Clusters of Galaxies and Early-Type Galaxies
The determination of cluster properties and their evolution provides fundamental insights into the formation of large scale structure in the universe. The following features make clusters ideal for testing cosmological models. Clusters are very luminous and are observed both optically and in X-rays to significant distances (z ~ 1). They have dynamical timescales which are a significant fraction of the age of the Universe so we can watch them evolve over even modest redshifts. Dynamical timescales for clusters are long so the imprint of the initial conditions has not yet been completely erased. The cluster mass spectrum is sensitive to the normalization and slope of the power spectrum on cluster scales and is one of the strongest tests of cosmological models. 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. Finally, clusters provide important information on the origin and abundance evolution of heavy elements in the universe.High redshift clusters
ASCA has made it possible to measure the X-ray temperatures of clusters sufficiently distant to exploit the sensitivity of their evolution to cosmological parameters. Using complete samples of 14 X-ray selected clusters in the redshift range 0.3--0.6 and 25 X-ray selected low redshift clusters, Henry (1997 ApJ 489, L1 and 1998, in preparation) finds Omega_0 = 0.46 +/- 0.10 if the universe is open and is 0.39 +/- 0.10 if it is flat.Additional constraints on cosmological models can be obtained by observing higher redshift clusters. Donahue and collaborators (Donahue et al. 1998 ApJ, in press; Voit & Donahue 1998 ApJ, in press; see also Tucker et al. 1998 ApJ, in press) have used ASCA observations of z ~ 0.8 clusters to show that their temperatures are much higher than expected in an Omega = 1, Lambda = 0 universe.
Studies are also continuing to determine H_0 from ASCA and ROSAT X-ray observations. Combining the measured values of the cluster gas temperature and electron number density with radio observations of the Sunyaev-Zel'dovich effect (the decrement in the radio flux which results as cosmic microwave background photons pass through the hot plasma of a cluster and are shifted to higher energy) yields a direct measure of H_0 to the clusters being studied. Using a sample of 8 clusters Hughes & Birkinshaw (1998 ApJ, in press) find an H_0 of 42 -- 61 km/s/Mpc with an additional random error of 16%.
Temperature maps
ASCA observations of clusters of galaxies are now being used to map the temperature of the hot intracluster medium. These observations show significant deviations from isothermality in a large number of clusters. These observations define the size of the cooling cores seen in some clusters and illustrate the effects of shocks, apparently due to recent mergers. Temperature maps allow the detection of shocks and other merger phenomena long after the X-ray surface brightness has returned to azimuthal symmetry (Churazov et al. 1998 ApJ, in press; Donnelly et al. 1998a,b ApJ, in press and submitted). ASCA temperature maps for about half of all clusters reveal merger shocks. In the A3266 cluster, the optical morphology is consistent with either an impending merger from the NE or a recent merger from the SW. The ASCA spectro-spatial map shows compression and shock heating of the gas perpendicular to the merger axis running from the main cluster to the SW, uniquely favoring the post-merger hypothesis (Henriksen & Donnelly 1998, in preparation). ASCA observations of A754 have been used to build a detailed model of a merger and estimate the amount of mixing of the merging components and the resulting non-thermal pressure support produced in the central regions of the cluster (Roettiger et al. 1998 ApJ 493, 62).
Figure 1: Temperature map of the cluster merger Abell 754 (Henriksen & Markevitch 1996 ApJ 466, L79).
Markevitch et al. (1998, ApJ in press and refs. therein) 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 nonisothermal, with spatial temperature variations (apart from cooling flows) by a factor of 1.3--2. Nearly all clusters show a significant radial 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-Omega_0 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.
ASCA spatially resolved analysis allowed, for the first time, a direct exclusion of the cooling flow regions from the average cluster temperature measurements. Such exclusion (Markevitch 1998 ApJ in press; Allen & Fabian MNRAS, in press) greatly reduces the scatter in the L_X-T and L_bol-T relations. These relations are important characteristics dependent on cosmology and cluster thermal history. The new, cooling flow-corrected cluster temperatures have been used to derive a more accurate cluster temperature function at low redshifts (Markevitch 1998 ApJ, in press). This is even more discrepant from standard Cold Dark Matter predictions than the earlier calculations.
ASCA observations are important in correlating galaxy evolution with the evolution of the cluster itself. Henriksen & Wang (1998 MNRAS, submitted) report results from an X-ray study of the Abell 2111 galaxy cluster (z = 0.23) which show that this classic Butcher-Oemler cluster is elongated and asymmetric, with clumpiness on arcminute scales. It also has a hot core and has probably undergone a recent merger, which may also be responsible for the high fraction of blue galaxies observed in the cluster.
Spatially-resolved temperature measurements have also been used to map the shape of the gravitational potential in clusters. 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). ASCA data have also been used to show the presence of an heirarchical arrangement of dark matter with a central cusp (Ikebe et al. 1995 Nature 329, 427; Xu et al. 1998 ApJ 500, 738; Ikebe et al. 1998 ApJ, submitted).
Elemental abundances
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 measured 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. 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 gravitational field of the cluster.Allen & Fabian (1998 MNRAS, in press) used ASCA and ROSAT data to show that cooling-flow clusters have higher emission-weighted metallicities than non-cooling-flow systems. This is likely due to the cooling flows having increased metal abundances in their cores, where the surface brightness is sharply peaked. As well as the sharply peaked metallicities in cooling flow clusters larger scale abundance gradients have been seen in the AWM7 (Ezawa et al. 1997 ApJ 490, L33) and Perseus clusters (Ezawa 1998 PhD thesis).
Mushotzky & Loewenstein (1997 ApJ 481, L63) measured the iron abundances for a large sample of clusters and found no evidence for evolution implying that most of the enrichment of the intra-cluster medium must have occurred at redshifts > 1. Indeed a z ~ 1 X-ray cluster observed by Hattori et al. (1997 Nature 388, 146) has an iron abundance consistent with local objects. This cluster is extraordinary in being a strong X-ray emitter with gravitational lensing without the expected population of optically-visible galaxies.
Poor Groups
Observations with ROSAT showed that small groups of galaxies often have extensive X-ray emission. While ROSAT observations can be used to map this emission and determine the gas temperature they cannot be used to determine the elemental abundances. Davis et al. (1998 ApJ, in press) have analyzed the ASCA data for a sample of 17 groups. They deduce that either star formation in poor groups is very different from that in clusters or groups lose much of their enriched material via winds during the early evolution of their elliptical members. If the latter is true then enrichment from poor groups makes up approximately 1/3 of the metals in the intergalactic medium.Early-Type Galaxies
ASCA observations of early-Type galaxies have provided important clues to the generally large scatter in the L_X/L_B ratio found by earlier studies. Matsumoto et al. (1997, ApJ 482, 133) report that the total X-ray emission from 12 early-type galaxies is a composite of a soft, 0.3 -- 1 keV and hard > 10 keV (or powerlaw) component. The X-ray luminosity of the hard component correlates well with the optical luminosity while the soft component is diffuse halo emission. Generally low elemental abundances with Solar ratios are found for the hot gas component. More detailed analysis using very long ASCA observations has been possible for individual galaxies. For example, the abundance map of NGC 4636 shows a steep radial abundance gradient which drops from Solar within 4' of the center to 0.2-0.3 Solar at 10' (Matsushita et al. 1997, ApJ 488, L125). Matsushita et al. (1998, ApJ 499, L13) report on a very long re-observation of NGC 4636 which shows that the emission extends out to > 25' (~ 300 kpc). The M/L ratio is 22 within 20 kpc increasing up to 300 at the observed extent of the galaxy. The extended nature of the emission and large M/L makes the galaxy a candidate ``dark group''.
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