

The nature of Dark Matter, despite its overwhelming evidence, is still intangible. It is nonetheless appealing, in these respects, that some astrophysical features of galaxy clusters and of galaxies might give information on the fundamental properties of the DM particles. 

Abstract We present the observational evidence and the theoretical indications for the presence of relativistic particles (cosmic rays) in galaxy clusters. We discuss the basic ideas for their origin and explore the astrophysical techniques to unveil their nature. 

Keywords Cosmology, Galaxy Clusters, Cosmic Rays... 

Several galaxy clusters show also an emission of extreme UV (Lieu et al. 1996, Durret et al. 2002) and soft X-ray (Bonamente et al. 2002, Kaastra et al. 2002) radiation in excess w.r.t. the thermal bremsstrahlung emission. This EUV emission excess may be consistent with both ICS of CMB photons off a non-thermal electron population (e.g., Lieu et al. 1999, Bowyer 2000) with Ee = 608.5 MeV (hv/keV)1/2 149 MeV for hv 60 eV, and with thermal emission from a warm gas at ksTe V 1 keV (Bonamente et al. 2002). In the case of Coma, the simple extrapolation of the ICS spectrum which fits the HXR excess down to energies 0.25 keV does not fit the EUV excess measured in Coma because it is too steep and yields a too high flux compared to the measured flux by the EUV satellite in the 0.065 — 0.245 keV band (Ensslin Biermann 1998). Thus, under the assumption that the HXR and the EUV emission of Coma is produced by ICS of CMB photons, the minimal requirement is that a break in the electron spectrum should be present in the range 0.3 — 2.8 GeV in order to avoid an excessive EUV contribution by the ICS emission and to be consistent with the radio halo spectrum. 

There is not yet, however, a definite detection of diffuse gamma-ray emission from galaxy clusters. While there is a preliminary evidence of gamma-ray emission from a dozen bright, radio-active clusters which host powerful radio galaxies and Blazars and are associated to unidentified EGRET sources (Co-lafrancesco 2002), many of the quiet, X-ray selected clusters only have upper limits for their emission at E > 100 MeV. 

Two distinct families of models for the CR origin of non-thermal phenomena in galaxy clusters have been proposed so far i) the electronic and the ii) hadronic models. 

Cosmic rays residing in galaxy clusters produce several astrophysical signatures among which there are diffuse synchrotron radio emission, ICS of CMB (and other background) photons which are then moved to higher frequencies... 

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