Clusters of Galaxies

31 400 Toulouse alain.blanchard ast.obs-mip.fr 

The first evidence for the existence of dark matter has been provided by dynamical measures performed on the Coma cluster, in 1930 by F. Zwicky. Since that time, our understanding of clusters has greatly increased. There is nearly 100 times more mass in clusters than in the stars that can be seen within them. However, there is much more baryons seen in X-ray clusters in form of hot gas than in stars. The discovery of this hot gas through its X-ray emission has revolutionized the study of clusters. Indeed X-ray observations allows to measure gas density and gas temperatures with a high accuracy and they are likely to provide the most accurate mass measurements. Clusters therefore provide a fascinating laboratory for cosmological studies their stellar, bary- 

However, the same difficulty that observers meet in defining a cluster exist for theorists to define clusters in a numerical simulation typical numerical simulations handled several millions dark matter particle and a similar number of gas particle when hydro-dynamical processes are taken into account the actual distribution of dark matter, at least on non linear scales is very much like a fractal, for which the definition of an object is somewhat conventional Different algorithms are commonly used to define clusters. Friend of friend is commonly used because of its simplicity, however its relevance to observations is very questionable, especially for low mass systems. On the analytical side 

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The Milky Way belongs to a cluster of galaxies called the Local Group that occupies about 3 Gly in space and contains over 30 galaxies, although the precise number is hard to determine. Placing the Milky Way at the centre (Figure 2.14),... 

A type II Cepheid variable is observed in the elliptical galaxy M49 in the Virgo cluster of galaxies with a period of 30 days. Calculate the absolute magnitude of this star. If the total flux on the Earth from the star is 4.2 x 10 22 W m-2, estimate the distance to the Virgo cluster. 

The nearest rich cluster of galaxies, in the constellation Coma Berenices. 

Advances in X-ray astronomy have led to good spectra of energetic objects like supernova remnants (see Fig. 3.30) and the hot gas in clusters of galaxies (see Fig. 3.31), from which some deductions about abundances can be made subject to realistic modelling, which is not always straightforward. The SNR shown in... 

Fig. 3.31. X-ray spectrum taken from the XMM-Newton and Chandra X-ray observatories of the inner part of the Centaurus cluster of galaxies, where the metallicity is roughly twice solar, showing the iron L- and K-shell features at energies of 1.2 and 6.8 keV repectively. The curve is a two-component fit to the continuum with temperatures of 0.7 and 1.5 keV. After Sanders and Fabian (2006). Courtesy Andy Fabian.

The intra-cluster gas in clusters of galaxies is generally hotter and in collisional ionization equilibrium, and the continuum is dominated by bremsstrahlung, making the interpretation of at least the hydrogen-like and helium-like K-shell emission lines relatively straightforward, but they are comparatively weak and an accurate determination of the temperature(s) is critical. 

Rich clusters of galaxies, characterized optically as concentrations of hundreds (or even thousands) of galaxies within a region spanning several Mpc, are among... 

Fig. 11.18. Gas mass in clusters of galaxies plotted against the total luminosity of elliptical and lenticular (SO) galaxies in the cluster. The la error in the slope a is 0.25. After Arnaud et al. (1992). Courtesy Monique Arnaud.

Fig. 12.15. Growth of metallicity with time in regions of differing overdensity, corresponding approximately to clusters of galaxies (5 = 103 thick curve), DLA and Lyman-limit systems (5 = 102 thinner curve), moderate-column-density Ly-a clouds (5 = 10 dotted curve) and low-column-density Ly-a clouds (8 = 0 dashed curve), The dot-dashed curve shows the global average. After Cen and Ostriker (1999). Courtesy Renyue Cen.

Every region of the Universe is evolving, but the most spectacular evolution concerns its geometry. Space is expanding between clusters of galaxies. However, this cosmic picture, no matter how generous it may appear, is still far too abstract. The question of the materiality of the Earth and the sky is left unanswered. Where is the world s flesh The search for the material origins of... 

Summing up the dynamical argument, telescopes reveal the architecture and motions of the cosmos on every scale. Planets, stars, galaxies, and clusters of galaxies are nested one within the other like Chinese boxes. But then the need for dark matter suddenly arises. Without it, stars at the edge of our Galaxy would fly off and the swarms of galaxies in clusters would scatter like birds. 

Let us examine this situation in more detail. It is quite clear that the density of matter in clusters of galaxies is significantly higher than the density of nuclear matter as deduced from primordial nucleosynthesis (2-5% of the critical density). If we assume that these structures are representative of the Universe as a whole, then in order to make up the difference, we are forced to resort to clouds of exotic elementary particles left over from the Big Bang. The fate of the Universe then lies in the hands of non-nuclear matter of unknown but not unknowable nature (e.g. neutralinos). 

M. Ixmgair, in Paul Davies (Ed.), The New Physics, Cambridge Univ. Press, New York, 1989 (see specifically Dark matter in galaxies and clusters of galaxies, p. 163). 

The theme of the conference was divided into three parts the first one concerned General Statements of Cosmological Theory. It was introduced by speeches by Lemaitre, on the Primaeval Atom Hypothesis and the Problem of Clusters of Galaxies, by Oscar Klein who developed Some Considerations Regarding the Earlier Development of the System of Galaxies, and by Hoyle on The Steady-State Theory. This was followed by a talk by Gold, on the Arrow of Time and another by Wheeler on Some Implications of General Relativity for the Structure and Evolution of the Universe. ... 

Z. Marie, M. Moles, and J. P. Vigier, Red shifting of light passing through clusters of galaxies A new photon property Lett. Nuovo Cimento 18(9) (Ser. 2), 269-276 (1977). 

G. Vettolani et al., Proc. Schloss Rindberg Workshop Studying the Universe with Clusters of Galaxies, 1994. 

As far as we can see into the Universe, we don t observe any primordial antimatter. Within the limits of our present observational horizon the Universe is seen to contain only matter and no antimatter. The presence of cosmic antimatter would lead to observable traces of annihilation however the measurements of the extragalactic 7 ray flux indicate an absence of annihilation radiation, and the microwave background spectrum lacks a corresponding distortion. These findings preclude the existence of a significant amount of antimatter within tens of Megaparsecs, which is the scale of super-clusters of galaxies. 

A different approach has been recently attempted by Battistelli (2002), based on an idea proposed by Fabbri Melchiorri (1978) the signal produced by CMB radiation when interacting with the hot gas in clusters of galaxies is extremely sensitive to the CMB temperature. 

Clusters of galaxies are the most massive collapsed systems in the Universe. A typical luminous cluster (e.g., Coma cluster) is filled with a hot, 100 million degree, low density (10-3 cm-3) gas. In addition to the optically luminous galaxies and diffuse X-ray gas, clusters are dominated by dark matter. The X-ray gas, relaxing on the relatively short sound crossing time, tmpc = 6.6 x 108(T/108) yrs(wherer = D/cs and cj. = fP/ p) is an effective tracer of this unseen dark matter. 

Abstract Clusters are useful cosmological tools which allow to understand how large scale structures and galaxies evolve in the Universe. I will review some of the methods based on optical observations of clusters of galaxies to derive cosmological properties, describing methods and limitations to find clusters and to study these structures depending on cluster characteristics. 

Keywords Dark matter galaxies clusters of galaxies... 

The mass discrepancy in clusters of galaxies was found by Zwicky (1933). He measured redshifts of galaxies in the Coma cluster and found that the total mass of the cluster calculated from the velocity dispersion using the virial theorem exceeds the sum of masses of visible galaxies more than tenfolds. He concluded that the cluster contains large amounts of invisible dark matter. 

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