Clusters of Galaxies: Setting the Stage

Abstract

Clusters of galaxies are self-gravitating systems of mass ∼ 10 14 − 10 15 h −1 M ⊙ and size ∼ 1 − 3h −1 Mpc. Their mass budget consists of dark matter (∼ 80%, on average), hot diffuse intracluster plasma ( 20%) and a small fraction of stars, dust, and cold gas, mostly locked in galaxies. In most clusters, scaling relations between their properties, like mass, galaxy velocity dispersion, X-ray luminosity and temperature, testify that the cluster components are in approximate dynamical equilibrium within the cluster gravitational potential well. However, spatially inhomogeneous thermal and non-thermal emission of the intracluster medium (ICM), observed in some clusters in the X-ray and radio bands, and the kinematic and morphological segregation of galaxies are a signature of non-gravitational processes, ongoing cluster merging and interactions. Both the fraction of clusters with these features, and the correlation between the dynamical and morphological properties of irregular clusters and the surrounding large-scale structure increase with redshift.In the current bottom-up scenario for the formation of cosmic structure, where tiny fluctuations of the otherwise homogeneous primordial density field are amplified by gravity, clusters are the most massive nodes of the filamentary large-scale structure of the cosmic web and form by anisotropic and episodic accretion of mass, in agreement with most of the observational evidence. In this model of the universe dominated by cold dark matter, at the present time most baryons are expected to be in a diffuse component rather than in stars and galaxies; moreover, ∼ 50% of this diffuse component has temperature ∼ 0.01 − 1 keV and permeates the filamentary distribution of the dark matter. The temperature of this Warm-Hot Intergalactic Medium (WHIM) increases with the local density and its search in the outer re-A. Diaferio

Kinematics and Mass Profile of AWM 7

“…Even within a single cluster, it is important to discriminate between the generally red, virialized population and the newly infalling bluer galaxies; including the latter in steady-state kinematical analyses will artificially inflate the mass estimate; we suggest that this effect accounts for the mass of 5.77 +1.80 −1.50 h −1 M ⊙ derived for AWM 7 by Girardi et al (1998). In principle, the non-virialized Em galaxies can be used as tracers of the escape velocity to arrive at an independent mass measurement (Diaferio & Geller 1997, Diaferio 1999. In practice, there are not enough Em galaxies in our sample for this method to yield a robust mass estimate; although the formal mass returned by this method for AWM 7 (kindly computed for us by A. Diaferio) is comparable to the ones we derive here, the fractional error in that mass is ∼0.9.…”

“…Although one can discern by eye where the caustics probably lie, they are in practice poorly defined. Their accurate determination requires many more galaxies than our sample contains (Diaferio 1999, Diaferio & Geller 1997, Geller, Diaferio & Kurtz 1999.…”

The Mass Profile of the Coma Galaxy Cluster

“…We can thus estimate the cluster mass profile from the inner halo to the outer infall regions. In spite of the assumption of spherical symmetry, the technique yields mass profiles accurate to ∼50% for a suite of N -body simulations (Diaferio & Geller 1997;Diaferio 1999).…”

“…Nbody simulations of Cold Dark Matter models which include a semi-analytic treatment of galaxy formation and evolution suggest that there is no velocity bias on the relevant scales Diaferio et al 1999). Diaferio (1999) uses these simulations to show that the mass estimation technique we apply here yields estimates to r = 5−10h −1 Mpc with a typical uncertainty of 50% for samples of a few hundred galaxies.…”

The LAMOST Complete Spectroscopic Survey of Pointing Area (LaCoSSPAr) in the Southern Galactic Cap. I. The Spectroscopic Redshift Catalog

“…With spectroscopic redshift, the accurate distance information can be obtained to reveal the intrinsic properties of those objects, e.g., the clustering of galaxies which can shed light on the important information about large-scale structure and cosmological models (Bahcall 1988;Postman et al 1992;Voit 2005;Diaferio et al 2008;Wen et al 2010), the pair counts and fraction which are the indicators of galaxy merger rate (Burkey et al 1994;Le Fèvre et al 2000;Kitzbichler & White 2008;Lotz et al 2011), the velocity dispersion of galaxies in groups and clusters which can be used to deduce the dark matter halo (Carlberg et al 1996;Jing et al 1998;Springel et al 2001;Berlind et al 2006; Thomas et al 2011), the external environment of field/cluster galaxies which is related to the formation and evolution of galaxies (Balogh et al 1999;Bell et al 2004;Kauffmann et al 2004;Elbaz et al 2007;Peng et al 2010), the galaxy population which is classified by the fundamental properties such as color, luminosity, metallicity, stellar mass (M * ), star formation rate (SFR), surface brightness and so on (Faber & Jackson 1976;Kennicutt 1998;Cole et al 2001;Baldry et al 2004;Bower et al 2006;Kewley et al 2006). Meanwhile, plenty of information about stellar population, emission line strength/ratio, SFR, metallicity and active galactic nucleus (AGN) activities also can be diagnosed by the spectrum itself (Baldwin et al 1981;Kennicutt 1983;Sanders et al 1988;Zaritsky et al 1994;Calzetti et al 2000;Bruzual & Charlot 2003).…”