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Cluster Research with X-Ray Observations RICCARDO GIACCONT AND RICHARD BURG Space Telescope Science Institute ABSTRACT Past X-ray surveys have shown that clusters of galaxies contain hot gas. Observations of this hot gas yield measurements of the fundamental properties of clusters. Results from a recent study of me X-ray luminosity function of local Abell clusters are described. Future surveys are discussed and the potential for studying the evolution of clusters is analyzed. INTRODUCTION The systematic study of clusters began with the surveys of Abell (1958) and Zwicly e! al. (1968) who each created well-defined catalogues according to specific definitions of the object class. In particular Abell defined clusters as overdensities of gal~es within a fixed physical radius around a center, classifying such objects as a function of their apparent magnitude (distance) and of their overdensity ("richness"~. The first X-ray survey of the sly by the UHURU X-ray satellite showed that "rich" nearby clusters were powerful X-ray sources (Gursly et al. 1971; Kellogg et al. 1972~. Subsequent spectroscopic studies detected X-ray emission lines of highly ionized iron and demonstrated that the X-ray emission was produced by thermal radiation of a hot gas with temperatures in the range of 30 to 100 million degrees (Mitchell et al. 1976; Serlemitsos et al. 1977~. With the launch of the HEAO1 and the Einstenl Observatories, surveys of significant samples of nearby clusters demonstrated that as a class, clusters of galaxies are bright X-ray sources with luminosities between 1042 112

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HIGH-ENERGY ASTROPHYSICS 113 and 1~5 ergs/see (Johnson et at. 1~3; Abramopoulos and Ku 19~; and Jones and Forman 1984~. The increased sensitivity of the Einstein imaging detectors also provided the capability to study clusters at large redshifts (z > 0.5) (Henry e! al. 1979~. The general problem one wishes to attack by means of X-ray obser- vations is the study of the formation and dynamic evolution of structures consisting of gravitationally bound galaxies. It has been pointed out by sev- eral authors (Kaiser 1986; Shaeffer and Sink 1988) that X-ray observations of such systems may offer important advantages with respect to studies other wavelength domains, particulars at early epochs of the universe. In Table 1 we list the fundamental properties of clusters that can be measured in X-ray surveys along with a brief description of the measurement. We also include, for comparison, the analogous measurement in the optical In order to be efficiently detected in X-rays, such systems can be empirically defined as having the following properties: 1. They must contain sufficient intergalactic gas (typically 1/10 of the cluster mass). The gas must have been heated to X-ray emitting temperatures typically larger than those corresponding to escape velocity from a single galaxy. It should be noted that the efficiency of X-ray emission depends on metallici~y. The gas must be centrally concentrated in the cluster, (Lo ~ p2), although not more so than the galaxies in nearby observed systems. Such properties have been shown to exist in Abell-~e clusters, as well as In much poorer systems such as cD groups (Kriss et al. 1980~. Thus the class of X-ray luminous clusters of galaxies which may be retrieved In future sensitive X-ray surveys in a sufficiently soft X-ray band (for example 0.1-2 Key), will include both optically defined classes of rich clusters (such as Abell or Zwicky) as well as poorer clusters or any gravitationally bound system of galaxies containing high-temperature gas. The Eu~steu' ObseIvatory Medium Sensitivity SuIvey which uses the IPC data (0.35 to 3.5 keV) has in fact detected a number of optically poor X-ray emitting clusters (Gioia et al. 19~. X-RAY LUMINOSITY [IJNC1ION We would like to briefly summarize some of the recent work by Forman, Jones, and ourselves on the X-ray luminosity function of Abell cluster as an introduction to the subject. The earliest determinations of the X-ray luminosity function for Abell- like clusters of galaxies were based on the UHURU and Ariel surveys.

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114 m~ to WE E id| c,,E E CJ Al ,C of s-~ ~ ~hi' Cot ~ ~ 8 ~ ~E ~8 -

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HIGH-ENERGY ASTROPHYSICS 115 Schwartz used a sample of 6 clusters and McHardy a sample of 20 clusters to derive luminosity functions. Extensions of these first attempts included the analysis of HEAO1-A2 data (Piccinotti et ale 1982) with samples of 30 clusters. More recent surveys with HEAO1-A2 were based on 128 detected clusters (Johnson e! al. 1983~. Finally, Abramopoulos and Ku (1983) used Eu~steu~ imaging observations of 74 nearby clusters with z < 0.27. It should be noted that while the UHURU, Ariel and HEAO1 surveys were X-ray flux limited surveys which, in principle, could have studied the X-ray luminosity of a more general cluster population, they were severely biased by their sensitivity and energy range to He detection of Abell-like rich, high-temperature clusters, although some X-ray emitting groups were observed (Schwarz et al. 1980~. These determinations of the X-ray luminosity function of Abell-like clusters were based on relatively small samples and had intrinsic limitations or deficiencies. In particular, previous determinations of the Abell-like clus- ter X-ray luminosity function did not take into account the incompleteness for richness O clusters (Abramopolos and Ku 1983), did not include richness O clusters (Kowalski et al. 1983), or were not sensitive to low temperatures because of their effective energy band (Piccinotti et al. 1982; Kowalski et al. 1983~. The last limitation could be quite important in attempting to understand the low end of the X-ray luminosity function since there ap- pears to be (at least for R > 1) a correlation between X-ray luminosity and temperature ~Iusho~zky 1988~. In our recent work we have investigated the statistical properties of the 226 Abell clusters with z < 0.15 observed by the Einstein Observatory (Burg et al. 1990, hereafter referred to as BFGJ). This sample is taken from the larger compilation of Einstein cluster observations analyzed by Jones and Forman. We show that this set of clusters form, for the purpose of this work an unbiased sample of Abell clusters that spans richness classes O to 2. We use the Einstein sample to derive an X-ray lum~nositr function which is free of some of the problems which beset previous analyses. The main advantages of this determination are: the ability to detect low- temperature clusters because of the energy band (0.5 to 4.5 keV) (in common with the Abramopoulos and Ku surveys; the larger sample which allows us to adopt stringent criteria to insure completeness and allows us to determine the X-ray luminosity function for different richness classes; and the higher sensitivity which allows us to explore the low-luminosity end of the luminosity function. The redshift limit of 0.15 was chosen for our sample since within this range the Atoll richness classification is distance independent Furthermore we have established that for redshifts < 0.15 there is no correlation between redshift and X-ray luminosity (see Figure 1~. Thus this subset of the Abell catalog can indeed be considered a proper sample to derive the shape of

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116 o o o o o o o a' Y o _ U. _ ~ - x J g O _ U. AMERICAN AND SOVIET PERSPECTIVES o o - ~ V 0.01 0.05 0.1 o.5 redshift FIGURE 1 Scatter diagram of X-ray luminosity versus redshift for richness 1 clusters. the X-ray luminosity function by richness class since the entire range of X-ray luminosity can be observed throughout the chosen volume. The IPC fluxes from the compilation of Jones and Fran (1990) have been obtained by integration over a region of 1 Mpc radius centered on the X-ray determined cluster center. These fluxes are computed in the 0.5~.5 keV (observed) band, from the observed counting rates, using the hydrogen column density and either the observed or estimated gas temperature. The estimated gas temperature is computed using the observed lummosi~r temperature relation, which we have redenved for our sample and which is given by Lee or T5/2 (Mushotzky 1988). The luminosity at the source is computed utilizing the measured X- ray flux and the measured or estimated redshifL K-corrections have been computed using Me Raymond-Smith model, assuming 0.5 solar metallici~, and they are of order 20% over the redshift and temperature range of the sample (Burg and Giacconi l990~. The method used for computing the

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HIGH-ENERGY ASTROPHYSICS 117 luminosity function is dictated by the criteria used in selecting the sample. In this work the undertring sample is defined by optical properties (he. the number of galaxies ~ an Abell radius) and not by X-ray properties. Thus the computed luminosity function is a bi-vanate function of both Lo and cluster richness R. The sample is volume limited in the sense that the Abell sample (with the same redshift cutoff is volume limited and with the same in- completeness problems. Therefore each cluster contributes 1/V to the luminosity function This is different from the methodology used for an X-ray flux limited survey where each cluster would contribute VmaX(s km, L=) 1b calculate the cumulative luminosity function, we use the Kaplan- Meier product limit estimate method (Cox and Oakes 1984; Schmitt 1985; Feigelson and Nelson 1985~. This is equivalent to the techniques developed by Avoi et al. (1980~. Specifically, the following probability is calculated: ( ~) 0` N < ' ~ This is the unnormalized cumulative luminosity function and is formally the minimum likelihood estimate of the luminosity function The results are shown in Figure 2a. Since our X-ray sample Is not an independently complete sample, we must rep on the understanding of the completeness characteristics of the Abell sample to derive the normalization. The Abell Catalogue is known to be incomplete for richness class 0. Abell recognized this and did not include the richness class 0 objects in his "statistical" sample. Later work by Bahcall (197~, see also Lucy 1983), based on analysis of the multiplicity function (the number of clusters per unit volume versus nchness) has shown that richness 0 clusters are incomplete by a factor of > 3. Quantitatively we fit Schechter functions to the data with the results shown in Figure 2b. It must be stressed that the normalization does not affect the shape of the luminosity function. Some aspects of our results are immediately apparent: a. The shape of the luminosity function is similar for each richness class, although there is a change in scale. b. Richer clusters are systematically brighter in X-rays. The value of Lid, the characteristic luminosity for each richness class is roughly proportional to (N*,a')~' (where N* is the characteristic number of galaxies per richness class, Lucy 1983), by = 1.6 ~ 0.4.

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118 CO tar CC -a: US ~ c c) Cat ._ Q ~(D _ - z 0 _ U. AMERICAN AND SOVIET PERSPECTIVES ~ _ Ro ~ in_ - R1 R2 $ `+` I'd fit ;7~ ~+ 1 and R _ 0. We show both cases since for R > O we are dominated by the

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HIGH-ENERGY ASTROPHYSICS 119 cry '' (D' FIGURE 2b Schechter function fits. - it-l? Ro \ \ R2 1 1 ~I 1 1 50 500 5000 50000 Luminosib(1 .0e40) normalization uncertainty. On the same plot we include previously reported luminosity functions. We agree at the high-luminosity end with previous results and we are not inconsistent at the low-luminosity end where use of the Bahcall normalization, to take into account R = 0 incompleteness, has the biggest effect. As mentioned above, the X-ray luminosity which we measure must be in all cases the sum of the X-ray luminosity of the individual galaxies plus a component due to intergalactic hot dense gas. For evolved systems we would elect this latter component to dominate. For instance a cluster such as Coma (R = 23 has an X-ray luminosity of 3.7 x 1044 ergs s~i, while the integrated emission from single galaxies is about 1043 erg sol. A1367, which was previously studied in some detail (Bechtold et al. 1983) could be an example of a cluster where summed galaxy emission is a large fraction of the total cluster luminosity. A1367 is a loose, seemingly unevolved structure, the relatively low temperature of 4 keV. In their work Bechtold et al. showed that the summed contribution of the 10 brightest galaxies was ~ 3 x 1042 ergs/sec. This is 5% of the total

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120 AMERICAN AND SOVIET PERSPECTIVES CD U' CD ID of - - * 1 1 1 50 60 70 80 90 N. L* oo N-(1-6 ~ OF) FIGURE 3 Charactenstic luminosity versus richness. luminosity of the cluster. Roughly 80% of the galaxies (brighter than L*) were not detected as individual sources (the high background due to the intracluster medium causes a high~etection threshold) but they may still have contributed significantly to the cluster emission. We note that in our data the minimum observed X-ray luminosities for R = 0, 1 and 2 clusters are 3 x 1042, and 8 x 1042 and 3 x 1~3 erg sec~i, respectively. In the three cases the expected summed contribution from individual galaxies (Forman et al. 1983) is roughly 4 x 1042, 6.4 x 1042 and 1043 erg sec~i respectively. This is evidence that there is a substantial number of clusters in which the X-ray emission from intracluster gas is not dominant. INTERPRETATION OF THE LUMINOSlXY FUNCTION Studying the wide range of X-ray luminosity of clusters selected by richness class (presumably therefore of given total mass) we are in a position to emphasize a fundamental proper of X-ray emission from clusters.

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HIGH-ENERGY ASTROPHYSICS 121 co ~, . 0 a' En Q~ ~ ~a) a' Cal ._ Cal ao~ Qa) A am ~,. Cl) * R>0 R> l + * O + o O ~ O + o o Kowalski et al o 1 1 1 1 1 00 1 000 1 0000 1 00000 Luminosity (1 e40 ergs/see) FIGURE 4 Summed non-parametric luminosities for R > 0 and R > 1. Also included is HEAOl-A1 R > 1 luminosity function. Specifically we can attempt to separate out the factors which determine the X-ray emission and relate them to initial conditions and dynamic evolution of the cluster. Using the thermal bremmstrahlung expression we can wnte: Lo ~ PgasM gaSTg/s Using the vinal theorem and equipartition we can relate T,a5 to the vinal gas mass, Mv, and density, pv, kT~ GMV Rv GM., MV ~ \4~PV J or To M213pi/3 and Lo ~ PgasMgasMv Pv

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122 We can then postulate: and then AMERICAN AND SOVIET PERSPECTIVES MU (O) fp (1 + I1 ((t)) pgas = VuLt) MgaS = MV(O)fp(1 +-((t)), fp LO ~ fp (1 Jr f1 ((t))2P716MV (0)4/3 p This allows us to separate the venous contributions to the X-ray luminosity namely: 0 the initial fraction fp of gas and any injection mechanism of gas after formation; a term which includes the effects of cosmology and dynamical evolution; the vinal mass of the cluster. We are currently working with ~ Cavaliere (Cavaliere et al. 1989) on a refinement of this approach, where we directly relate these venous quantities to the density fluctuation spectrum, ~2 ~ kn. This results in an expression for Lo of the form LO = A.,a+b+cM< in which a represents the cosmological term, b the dynamical term, and c the gas injection or stripping mechanisms. In summary: The X-ray emission of a cluster depends not only on its mass but also on the initial conditions, the subsequent dynamical evolution of the system and the mechanism and history of the gas injection. Studying the luminosity function at the current epoch is not suffi- cient to disentangle the various contributions. On the other hand, coupling the local luminosity function for each range of masses win luminosity functions for the same mass objects at different redshifts will allow us to study in detail the various processes ~ We evolution of the gas.

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HIGH-ENERGY ASTROPHYSICS 123 Future X-ray surveys will allow us lo obtain direct information on the initial density fluctuation spectrum. F[TrURE SURVEYS We focus our attention in this discussion on only two missions: RO SAT, the precursor survey mission to be launched in 1990 and AXAF, the follow- on mission which will dominate X-ray astronomy for many years after its planned launch in 1996. In Able 2, we summarize the principal char- acteristics of the two missions from the point of view of the relevant instrumentation. Figure 5 shows the effective area of ROSAT and AXAF as a function of energy. It is clear that with these capabilities many of the investigations initiated with Einstein can be pursued with greater depth and scope. They can be roughly divided in a few general headings. (See Able 3) A good starting point to understand the impact of the new X-ray missions on the topics of cluster research is the review "The Advanced X-ray Astrophysics Facility", a special volume of Astrophysical Letters and Communications (VoL bf 26, 1987), and references herein. We would like to discuss here in some detail only one aspect of the program in which we are personally interested. It deals with the study of cluster formation and evolution by means of surveys win ROSAT and AXAF. The Pro missions diner in a fundamental way. For each cluster the X-ray observations can yield two basic quantities: 1. The X-ray surface brightness distn~ution at the source as well as its integral. 2. The spectrum (and/or temperature) at each point or (for weaker sources) the spectrum (or temperature) of the integrated source emission. The detection of redshifted line emission from heavy el- ements may permit direct determination of the redshift and metal- licity. 1 and 2 together yield a direct measure of Mv (the vinal mass). As we discussed above, X-ray emission from a cluster will ul~natel~r depend both on the initial conditions at formation and on its state of chemical and dynamic evolution. Thus, at least two obsenables will be required to characterize each cluster, in order to derive its properties without a very large number of simplifying assumptions. Basically, the ROSAT experunents only yield one quantity (the distribution of Lay. Future missions such as ASTRO-D, Jet X and XMM will yield spectra with some angular resolution for the nearest systems but only integral luminosity and integral spectra for the more distant ones. Only with AXAF will one have

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124 TABLE 2 AMERICAN AND SOVIET PERSPECTIVES Telescope ROSAT AXAF . . Diameter 84 cm 120 cm diameter Number of surfaces 4 6 field of view 2 degrees l degree geometric area 1141 Cm2 1700 cm2 Angular resolution Center of field ~3" - ~.5" 8' off axis ~5" ~3.5,, 30' off ems ~60" ~60,' Imaging Instruments (field, spectral resolution) HRI~40', none HRI~30', none PSPC~12~. CCD~14' ~E~.43 at 1 kev AE~150 eV (.5~) keV Spectroscopy Transmission gratings high energy R~100 at 4.5 keV, R~700 at .4 keV low energy R~100 at 1.5 keV, R~750 at .1 keV Calorimeter .78 sq. arc min. AE- 12eV

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HIGH-ENERGY ASTROP~ICS 125 i. FIGURE 5 Companson of ROSAT and AXAF. .. .. . ... . AXAF ROSAT .. ... .. . . . ., \ \ \ , . 0.1 0.5 1.0 5.0 Energy (koV) high-anguLdr resolution coupled with spectral resolution for the most distant detectable clusters (A z ~ 1~. The ROSAT all-sky survey is an important precursor to AXAF. No cluster for which one plans to do detailed spectral or morphological studies with AXAF can be much fainter than those detected in the ROSAT all- sly survey. The detection capabilities of ROSAT are such that we expect to detect a large sample of clusters. Figures 6 and 7 show the Log N- I-og S relation derived by Burg and Giacconi (1990) for z < 0.5 and 1.0 respectively. These figures are derived from the luminosity function of Burg et al. 1989 for RgeO Abell clusters at z < 0.15. It is important to note that uncertainties in the normalization parameter (of the Schechter functions fitted to the local luminosity functions) result in large uncertainties in Log N - Log S. However, the relative z distribution of clusters by richness class (Figure 8) assuming no evolution is a direct consequence only of the shape of the luminosity function, which is much better established. This means that we expect to detect in the ROSAT surveys a substantial fraction of

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126 TABLE 3 AMERICAN AND SOVIET PERSPECTIVES Individual galaxies-interaction with the intracluster medium - Gamy halos - Galaxy stripping (M86) - Cooling flows (M87) The intracluster medium - p, p (r) - metallicity (spectra from SSS Einstein) Cluster emission :Morphology and state of dynamic evolution (double clusters) :S(r) = S(O)(1 + r2/a2~~34+l/2 surface brightness density profiles Correlations : Lo vs N : Lo vs No : L: vs Tga~ : Lo vs % spiral : is= ~ me IC Sagas Cluster evolution evolution of intergalactic medium with z (evolution of a single cluster in time) evolution of the luminosity function evolution of metallicity Cluster formation and cosmology dark mass correlation functions protoclusters primordial gas Zeldovich-Sunyaev effect, Krolik-Raymond method: Ho, qO clusters at 0.5 < z < 1.0. However, the angular resolution of the PSPC, the detector with which the survey will be conducted, is of order ~ 30" and except for nearby clusters (z < 0.2) it will be difficult from X-ray observations alone to classifSr me objects as clusters rather than stars or AGNs. A collaboration between the Max Planck Institute, STScI and ROE is planning a rapid (quasi real-time) classification of X-ray sources in the all sly ROSAT survey by utilizing existing ground based optical sly surveys. The purpose of this program is to petit the follow up of PSPC detections

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HIGH-ENERGY ASTROPHYSICS to ._ as Ct a' - ~n O Q cn A __ By 127 US 3K OF ++ + 3- - ~ MSS ~,, 7 0~ i_ ~ J ATE- 7~ 7~ Lo my ~ ~ O , ~ ~ ~7 am 7rx~L7 A,` Total Piccinotti 3< 1: V art e-15 e-14 e-13 Lo 7~ 1 1 1 1 e-1 2 e-1 1 e-1 0 e-9 S (ergs/s/sq. cm) Zcut=.5 FIGURE 6 Log N-Log S for clusters using BEGJ luminosity functions (Burg et al. 1989) with no evolution and cutoff redshift of 0.5. Of particularly interesting clusters with HRI high angular resolution obser- vations capable of resolving the details of the cluster morphology. This classification technique relies on utilizing all available X-ray and optical information and the observational constraints derived from the MSS survey to divide the ROSAT survey sources into appropriate bins. The subset tional base for discrimination is given by studies such as those of Maccacaro and Gioia on the MSS sources which are shown in Figure 9. In Figure lo we show a tentative conceptual flow diagram which will be refined and implemented in an automated expert system. We hope by these means to isolate an enriched sample of distant clusters which could then be further studied with later X-ray missions. What we hope to achieve from the ROSAT all-sly survey is the following:

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128 CC ._ ~ 8 a) _ - a) Q o - - z AMERICAN AND SOVIET PERSPECTIVES A o o A A A A o A A o _ 0; + + I___ MSS 9~ _ ~ ~ R~ Total I Piccinohi ;: | L PHI o ~, , , ~ ~9 o o 1 e-15 e-14 e-13 FIGURE 7 Log N-Log S with redshift cutup of 1.0. e-1 2 e-1 1 e-1 0 e-9 S (ergs/s/sq. cm) Zcut=1 .0 o X-ray flux measurements (or upper tidbits) for all classes of optically defined clusters (whether Abell, Zwicly or poor groups). We will be able to derive bivariate lutnitlosity functions with methods similar to those used for the Einstein sources (Burg et al. 1990~. A flux limited X-ray survey for all types of clusters defined as gravitationally bound systems of galaxies containing intracluster gas with a high degree of metallicity and central condensation. Such surveys are naturally biased toward systems which are both chemically and dynamically evolved. Detailed study of morphology and rough temperature determina- tion (for low-temperature systems) for all nearby systems. Point lo point correlations (with a large sample of X-ray defined clusters) to study large-scale structure.

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HIGH-ENERGY ASTROPHYSICS RO /\ 129 R1 o o ~ ~ ._ C: o l - 0.0 0.2 0.4 0.6 0.8 1.0 redshift R2 Cat o L o . CO Cat o o to to l - 1 0.0 0.2 0.4 0.6 0.8 1.0 redshift / / / ~ 0.0 0.2 0.4 0.6 0.8 1.0 redshift FIGURE 8 Redshift distribution of clusters to Smin = 10 - 13 ergs s-1 cm-2 Study of prevalence of cooling flows, galactic halos, (both in clusters and in isolated systems), etc. for nearby systems. Confrontation between model-predicted redshift distn~ut~ons and observed redshift distn~utions for optically characterized subsam- ples. The tremendous advantage of AXAF with respect to ROSAT is glen by two specific technical improvements coupled together: the high~uantum efficiency and spectral resolution of the CCD detectors coupled to a high- angular resolution telescope. within the field of view of the CCD, any cluster, at any z, which is detected, can be resolved as an extended structure.

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30 1 o -2 -3 Sit 4t . ' ~ I 6 8 10 12 14 16 163 AMERICAN AND SOVIET PERSPECTIVES 2 am 8 Cc C ~ ~ ,, K and M stars _' O I,, aim. _ f km 1 0-13 ergs s-1 cm 2 At' = ~normal. star ~ = quasar C = cluster of galaxies G = "normaJ. galaxy ~ = BL Lac CV = camclysmic variable M`, FIGURE 9 Classification of Einstein Medium Sensitivitr Survey. 20 22 For given models of cluster evolution we can estimate the characteristic core radius of clusters at different z's: For n = -1 and Friedman cosmology with q* = 1/2 and H$ = 50 lans~i Mpc-i the linear dimensions of clusters at Z = 1 and 2 would correspond lo angular diameters of 15 to 7 arc seconds. This angular extent can easily be measured by AXAF within the field of view of the CCD. AXAF therefore possesses He angular resolution and spectroscopic capability to directly determine the angular extent of a cluster as well as the ability to directly measure its redshift (if the X-ray emitting gas is enriched). Figure 11 shows a simulation carried out by G. Garmire and colleagues of the expected surface brightness profile and spectrum which drill be obtained with the AXAF COD X-ray camera for a rich cluster at different redshifts. SUMMARY With ROSAT observations we will obtain the only all-sly X-ray flux limited sample of clusters for years to come. We might be able to show the existence of evolutionary effects by extensive

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HIGH-ENERGY ASTROP~ICS g4500 1 Sx22x 10 13e~cm2 so 1.1 -2keV- 100.000sourcesl r Point-like in X-ray ; 51500 ~ Star or Galaxy ma 15.5 Star or Gal. mV c 15.5 (3% false coinc. stars) ! it, N.: N21l ~34~3 49600 1400 500 ~ r27~ Moo 1 Extended In X-ray 1~ N . 1 N >~1 34 1 00 5000 10000 ~ Stellar Object Galaxies 1 9 < mV ~ 20.5 15.5 ~ TV < 20.5 Stellar Object 1 5.5 < ~ ~ 1 9.0 ~ , ~ ~- P~ 1 rO Proper | Mown J I Motion I 1 2~ 131 N.8.: ID includes - 1300 False Coinc. ]6000 _ / 3400 \ / LLAGN's \ \ 8 / \ 1300 / \Clusters/ 1 N.1 N>~1 1 ~ _ 1000 16000 450 1000 8950 10000 4700 1300 - ~ -' ~11 ~- 1 - ~s-1 11 - ~ o 1 1 -1 slog fx/tv<0 1 1 FIGURE 10 Proposed categorization algorithm for ROSA:I: optical follow-up (to measure z) and by studying the z distribution of carefully selected subsamples. However, the causes for the apparent evolutionary effects, if any, will be difficult to determine. In particular, given the lack of X-ray spectroscopy we will be unable to measure the evolution of metallicity and gas of temperature and mass (except with very restrictive assumptions).

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132 - a, ~o N N N O O 0 0+ 0 0 _ _ ~ l - l A91-~S- zWO;SlNnOO - (:, 2 ~ O ,

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HIGH-ENERGY ASTROPHYSICS 133 With the advent of ASTRO-D and Jet X, etc., which will combine moderate angular and spectral resolution capabilities with a wide bandwidth, we will be able to directly measure redshift, metallicity and temperature. For low-redshift systems (z < 0.25) we will be able to also measure the distribution of surface brightness, temperature and metallici~. This will enable us to determine the state of dynamic and chemical evolution and directly measure the dark mass. a Similar measurements can be extended by AXAF to very large redshifts, from z > 0.25 to z of order of ~ When this program is completed there will emerge a new understanding of the formation and evolution of these objects, among the most massive gravitationally bound structures in the Universe. Furthermore, we hope this will lead to a more complete picture of the structure of the Universe. REFERENCES Abell, G.O. 1958. Astrophys J. Suppl. 3: 211. Abramopoulos, F., and W. Ku. 1983. Astrophys. J. 271: 446. Avni, Y., A. Soltan, H. Tannenbaum, and G. Zamorani. 1980. Ap. J. 238: 800. Bahcall, N. 1979. Ap. J. 232, 689. Bechtold, J., W. Forman, R. Giacconi, C. Jones, J. Schwarz, W. Itcker, and ~ VanSp~r broeck. 1983. Astrophys J. 265: 26. Burg, R., and R Giacconi. 1990. In preparation. Burg, R., W. Foreman, R. Giacconi, and C Jones. 1990. In preparation, (BFGJ). Cavaliere, A., R. Burg, and R. Giacconi. 1989. In preparation. Con, D.R, and D. Oakes 1984. Analysis of Survival Data, Chapman and Hall, Cambridge. Feigelson, E.D., and PI. Nelson. 1985. Ap. J. 293: 192 Forman, W., C Jones, and W. Ill en 1983. Astrophys J. 293, 102. Gioia, I.M., MJ. Geller, J.P. Huchra, ~ Maccacaro, J.K Steiner, and J. Stocke. 1982 Ap. J. Otters), 255: L 17. Gursly, H., ~M. Kellog, S. Murray, C Leoug, H. linanbaum, and R Giacconi. 1971. Ap. J. (Letters) 169: L81. Henry, J.P., G. Brandvardi, V. Bnel, D. Fabn<:ant, E. Feigelson, S. Murray, ~ Soltan, and H. ~ - nbaum. 1979. Ap. J. fretters) 238: L15. Johnson, M.W., RAG Cruddace, M.P. IJlmer, M.P. Kowalski, and KS. Wood. 1983. Ap. J. 266: 425. Jones, C, and W. Forman. 1984. Ap. J. Z76: 38. Jones, C, and F. Forman. 1990. In preparation. Kaiser, N. 1986. M.N.RAS. 222: 3~. Kellog, E.M., H. Gursly, H. Tanabaum, R Giacconi, and K Pounds 197Z 174: L65. Kowalski, M.P., M.P. Ulmer, RG. Guddace. 1983. Ap. J. 268, 540. Kriss, GA, CE. Canizares, J.E. Mcaintock, and E.D. Feigelson, 19~, Ap. J. (Letters3 235: L61. LO I, J.R. 1983. M.N.RAS. 204: 33. Mitchell, RJ., J.I~ Culhane, PJ. Davison, and J.C Ives. 1976. M.N.RAS. 176: 29. Mushotzly, R. 1988. Proceedings of the NATO summer school on Hot Astrophysical Plasmas, Pallu~cani, R., Editor. Piccinotti, G., RF. Mushotzly, E.F. Boldt, S.S. Holt, F.E. Marshall, PJ. Serlenitsos, and RAT Shafer. 19~ Ap. J. 253: 485. Schmitt, J.H.M.M. 1985. 293: 198. Serlemitsos PJ., B.W. Smith, E.A. Boldt, S.S. Holt, and JA Swank. 1977. 211: L63. Shaeffer, R. and J. Silk. 1988. Ap. J. 333: S09. Shwartz, D^, M. Davis, RE. Doxsey, R.~ Gnffiths, J. Huchra, M.D. Johnston, RF. Mushotzly, J. Swank, and J. Tony. 1980. Ap. J. (Letters) 238: LS3. Stocke, J.T et al. 1983. Ap. J. 273: 458. Zwicly F., ~ Hotdog, P. Wild, h1. Karpowicz, and C:C Kowal. 1961-1968. Catalogue of Galaxies and Clustets of Galaxies Volumes 1~. Caltech, Pasadena.