1. Scientific objectives for XEUS: Galaxies Groups and Clusters at z~2 Study of the Evolution of clusters in the mass range kT > 2 keV up to z=2. Dynamics, Cooling Processes, Mixing in moderate Z clusters. Groups (kT < 2 keV) detection and study. mainly from the S.R.D. (M.Arnaud)

2. Figure 6.1 A Spectrum of a kT = 2 keV cluster (L bol = 5.3 10 43 ) ergs/s at z= 2 convolved with the XEUS STJ response. The back line is the corresponding background in the extraction region (15” in radius). Figure 6.1 B Simulated XEUS STJ spectrum (after subtraction of KNOWN BACKGROUND) of a kT = 2 keV cluster (L bol = 5.3 10 43 ) ergs/s at z= 2 observed for 100 ksec. The abundances and temperature are measured with accuracies of ±2.7% for kT, ±20% for O, ±25% for Ne, Mg, Si and ±11% for Fe (1  errors). a 510 43 erg/s, kT=2 keV Cluster with STJ Comparison with Background WARNING: Confusion with Sky Lines  Need GOOD BKG Subtraction…

3. What Field of View for XEUS ? Figure 6.2: left: Typical cluster diameter as a function of redshift for 3 temperatures. The bottom corner defined by the thin lines corresponds to the most important range for evolution study (kT < 5 keV, 0.5 < z < 2). Right: core size. The cluster diameter corresponds to a density contrast of 500 (2 r 500 ) as compared to the critical density at the cluster redshift (roughly the virial radius). The core size is defined as 0.07 r 500 (0.1r 200 ). A standard self-similar evolution for a  CDM cosmology is assumed (see Appendix for details). FOV is the main limitation of the reference XEUS mission for cluster studies. Extending the FOV (especially of the NFI instrument) is crucial. Diameter (Arcmin)Cluster Core (arcsec) FOV 1.7’ PSF~5”

4. -I- Requirements for Clusters Field of View (Baseline BAD) Requirement: 1.7’ X 1.7’ for the NFI Science Requirement: Cover half of the virial radius of clusters (kT < 5 keV) above z=0.8. The NFI instrument is required to measure α-element abundances beyond that redshift. Comments: This is also required for spatially resolved high resolution spectroscopy of the cooling core of moderate redshift clusters, as well as velocity (Turbulences) studies. Spectral resolution (Baseline ~OK) Requirement: 2 eV below E= 1 keV Science Requirement: separate the cluster lines and the sky lines and be roughly photon limited for the α element lines up to z=2. Comments: This depends on the actual effective area The limiting factor for line detection is the sky background, especially at lowest energies (ie where the O line stands). It is particularly important to detect the O and Mg lines, which provides the most sensitive diagnostics on relative SNI and SNII enrichment. Energy Range (Baseline ~OK) Requirement: 0.2 – 10 keV Science Requirement: detect the OVII line up to z=2 (E=0.218 keV). An upper limit of 10 keV is required for kT estimates. Comments: An extension to higher energy is highly desirable to study hard tails (non thermal phenomena) and FeK for clusters at low Z.

5. Effective area (Baseline OK) Requirement: 10 m 2 @ 1 kev Science Requirement: 5 sigma detection in 100 ksec of the Mg line down to kT= 2 keV cluster and up to z=2. Angular Resolution (Baseline OK) Requirement: 5” in the center, 20” in the outer part of the WFI. Goal: 2” Science Requirement: resolve the cluster core down to kT = 2 keV and up to z=2. Resolve high z groups serendipitously found by the WFI. Comments: higher angular resolution is important to limit confusion, not only with field point sources, but with AGNs within the clusters Sensitivity (Baseline OK) Not a unique number for extended sources like clusters. Typically we aim at detecting a group at z=2 with a flux of 1.2 10 -16 ergs/s/cm 2 in the [0.2 – 2] keV band. Count rate capability Not a driver. High z clusters are faint and extended sources (no pile-up problem). -II- Requirements for Clusters

6. High Energy+High Spectral Resolution and “nearby” Clusters. Hot gas velocity structure Gas bulk flows on mergers, Bulk motions of the cold front Fe-K in merging clusters, relative radial velocity >2000 km/s. Turbulence: Line Broadening in merger. Cooling Flows : where is the cool gas ? Gas Mixing via Abundances Mapping Resonant Scattering of Fe, Si, and O in Cluster Core