Annu. Rev. Astron. Astrophys. 2002. 40: 539-577 Copyright © 2002 by Annual Reviews. All rights reserved

3.2. X-ray Cluster Surveys

The Uhuru X-ray satellite, which carried out the first X-ray sky survey (Giacconi et al. 1972), revealed a clear association between rich clusters and bright X-ray sources (Gursky et al. 1971, Kellogg et al. 1971). Uhuru observations also established that X-ray sources identified as clusters were among the most luminous in the sky (10^43-45 erg s^-1), were extended and showed no variability. Felten et al. (1966) first suggested the X-ray originated as thermal emission from diffuse hot intra-cluster gas (Cavaliere et al. 1971). This was later confirmed when the first high quality X-ray spectra of clusters were obtained with the HEAO-1 A2 experiment (e.g. Henriksen and Mushotzsky, 1986). These spectra were best fit by a thermal bremsstrahlung model, with temperatures in the range 2 × 10⁷ - 10⁸ keV, and revealed the 6.8 keV iron K line, thus showing that the ICM was a highly ionized plasma pre-enriched by stellar processes.

The HEAO-1 X-ray Observatory (Rothschild et al. 1979) performed an all-sky survey with much improved sensitivity compared to Uhuru and provided the first flux-limited sample of extragalactic X-ray sources in the 2-10 keV band, with a limiting flux of 3 × 10^-11 erg cm^-2 s^-1 (Piccinotti et al. 1982). Among the 61 extragalactic sources discovered outside the galactic plane (| b| > 20°), 30 were identified as galaxy clusters, mostly in the Abell catalog. This first X-ray flux-limited sample allowed an estimate of the cluster X-ray luminosity function (XLF) in the range L_X = 10⁴³ - 3 ^. 10⁴⁵ erg s^-1. The derived space density of clusters (all at z < 0.1) is fairly close to current values. An earlier determination of the XLF based on optically selected Abell clusters (McKee et al. 1980) and the same HEAO-1 A2 data gave similar results.

The Piccinotti et al. sample was later augmented by Edge et al. (1990), who extended the sample using the Ariel V catalog (McHardy et al. 1981) and revised the identifications of several clusters using follow-up observations by the Einstein Observatory and EXOSAT. With much improved angular resolution, these new X-ray missions allowed confused sources to be resolved and fluxes to be improved. The resulting sample included 55 clusters with a flux limit a factor of two fainter than in the original Piccinotti catalog.

Confusion effects in the large beam ( 1°) early surveys, such as HEAO-1 and Ariel V, had been the main limiting factor in cluster identification. With the advent of X-ray imaging with focusing optics in the 80's, particularly with the Einstein Observatory (Giacconi et al. 1979), it was soon recognized that X-ray surveys offer an efficient means of constructing samples of galaxy clusters out to cosmologically interesting redshifts.

First, the X-ray selection has the advantage of revealing physically-bound systems, because diffuse emission from a hot ICM is the direct manifestation of the existence of a potential well within which the gas is in dynamical equilibrium with the cool baryonic matter (galaxies) and the dark matter. Second, the X-ray luminosity is well correlated with the cluster mass (see right panel of Figure 2). Third, the X-ray emissivity is proportional to the square of the gas density (Section 2), hence cluster emission is more concentrated than the optical bidimensional galaxy distribution. In combination with the relatively low surface density of X-ray sources, this property makes clusters high contrast objects in the X-ray sky, and alleviates problems due to projection effects that affect optical selection. Finally, an inherent fundamental advantage of X-ray selection is the ability to define flux-limited samples with well-understood selection functions. This leads to a simple evaluation of the survey volume and therefore to a straightforward computation of space densities. Nonetheless, there are some important caveats described below.

Pioneering work in this field was carried out by Gioia et al. (1990a) and Henry et al. (1992) based on the Einstein Observatory Extended Medium Sensitivity Survey (EMSS, Gioia et al. 1990b). The EMSS survey covered over 700 square degrees using 1435 imaging proportional counter (IPC) fields. A highly complete spectroscopic identification of 835 serendipitous sources lead to the construction of a flux-limited sample of 93 clusters out to z = 0.58. By extending significantly the redshift range probed by previous samples (e.g. Edge et al. 1990), the EMSS allowed the cosmological evolution of clusters to be investigated. Several follow-up studies have been undertaken such as the CNOC survey (e.g. Yee et al. 1996), and gravitational lensing (Gioia & Luppino 1994).

The ROSAT satellite, launched in 1990, allowed a significant step forward in X-ray surveys of clusters. The ROSAT-PSPC detector, in particular, with its unprecedented sensitivity and spatial resolution, as well as low instrumental background, made clusters high contrast, extended objects in the X-ray sky. The ROSAT All-Sky Survey (RASS, Trümper 1993) was the first X-ray imaging mission to cover the entire sky, thus paving the way to large contiguous-area surveys of X-ray selected nearby clusters (e.g. Ebeling et al. 1997, 1998, 2000, 2001; Burns et al. 1996; Crawford et al. 1995; De Grandi et al. 1999; Böhringer et al. 2000, 2001). In the northern hemisphere, the largest compilations with virtually complete optical identification include, the Bright Cluster Sample (BCS, Ebeling et al. 1998), its extension (Ebeling et al. 2000b), and the Northern ROSAT All Sky Survey (NORAS, Böhringer et al. 2000). In the southern hemisphere, the ROSAT-ESO flux limited X-ray (REFLEX) cluster survey (Böhringer et al. 2001) has completed the identification of 452 clusters, the largest, homogeneous compilation to date. Another on-going study, the Massive Cluster Survey (MACS, Ebeling et al. 2001) is aimed at targeting the most luminous systems at z > 0.3 which can be identified in the RASS at the faintest flux levels. The deepest area in the RASS, the North Ecliptic Pole (NEP, Henry et al. 2001) which ROSAT scanned repeatedly during its All-Sky survey, was used to carry out a complete optical identification of X-ray sources over a 81 deg² region. This study yielded 64 clusters out to redshift z = 0.81.

In total, surveys covering more than 10⁴ deg² have yielded over 1000 clusters, out to redshift z 0.5. A large fraction of these are new discoveries, whereas approximately one third are identified as clusters in the Abell or Zwicky catalogs. For the homogeneity of their selection and the high degree of completeness of their spectroscopic identifications, these samples are now becoming the basis for a large number of follow-up investigations and cosmological studies.

After the completion of the all-sky survey, ROSAT conducted thousands of pointed observations, many of which (typically those outside the galactic plane not targeting very bright or extended X-ray sources) can be used for a serendipitous search for distant clusters. It was soon realized that the good angular resolution of the ROSAT-PSPC allowed screening of thousands of serendipitous sources and the selection of cluster candidates solely on the basis of their flux and spatial extent. In the central 0.2 deg² of the PSPC field of view the point spread function (PSF) is well approximated by a Gaussian with FWHM = 30 - 45". Therefore a cluster with a canonical core radius of 250 h^-1 kpc (Forman & Jones 1982) should be resolved out to z ~ 1, as the corresponding angular distance always exceeds 45" for current values of cosmological parameters (important surface brightness biases are discussed below).

ROSAT-PSPC archival pointed observations were intensively used for serendipitous searches of distant clusters. These projects, which are now completed or nearing completion, include: the RIXOS survey (Castander et al. 1995), the ROSAT Deep Cluster Survey (RDCS, Rosati et al. 1995, 1998), the Serendipitous High-Redshift Archival ROSAT Cluster survey (SHARC, Collins et al. 1997, Burke et al. 1997), the Wide Angle ROSAT Pointed X-ray Survey of clusters (WARPS, Scharf et al. 1997, Jones et al. 1998, Perlman et al. 2002), the 160 deg² large area survey (Vikhlinin et al. 1998b), the ROSAT Optical X-ray Survey (ROXS, Donahue et al. 2001). ROSAT-HRI pointed observations, which are characterized by a better angular resolution although with higher instrumental background, have also been used to search for distant clusters in the Brera Multi-scale Wavelet catalog (BMW, Campana et al. 1999).

A principal objective of all these surveys has been the study of the cosmological evolution of the space density of clusters. Results are discussed in Section 4 and 5, below. In Figure 4, we give an overview of the flux limits and surveyed areas of all major cluster surveys carried out over the last two decades. RASS-based surveys have the advantage of covering contiguous regions of the sky so that the clustering properties of clusters (e.g. Collins et al. 2000, Mullis et al. 2001), and the power spectrum of their distribution (Schücker et al. 2001a) can be investigated. They also have the ability to unveil rare, massive systems albeit over a limited redshift and X-ray luminosity range. Serendipitous surveys, or general surveys, which are at least a factor of ten deeper but cover only a few hundreds square degrees, provide complementary information on lower luminosities, more common systems and are well suited for studying cluster evolution on a larger redshift baseline. The deepest pencil-beam surveys, such as the Lockman Hole with XMM (Hasinger et al. 2001) and the Chandra Deep Fields (Giacconi et al. 2002, Bauer et al. 2002), allow the investigation of the faintest end of the XLF (poor clusters and groups) out to z ~ 1.

Figure 4. Solid angles and flux limits of X-ray cluster surveys carried out over the last two decades. References are given in the text. Dark filled circles represent serendipitous surveys constructed from a collection of pointed observations. Light shaded circles represent surveys covering contiguous areas. The hatched region is a predicted locus of future serendipitous surveys with Chandra and Newton-XMM.