Clusters of Galaxies

Cluster Surveys

The determination of cluster properties and their evolution provides fundamental insights into the formation of large scale structure in the universe. Clusters are ideal for testing cosmological models. They are sufficiently luminous to be observed to z ~ 1. They have dynamical time-scales that are a significant fraction of the age of the Universe so we can watch them evolve over even modest redshifts. Further, the masses and scales of clusters are such that they should comprise a fair sample of the mass of the Universe, with representative fractions of the different components. Since cluster formation by gravitational collapse is fairly well understood, comparisons of cluster properties with theoretical predictions for different cosmological models can be used to distinguish among these models.

The ROSAT PSPC detector had a 2° field-of-view and excellent sensitivity to low surface brightness structures so the ROSAT archive has proved particularly fertile ground for building unbiased samples of clusters. Major projects have included the SHARC (Collins et al. 1997, ApJ 479, L117; Romer et al. 2000, ApJ in press), WARPS (Scharf et al. 1997, ApJ 477, 79), RIXOS (Castander et al. 1995, Nature 281, 59), RDCS (Rosati et al. 1998, ApJ 492, 21), and CfA-ROSAT (Vikhlinin et al. 1998, ApJ 502, 558) surveys. All these groups work by searching large numbers of PSPC fields using various algorithms to detect extended sources. Easy access to the data enables different algorithms to be tried and compared leading to more reliable results. The consensus result from all these surveys is that there is little evidence for evolution in the cluster luminosity function out to redshifts of ~ 0.75 and that massive, virialized clusters exist at this redshift (Fig. 3). This provides evidence independent of the SNIa and CMB results for either a low OMEGA or a non-zero cosmological constant. This work will be extended as the Chandra and XMM-Newton serendipitous survey data become available leading to tighter constraints on, and testing of, cosmological models.

luminocity function
The X-ray luminosity function of z=0.3-0.7 clusters from SHARC. The hashed band is the local luminosity function. The figure is from Nichol et al. (1999, ApJ 521, L21).

Temperature maps

The HEASARC archive infrastructure has made possible detailed studies of large samples of nearby clusters of galaxies. Markevitch et al. (1998, ApJ 503, 77; 1999, ApJ 527, 545) analyzed the ASCA spatially resolved spectroscopic observations for a sample of 30 bright, nearby clusters and derived their projected gas temperature profiles and, on a coarse spatial scale, their two-dimensional temperature maps. All clusters were found to be non-isothermal, with spatial temperature variations (apart from cooling flows) of a factor of 1.3--2. Nearly all clusters show a significant temperature decline at large radii. This decline corresponds to the total mass within 1 and within 6 core radii being approximately 1.35 and 0.7 times the isothermal beta-model estimates, respectively. Thus the gas fraction at large radii is larger than had been estimated under the assumption of isothermality. This result strengthens the argument for a low-OMEGA0 cosmology, based on the high baryon fraction in clusters. It also implies a strong segregation of gas and dark matter, possibly indicating that sources other than gravity have produced a significant fraction of the gas thermal energy. The decline in temperature with radius is steeper than predicted by any published hydrodynamical simulations.

The easy availability of archive data has also enabled this result to be checked resulting in a lively controversy. White (2000, MNRAS 312, 663) has analyzed archival ASCA data, Irwin, Bregman & Evrard (1999, ApJ 519, 518) archival ROSAT PSPC data, and by Irwin & Bregman (2000, ApJ in press) archival BeppoSAX data. All these authors find that in most cases the temperature does not decrease significantly at large radii. Chandra and XMM-Newton are eagerly awaited to resolve this disagreement.

Spatially-resolved temperature measurements have also been used to map the shape of the gravitational potential in cluster cores. Allen (1998 MNRAS 296, 392) used ASCA and ROSAT data on a sample of 13 clusters to compare X-ray and gravitational lensing mass measurements. He found excellent agreement in cooling flow clusters (which have strong central X-ray surface brightness peaks) but in non-cooling flow clusters the central masses determined from the X-ray data are 2 -- 4 times smaller than those from strong gravitational lensing. In these latter cases there is evidence for the X-ray emitting gas being in a complex dynamical state so the assumptions used to calculate the gravitational potential from the X-ray data will not be valid (see also Ota et al. 1998 ApJ 495, 170; Boehringer et al. 1998 A & A 334, 789).

Elemental abundances

Measuring elemental abundances in clusters of galaxies provides important clues to their evolution and star formation histories. The total elemental abundance is an indicator of the total star formation rate while the relative abundances of different elements can be used to discriminate between different types of supernovae and even between different supernova nucleosynthetic calculations. ASCA was the first X-ray astronomy satellite capable of measuring the abundance of elements other than iron in large numbers of clusters. Fukazawa et al. (1998 PASJ 50, 187) have used the ASCA archive to measure the Fe and Si abundance ratio in a sample of 40 clusters. They confirm earlier results that most of the intra-cluster medium enrichment was due to type II supernovae (SNe II).

However, the Si-to-Fe ratio decreases in poorer clusters, indicating that for these objects some of the SNe II products were able to escape from the cluster. It is likely that these SNe II occurred following a burst of star formation at high redshifts. Evidence for this has come from measurements of Fe in a large number of clusters (Mushotzky & Loewenstein 1997, ApJ 481, L63). These results are consistent with no evolution and imply that most of the enrichment of the intra-cluster medium must have occurred at redshifts > 1.


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