Definición de métricas existentes a ser usadas

The success story of X-ray surveys for clusters of galaxies is largely built upon the German- American ROSAT (ROentgenSATellit) mission from 1990–1999. The X-ray imaging ca- pabilities of its main instrument, the Position Sensitive Proportional Counter (PSPC), allowed for the first time a cluster selection based on source extent, in particular for the mission phase of pointed observations after the completion of the ROSAT All-Sky Survey (RASS). The central 0.2 deg2 _{of the PSPC field-of-view achieved a point spread function}

with a FWHM resolution of 3000_–4500 _{enabling the resolved detection of extended emission} of clusters with fiducial core radii of 250h−1_{kpc out to z∼1 (e.g. Rosati, Borgani & Nor-}

man, 2002). Considering that the extragalactic AGN point source population outnumbers clusters by approximately a factor of 10 : 1 at faint flux levels (fX∼10−14erg s−1cm−2), the

selection based on the extent criterion greatly enhances the cluster survey efficiencies. 2_{Additional correction terms might be necessary, e.g.to take the detection efficiency as a function of}

XDCP

Figure 4.1: Comparison of X-ray cluster surveys over the past two decades. The approximate survey flux limits and solid angles are plotted for different contiguous area surveys (light shaded circles) and serendipitous surveys (dark circles). The position of the XDCP survey has been added. Plot adapted from Rosati, Borgani & Norman (2002).

Two basic types of X-ray surveys can be distinguished. (i) Contiguous sky area sur- veys covering a single connected region, and (ii) serendipitous surveys making use of the ensemble of sky patches of numerous individual pointed observations. Both survey types have their merits and strengths for different science applications. Contiguous surveys, in particular with all-sky coverage, span large solid angles with typically relatively bright flux limits. They are well-suited to detect the rarest, most luminous systems in the large survey volume and to investigate their clustering properties. Serendipitous surveys, on the other hand, are typically sensitive to significantly fainter flux limits, thus providing complementary information on more common systems with lower luminosities and more distant objects. Samples compiled from this survey type are particularly fit for cluster evolution studies.

Figure 4.1 provides an overview of the major X-ray cluster projects of the last two decades. Contiguous area surveys are indicated by the light shaded symbols, whereas serendipitous surveys are represented by dark circles. The approximate location of the XMM-Newton Distant Cluster Project in the solid angle versus flux limit plane has been added in blue. First, we will have a short look at the major all-sky survey samples providing a local cluster census and reference system.

Figure 4.2: Important results of local cluster surveys. Left: The empirical LX–M relation of Reiprich & B¨ohringer (2002) based on the HIFLUGCS sample of the 63 brightest galaxy clusters in the sky (solid line) and an extended sample of 106 objects (dashed line). Right: Galaxy cluster power spectrum from the REFLEX survey (top filled symbols) in comparison to a galaxy power spectrum (bottom open symbols). The power spectra amplitude of the two object classes differ by thebiasing factorb2_{'6.8. Plot from Schuecker et al.(2001).}

Figure 4.3: X-ray luminosity functions (XLF) as determined by different cluster surveys. Left: The local (z <0.3) X-ray luminosity functions of eight flux-limited surveys show excellent agreement and serve as the no-evolution baseline for comparisons with distant cluster samples. Right: XLFs derived from cluster samples of intermediate redshifts (0.25< z <0.95). The shaded regions indicates the local XLF of the left panel. Significant evolution effects have only been observed for the highest luminosity clusters manifested in a lower observed space density at z∼0.8. Plots from Mullis et al. (2004).

Figure 4.4: The NORAS 2 cluster of galax- ies RXC J0834.9+5534 atz= 0.239 as a typ- ical example of a low-redshift object detected in the ROSAT All-Sky Survey. The red X-ray contours are shown on top of the120_×120 _op- tical DSS image. The circles indicate the lo- cations of spectroscopically confirmed cluster galaxies of early type (red), late type (blue), and Seyfert type 1 (yellow).

Figure 4.5: Typical spectra of galaxy members of the cluster RXC J0834.9+5534 atz= 0.239shown above. Important spectral features are indicated by dashed blue lines with corresponding labels, atmospheric features by red dotted lines. Left: Early type galaxy spectra with their characteristic D4000 break redshifted to about 5 000 ˚A and other labelled absorption features. Right: Late-type galaxy spectrum (top) and a Seyfert type 1 spectrum (bottom) with indicated emission lines.

HIghest X-ray FLUx Galaxy Cluster Sample (HIFLUGCS): This sample in- cludes the 63 brightest X-ray clusters in the (extragalactic) sky and has a flux limit of fX = 2×10−11erg s−1cm−2 in the ROSAT 0.1–2.4 keV band (Reiprich, 2001).

The major achievement with this brightest cluster sample has been the empirical calibration of the (local) LX–M relation as shown in the left panel of Fig. 4.2

(Reiprich and B¨ohringer, 2002). This relation establishes the crucial mass proxy and hence the link to cosmological applications for large X-ray survey samples.

ROSAT-ESO Flux-Limited X-ray cluster survey (REFLEX): The REFLEX sam- ple (B¨ohringer et al., 2004) is currently the largest statistically complete published compilation of X-ray clusters. It contains 447 objects in the Southern sky (δ <+2.5◦₎ down to the flux limit of fX= 3×10−12erg s−1cm−2 [0.1–2.4 keV]. REFLEX provides

the local reference of the luminosity function (left panel of Fig. 4.3) (B¨ohringeret al., 2002a) and has been used for a wide variety of cosmological studies (e.g. Schuecker et al., 2003a, Schuecker et al., 2003b). A major achievement has been the measure- ment of the galaxy cluster power spectrum on scales of 15–800h−1_{Mpc as shown in}

the right panel of Fig. 4.2 (Schuecker et al., 2001). An extension of the survey, RE- FLEX 2, is in preparation and will roughly double the number of objects by lowering the flux limit to fX= 1.8×10−12erg s−1cm−2.

NOrthern ROSAT All-Sky survey (NORAS): NORAS (B¨ohringer et al., 2000) is the counterpart of REFLEX in the Northern hemisphere containing 378 clusters out toz <_∼0.3. The survey extension NORAS 2 is in preparation. The combined extended samples NORAS 2 and REFLEX 2 with a total of approximately 1 800 X-ray clusters will provide the next milestone for a local power spectrum analysis. This thesis work also included the spectroscopic analysis of 35 clusters for the extended NORAS 2 survey. Figures 4.4 & 4.5 show one example of a low-redshift cluster at z = 0.239 and some typical cluster galaxy spectra as local reference for the discussions on the spectral energy distribution (SED) in Chap. 7 and spectroscopic features (Chap. 8) of distant cluster galaxies.