Einstein Extended Medium Sensitivity Survey Research Articles

Hubble Space TelescopeAdvanced Camera for Surveys Weak‐Lensing andChandraX‐Ray Studies of the High‐Redshift Cluster MS 1054−0321

We present Hubble Space Telescope/Advanced Camera for Surveys (ACS) weak-lensing and Chandra X-ray analyses of MS 1054-0321 at z=0.83, the most distant and X-ray luminous cluster in the Einstein Extended Medium-Sensitivity Survey (EMSS). The high-resolution mass reconstruction through ACS weak-lensing reveals the complicated dark matter substructure in unprecedented detail, characterized by the three dominant mass clumps with the four or more minor satellite groups within the current ACS field. The direct comparison of the mass map with the Chandra X-ray image shows that the eastern weak-lensing substructure is not present in the X-ray image and, more interestingly, the two X-ray peaks are displaced away from the hypothesized merging direction with respect to the corresponding central and western mass clumps, possibly because of ram pressure. In addition, as observed in our previous weak-lensing study of another high-redshift cluster CL 0152-1357 at z=0.84, the two dark matter clumps of MS 1054-0321 seem to be offset from the galaxy counterparts. We examine the significance of these offsets and discuss a possible scenario, wherein the dark matter clumps might be moving ahead of the cluster galaxies. The non-parametric weak-lensing mass modeling gives a projected mass of M(r<1 Mpc)=(1.02+-0.15)x 10^{15} solar mass, where the uncertainty reflects both the statistical error and the cosmic shear effects. Our temperature measurement of T=8.9_{-0.8}^{+1.0} keV utilizing the newest available low-energy quantum efficiency degradation prescription for the Chandra instrument, together with the isothermal beta description of the cluster (r_c=16"+-15" and beta=0.78+-0.08), yields a projected mass of M(r<1 Mpc)=(1.2+-0.2) x 10^{15} solar mass, consistent with the weak-lensing result.

An X-ray review of MS 1054–0321: Hot or not?

XMM-Newton observations are presented for the z = 0.83 cluster of galaxies MS 1054−0321, the highest redshift cluster in the Einstein Extended Medium Sensitivity Survey (EMSS). The temperature inferred by the XMM-Newton data, T = 7.2 +0.7 −0.6 keV, is much lower than the temperature previously reported from ASCA data, T = 12.3

Radio spectra of a sample of X-ray selected BL Lacs

We present simultaneous multifrequency radio observations for a complete subsample of 26 XBLs from the Einstein Extended Medium-Sensitivity Survey, obtained with the Very Large Array (VLA). Spectra are computed using fluxes at 20, 6 and 3.6 cm. Unlike many radio selected samples, the EMSS did not impose any criterion on the radio spectrum to identify BL Lac objects. It is therefore possible to investigate the intrinsic radio spectral slope distribution and to determine the effect produced by this selection criterion. We find that 15% of the observed objects do not meet the flat-spectrum criterion imposed on some other BL Lac samples. A dataset that includes non-simultaneous data (that are also taken with different VLA configu- rations) shows an even higher percentage of steep spectrum sources. This effect can be ascribed to a larger fraction of extended flux detected with the more compact VLA configuration. Possible biases introduced by the flat-radio-spectrum criterion in the radio-selected BL Lac samples cannot explain the discrepancies observed in the evolutionary properties of Radio and X-ray selected samples of BL Lacs.

X‐Ray Temperatures for the Extended Medium‐Sensitivity Survey High‐Redshift Cluster Sample: Constraints on Cosmology and the Dark Energy Equation of State

We measure the X-ray temperature (and luminosity) with ASCA of all but one cluster in the Einstein Extended Medium-Sensitivity Survey (EMSS) high-redshift (z ≥ 0.3) sample. We compare these data to a complete sample of low-redshift clusters that also has temperature measurements, thereby providing cosmological constraints. Improvements over our previous work include (1) an enlarged high-redshift sample; (2) temperatures for the low-redshift comparison sample that come from the same instrument as the high-redshift sample; (3) the elimination of three EMSS clusters with the same redshift as the target (i.e., not truly serendipitous) and a fourth with an ASCA flux well below the completeness limit; (4) using a theoretical cluster mass function that more closely matches N-body simulations (the Sheth-Torman function); (5) using a cold dark matter power spectrum instead of a power law; (6) using a general cosmology with arbitrary matter density and cosmological constant; (7) using a cosmology that generalizes the cosmological constant to quintessence; (8) including the effects of temperature measurement errors and scatter in the cluster luminosity-temperature relation; and (9) marginalizing over the poorly known normalization of the mass-temperature relation. We find an allowed band in the Ωm0-ΩΛ0 plane of different orientation to the band of constraints provided by the supernovae Ia Hubble diagram and the cosmic microwave background fluctuations. All three bands intersect at the same place: Ωm0 ≈ 0.3, ΩΛ0 ≈ 0.7. We measure the quintessence equation-of-state parameter to be w = -(0.42 ± 0.21) (68% confidence for one interesting parameter), consistent with previously determined upper limits. We measure the normalization of the mass fluctuation power spectrum to be σ8 = 0.66 ± 0.16 (68% confidence for three interesting parameters). Systematic errors are larger than the statistical errors only for σ8 with our sample; thus the errors for it depend on the details of the marginalization over the temperature-mass normalization.

Improvements in theM‐TRelation, Mass Function, and Measured Value of \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape $\Omega _{m}$ \end{document} through Cluster Evolution

In this paper I revisit the constraints obtained by several authors on the estimated values of Ωm, n, and σ8 in light of recent theoretical developments: (1) new theoretical mass functions and (2) a more accurate mass-temperature relation, also determined for arbitrary Ωm and ΩΛ. First, using the quoted improvements, I rederive an expression for the X-ray luminosity function (XLF) similarly to Reichart and coworkers, and then I obtain constraints on Ωm and n by using the ROSAT Brightest Cluster Sample, an Einstein Extended Medium-Sensitivity Survey sample, and maximum likelihood analysis. Then I rederive the X-ray temperature function (XTF) similarly to Henry and to Eke and coworkers, reobtaining the constraints on Ωm, n, σ8. In the cases of both the XLF and XTF, the changes in the mass function and M-T relation produce an increase in Ωm of 20% and similar results in σ8 and n.

The ROSAT North Ecliptic Pole Survey: the Optical Identifications

The X-ray data around the North Ecliptic Pole (NEP) of the ROSAT All Sky Survey have been used to construct a contiguous area survey consisting of a sample of 445 individual X-ray sources above a flux of ~2 × 10-14 ergs cm-2 s-1 in the 0.5-2.0 keV energy band. The NEP survey is centered at α2000 = 18h00m, δ2000 = +66°33' and covers a region of 80.7 deg2 at a moderate Galactic latitude of b = 298. Hence, the NEP survey is as deep and covers a comparable solid angle to the ROSAT serendipitous surveys but is also contiguous. We have identified 99.6% of the sources and determined redshifts for the extragalactic objects. In this paper we present the optical identifications of the NEP catalog of X-ray sources including basic X-ray data and properties of the sources. We also describe with some detail the optical identification procedure. The classification of the optical counterparts to the NEP sources is very similar to that of previous surveys, in particular the Einstein Extended Medium Sensitivity Survey (EMSS). The main constituents of the catalog are active galactic nuclei (AGNs) (~49%), either type 1 or type 2 according to the broadness of their permitted emission lines. Stellar counterparts are the second most common identification class (~34%). Clusters and groups of galaxies comprise 14%, and BL Lacertae objects 2%. One non-AGN galaxy and one planetary nebula have also been found. The NEP catalog of X-ray sources is a homogeneous sample of astronomical objects featuring complete optical identification.

A reanalysis of the X-ray luminosities of clusters of galaxies with 0.3&lt;z&lt;0.6 in the EMSS sample

The X-ray luminosities of the Einstein Extended Medium Sensitivity Survey (EMSS) clusters of galaxies with redshifts 0.3 250kpc (the constant value assumed in the EMSS), the new luminosities are 2.2 ′ 0.15 times the previous measurements. The X-ray luminosity function (XLF) at 0.3 < z < 0.6 is recalculated and is found to be consistent with the local XLF. The constraints on the updated properties of the 0.3 < z < 0.6 EMSS sample, including a comparison with the number of clusters predicted from local XLFs, indicate that the space density of luminous, massive clusters either has not evolved or has increased by a small factor 2 since z = 0.4. The implications of this result are discussed in terms of constraints on the cosmological parameter Ω 0 .

Hubble Space Telescope observations of X-ray-selected active galactic nuclei

We report on the initial results of a comprehensive Hubble Space Telescope (HST) snapshot imaging survey of 76 low-redshift (z<0.15) X-ray-selected active galactic nuclei (AGN) in the Einstein Extended Medium Sensitivity Survey. This survey is expected to show no bias with respect to host galaxy types, and so is arguably one of the best available samples with HST imaging for the study of the host galaxies. The HST observations in the F814W band are complemented by deeper ground-based observations in the B and R bands for most AGN. The absolute magnitudes for AGN in this sample lie in the range −24<MB(AB)<−18, bracketing the extrapolated break in the QSO luminosity function MB(AB)=−22.3] at these low redshifts. We find a weak correlation between the luminosity of the host galaxy and the central AGN. We find no convincing cases of an AGN with no detectable host galaxy, although the faintest host galaxies of moderately luminous AGN do extend as faint as MB(AB)=−18. We find no evidence for strong interaction/merger activity in any of the AGN in this sample. The median ratio of AGN to host galaxy luminosity (LAGNLHost=0.2) is lower than previously observed, although the observed scatter is large. Approximately 55 per cent of these radio-quiet AGN have host galaxies that are fitted best by a ‘bulge-only’ model (or alternatively are classified visually as ellipticals/S0 galaxies), confirming the result by McLure et al. that radio-quiet AGN are not exclusively found in spiral galaxies. A comparison with the Autofib field galaxy survey shows that the morphological type distribution is skewed toward earlier types than a field galaxy sample drawn at random with the same distribution of luminosities. This is consistent with the observation that the luminosity of the host galaxies is higher by 0.75±0.25 mag than a matching sample drawn from the Autofib survey. Given the bias toward early-type galaxies, the AGN host galaxies are consistent with a luminosity and size distribution identical to normal galaxies. In every respect these galaxies are intermediate in their properties between the large, luminous host galaxies found around high-luminosity AGN in the local Universe and the fainter host galaxies identified around lower luminosity Seyfert galaxies. These results suggest that, with the exception of a bias toward early spectral types, host galaxies of AGN are drawn at random from the overall galaxy population, with the nuclear properties governed (weakly) by spheroid mass.

The Second Most Distant Cluster of Galaxies in the Extended Medium Sensitivity Survey

We report on our ASCA, Keck, and ROSAT observations of MS 1137.5+6625, the second most distant cluster of galaxies in the Einstein Extended Medium Sensitivity Survey (EMSS), at redshift 0.78. We now have a full set of X-ray temperatures, optical velocity dispersions, and X-ray images for a complete, high-redshift sample of clusters of galaxies drawn from the EMSS. Our ASCA observations of MS 1137.5 +6625 yield a temperature of 5.7 (+2.1)(-1.1) keV and a metallicity of 0.43 (+40)(-3.7) solar, with 90% confidence limits. Keck II spectroscopy of 22 cluster members reveals a velocity dispersion of 884 (+185)(-124) km 24/s. This cluster is the most distant in the sample with a detected iron line. We also derive a mean abundance at z = 0.8 by simultaneously fitting X-ray data for the two z = 0.8 clusters, and obtain an abundance of Z(sub Fe) = 0.33 (+.26)(-.23). Our ROSAT observations show that MS 1137.5+6625 is regular and highly centrally concentrated. Fitting of a Beta model to the X-ray surface brightness yields a core radius of only 71/h kpc (q(sub o) = 0.1) with Beta = 0.70(+.45)(-.15) The gas mass interior to 0.5/h Mpc is thus 1.2 (+0.2)(-0.3) X 10(exp 13) h(exp - 5/2) Solar Mass (q(sub o) = 0.1). If the cluster's gas is nearly isothermal and in hydrostatic equilibrium with the cluster potential, the total mass of the cluster within this same region is 2.1(+1.5)(-0.8) X 10exp 14)/h Solar Mass, giving a gas fraction of 0.06 +/-0.04 h (exp -3/2). This cluster is the highest redshift EMSS cluster showing evidence for a possible cooling flow (about 20-400 Solar Mass/yr). The velocity dispersion, temperature, gas fraction, and iron abundance of MS 1137.5+6625 are all statistically the same as those properties in lower red- shift clusters of similar luminosity. With this cluster's temperature now in hand, we derive a high-redshift temperature function for EMSS clusters at 0.5 < z < 0.9 and compare it with temperature functions at lower redshifts, showing that the evolution of the temperature function is relatively modest. Supplementing our high-redshift sample with other data from the literature, we demonstrate that neither the cluster luminosity-temperature relation, nor cluster metallicities, nor the cluster gas evolved with redshift. The very modest degree of evolution in the luminosity-temperature relation inferred from these data is inconsistent with the absence of evolution in the X-ray luminosity functions derived from ROSAT cluster surveys if a critical density structure formation model is assumed.

The RGB Sample of Intermediate BL Lacertae Objects

Combining newly identified and previously known BL Lacertae objects from the ROSAT All-Sky Survey-Green Bank (RGB) catalog, we present a sample of 127 BL Lacertae objects, the largest ever derived from a single uniform survey. A complete sample of 33 objects brighter than O = 18.0 mag is also presented. These samples are compared to other known BL Lac samples and are generally found to exhibit properties intermediate between those of the previously disparate classes of high- and low-energy-peaked BL Lacertae objects (HBLs and LBLs, respectively). This result is most dramatic in the distribution of the X-ray to radio logarithmic flux ratios, where the RGB BL Lacertae objects are shown to peak precisely where the sharp dichotomy between the two subclasses was previously seen. The αro versus αox diagram also shows the RGB sample smoothly bridges the gap between the previously distinct subclasses of LBLs and HBLs. The range of broadband spectral energy distributions (SEDs) exhibited by the RGB objects also shows that, contrary to prior claims, searches based on relatively deep surveys cannot limit follow-up spectroscopy to targets with a narrow range of SEDs since BL Lacertae objects clearly constitute a homogeneous population with a wide range of SEDs. As in results based on the Einstein Extended Medium Sensitivity Survey (EMSS) and 1 Jy BL Lac samples, we find a weak but statistically significant correlation between the composite X-ray spectral index αxox and the radio-optical spectral index αro. This implies that the more LBL-like RGB BL Lacertae objects have secondary sources of X-ray emission, possibly from inverse Compton components. This result, in addition to other characteristics of the RGB sample, indicates that the simple unified scheme, which postulates that HBLs and LBLs differ solely by orientation, may be in need of revision. We also present both the X-ray and radio log N- log S distributions for which the competing HBL/LBL unification scenarios have differing predictions. The unknown effects of the triple flux limit inherent in the RGB Complete sample makes quantitative analysis uncertain, but the characteristics of the RGB sample compare well both with results obtained from previous samples and with general theoretical predictions based on a simple Monte Carlo simulation. Our analysis indicates that the unimodal distribution of BL Lac properties found in the RGB sample likely reliably reflects the underlying population, while the bimodal distribution found in earlier studies arose primarily from observational selection effects. The presence of not only intermediate but also extreme HBL and LBL objects is the RGB survey's unique strength and offers clear avenues for future studies that can undoubtedly address the question of how HBLs and LBLs are related.

The Evolution of Cluster Radio Galaxies from [CLC][ITAL]z[/ITAL][/CLC] = 0 to [CLC][ITAL]z[/ITAL][/CLC] = 0.8

A radio continuum survey of a complete sample of distant (z = 0.3?0.8) X-ray?selected clusters has been conducted at 20 cm with the Very Large Array (VLA). The targets for this survey are the complete sample of 19 distant clusters found in the X-ray?selected Einstein Extended Medium Sensitivity Survey (EMSS) at z ? 0.3 and ? ? -20?. This survey is complete to a limiting point-source radio power level of log P ? 23.5 W Hz-1 (H0 = 75 km s-1 Mpc-1 and q0 = 0.1) out to ? Abell cluster radius from the brightest cluster galaxy close to the X-ray emission centroid. Of the total of 41 radio sources detected within ? Abell radius, 23 are associated with galaxies in these clusters. Higher angular resolution mapping of these cluster radio galaxies provides details of their structure and morphology. A comparison of the radio power levels, radio structure, position in the cluster, and the total number of these radio galaxies finds no evidence for any significant differences compared to nearby rich cluster radio galaxies. The radio luminosity function of distant cluster radio galaxies is statistically indistinguishable from that seen nearby. Taken together with the minimal evolution seen in the number of rich clusters over the same time period, this provides substantial evidence for no evolution in the population of rich cluster radio galaxies since z ~ ?. The results of this radio continuum survey differ from those studies that took the opposite approach, in which a radio-loud quasar or FR 2 radio galaxy was used as a target for a rich cluster search. While those studies found numerous FR 2's and quasars in rich clusters at z ~ ?, this survey has found only FR 1?type sources. The difference between these two approaches may lie in the density of the intracluster medium (ICM) and the galaxy richness of the clusters in these two samples, with the current sample having the demonstrably denser ICMs and richer galaxy populations.

A Search for Low‐Luminosity BL Lacertae Objects

Many properties of BL have become explicable in terms of the relativistic beaming hypothesis whereby BL are FR-1 radio galaxies viewed nearly along the jet axis. However, a possible problem with this model is that a transition population between beamed BL and unbeamed FR-1s has not been detected. A transition population of low-luminosity BL Lacs was predicted to exist in abundance in X-ray-selected samples such as the Einstein Extended Medium Sensitivity Survey (EMSS) by Browne & Marcha. However, these BL may have been misidentified as clusters of galaxies. We have conducted a search for such objects in the EMSS with the ROSAT HRI; and here we present ROSAT HRI images, optical spectra and VLA radio maps for a small number of BL which were previously misidentified in the EMSS catalog as clusters of galaxies. While these objects are slightly lower in luminosity than other EMSS BL Lacs, their properties are too similar to the other BL in the EMSS sample to bridge the gap between BL and FR-1 radio galaxies. Also, the number of new BL found are too few to alter significantly the X-ray luminosity function or value for the X-ray-selected EMSS BL Lac sample. Thus, these observations do not explain fully the discrepancy between the X-ray and radio-selected BL Lac samples.

The X-Ray Luminosity Function of Bright Galaxy Clusters in the Local Universe

We present the X-ray luminosity function (XLF) for clusters of galaxies derived from the RASS1 Bright Sample. The sample, selected from the ROSAT All-Sky Survey in a region of 2.5 sr within the southern Galactic cap, contains 130 clusters with flux limits in the range ~3-4×10−12 ergs cm-2 s-1 in the 0.5-2.0 keV band. A maximum likelihood fit with a Schechter function of the XLF over the entire range of luminosities (0.045-28.0×1044 ergs s-1) gives α=1.52+0.11−0.11, L*=3.80+0.70−0.55×1044 ergs s-1, and A=5.07±0.45×10−7 Mpc-3 (1044 ergs s-1)α-1. We investigate possible evolutionary effects within the sample, out to our redshift limit (z~0.3), finding no evidence for evolution. Our results are in good agreement with other local estimates of the XLF, implying that this statistic for the local universe is now well determined. Comparison with XLFs for distant clusters (0.3 < z < 0.6) shows that no evolution is present for LX 1044 ergs s-1. However, we detect differences at the 3 σ level between our local XLF and the distant one estimated by Henry et al. for the Einstein Extended Medium-Sensitivity Survey (EMSS) sample. This difference is still present when considering the EMSS sample revised by Nichol et al.

A Very Hot High‐Redshift Cluster of Galaxies: More Trouble for Ω0= 1

We have observed the most distant (= 0.829) cluster of galaxies in the Einstein Extended Medium Sensitivity Survey (EMSS), with the ASCA and ROSAT satellites. We find an X-ray temperature of 12.3 (sup +3.1) (sub -2.2)keV for this cluster, and the ROSAT map reveals significant substructure. The high temperature of MS1054-0321 is consistent with both its approximate velocity dispersion, based on the redshifts of 12 cluster members we have obtained at the Keck and the Canada-France-Hawaii telescopes, and with its weak lensing signature. The X-ray temperature of this cluster implies a virial mass approx. 7.4 x 10 (sup 14) h (sup -1) M (circle dot), if the mean matter density in the universe equals the critical value (OMEGA (sub 0) = 1), or larger if OMEGA (sub 0) is less than 1. Finding such a hot, massive cluster in the EMSS is extremely improbable if clusters grew from Gaussian perturbations in an OMEGA (sub 0) = 1 universe. Combining the assumptions that OMEGA (sub 0) = 1 and that the initial perturbations were Gaussian with the observed X-ray temperature function at low redshift, we show that this probability of this cluster occurring in the volume sampled by the EMSS is less than a few times 10 (sup -5). Nor is MS1054-0321 the only hot cluster at high redshift; the only two other z greater than 0.5 EMSS clusters already observed with ASCA also have temperatures exceeding 8 keV. Assuming again that the initial perturbations were Gaussian and OMEGA (sub 0) = 1, we find that each one is improbable at the less than 10 (sup -2) level. These observations, along with the fact that these luminosities and temperatures of the high-z clusters all agree with the low-z L (sub X) - T (sub X) relation, argue strongly that OMEGA (sub 0) less than 1. Otherwise, the initial perturbations must be non-Gaussian, if these clusters' temperatures do indeed reflect their gravitational potentials.

A Catalog of 203 Galaxy Clusters Serendipitously Detected in theROSATPSPC Pointed Observations

We present a catalog of 203 clusters of galaxies serendipitously detected in 647 ROSAT PSPC high Galactic latitude pointings covering 158 deg2. This is one of the largest X-ray-selected cluster samples, comparable in size only to the ROSAT All-Sky Survey sample of nearby clusters (Ebeling et al. 1997). We detect clusters in the inner 175 of the ROSAT PSPC field of view using the spatial extent of their X-ray emission. Fluxes of detected clusters range from 1.6 × 10-14 to 8 × 10-12 ergs s-1 cm-2 in the 0.5-2 keV energy band. X-ray luminosities range from 1042 ergs s-1, corresponding to very poor groups, to ~5 × 1044 ergs s-1, corresponding to rich clusters. The cluster redshifts range from z = 0.015 to z > 0.5. The catalog lists X-ray fluxes, core radii, and spectroscopic redshifts for 73 clusters and photometric redshifts for the remainder. Our detection method, optimized for finding extended sources in the presence of source confusion, is described in detail. Selection effects necessary for a statistical analysis of the cluster sample are comprehensively studied by Monte Carlo simulations. We have optically confirmed 203 of 223 X-ray sources as clusters of galaxies. Of the remaining 20 sources, 19 are likely false detections arising from blends of unresolved point X-ray sources. Optical identifications of the remaining object are hampered by a bright nearby star. Above a flux of 2 × 10-13 ergs s-1 cm-2, 98% of extended X-ray sources are optically confirmed clusters. The number of false detections and their flux distribution are in perfect agreement with simulations. The log N-log S relation for clusters derived from our catalog shows excellent agreement with counts of bright clusters derived from the Einstein Extended Medium Sensitivity Survey and ROSAT All-Sky Survey. At fainter fluxes, our log N-log S relation agrees with the smaller area WARPS survey. Our cluster counts appear to be systematically higher than those from a 50 deg2 survey by Rosati et al. In particular, at a flux of 2 × 10-13 ergs s-1 cm-2, we find a surface density of clusters of 0.57 ± 0.07 deg-2, which is a factor of 1.3 more than was found by Rosati et al. This difference is marginally significant at the ~2 σ level. The large area of our survey makes it possible to study the evolution of the X-ray luminosity function in the high luminosity range inaccessible with other, smaller area ROSAT surveys.

The WARPS Survey. II. The logN–logSRelation and the X‐Ray Evolution of Low‐Luminosity Clusters of Galaxies

The strong negative evolution observed in previous X-ray-selected surveys of clusters of galaxies is evidence in favor of hierarchical models of the growth of structure in the universe. A large recent survey has, however, contradicted the low-redshift results, finding no evidence for evolution at z < 0.3. Here we present the first results from an X-ray-selected, flux- and surface brightness-limited deep survey for high-redshift clusters and groups of galaxies based on ROSAT PSPC pointed data. The log N-log S relation of all clusters in this survey is consistent with that of most previous surveys, but it occupies a flux range not previously covered (greater than 6 × 10-14 ergs cm-2 s-1 total flux in the 0.5-2 keV band). At high redshifts (z > 0.3) the cluster luminosities are in the range 4 × 1043-2 × 1044 h−250 ergs s-1, the luminosities of poor clusters. The number of high-redshift, low-luminosity clusters is consistent with no evolution of the X-ray luminosity function between redshifts of z ≈ 0.4 and z = 0, and it places a limit of a factor of less than 1.7 (at 90% confidence) on the amplitude of any pure negative density evolution of clusters of these luminosities, in contrast with the factor of ≈ 3 [corresponding to number density evolution ∝(1 + z)-2.5] found in the Einstein Extended Medium-Sensitivity Survey at similar redshifts but higher luminosities. Taken together, these results support hierarchical models in which there is mild negative evolution of the most luminous clusters at high redshift, but little or no evolution of the less luminous but more common optically poor clusters. Models involving preheating of the X-ray gas at an early epoch fit the observations, at least for Ω0 = 1.

Results from the ROSAT Deep Cluster Survey

AbstractDeep X‐ray surveys of galaxy clusters covering large solid angles offer ideal samples for studying the evolution of the large scale structure in the Universe. I review here some recent results from the ROSAT Deep Cluster Survey (RDCS), particularly, on the study of the evolution of the abundance of galaxy clusters over a large redshift range. The optical identification of RDCS sources have revealed a significant fraction of clusters at &gt;z &gt; 0.5. The observed X‐ray Luminosity Function out to z ⋍ 0.8 and the faint cluster number counts from the RDCS, when combined with previous results of the Einstein Extended Medium Sensitivity Survey, provide a much improved observational picture of the evolution of clusters in the X‐ray band, revealing a slowly evolving population of galaxy clusters since at least z ⋍ 0.8. The theoretical interpretation of these findings in the framework of theories of structure formation will still be subject of much debate in the years to come. The construction of large samples of clusters out to z ∽ 1 and beyond appears to be a challenging task even with the next generation of X‐ray satellites (AXAF, XMM).

The [ITAL]ROSAT[/ITAL] Deep Cluster Survey: The X-Ray Luminosity Function out to [CLC][ITAL]z[/ITAL][/CLC] = 0.8

We present the X-ray luminosity function (XLF) of the ROSAT Deep Cluster Survey sample over the redshift range 0.05-0.8. Our results are derived from a complete flux-limited subsample of 70 galaxy clusters, representing the brightest half of the total sample, which have been spectroscopically identified down to the flux limit of 4×10 -->−14 ergs m-2 s-1 (0.5-2.0 keV) and have been selected via a serendipitous search in ROSAT PSPC pointed observations. The redshift baseline is large enough that evolutionary effects can be studied within the sample. The local XLF (z≤0.25) is found to be in excellent agreement with previous determinations using ROSAT All-Sky Survey data. The XLF at higher redshifts, when combined with the deepest number counts constructed to date (f>2×10 -->−14 ergs cm -->−2 s -->−1), reveals no significant evolution at least out to z=0.8, over a luminosity range of 2 × 1042 to 3 × 1044 ergs s -->−1 in the 0.5-2 keV band. These findings extend the study of cluster evolution to the highest redshifts and the faintest fluxes probed so far in X-ray surveys. They complement and do not necessarily conflict with those of the Einstein Extended Medium-Sensitivity Survey, leaving open the possibility of negative evolution of the brightest end of the XLF at high redshifts.

The X-ray flux and spectral variability of the Einstein Extended Medium Sensitivity Survey AGN

The X-ray flux and spectral variability of the Einstein Extended Medium Sensitivity Survey AGN

Fluctuations in the diffuse X-ray background observed with Ginga

We present Ginga measurements of the spatial fluctuations in the diffuse X-ray background. When combined with earlier results, the new data constrain the extragalactic log N - log S relation in the 2-10 keV energy band to a form close to the Euclidean prediction over the flux range 1E-10 - 5E-13 erg/cm2/s. The normalisation of the 2-10 keV source counts is a factor 2-3 above that derived in the softer 0.3-3.5 keV band from the Einstein Extended Medium Sensitivity Survey if a spectral conversion is assumed which ignores X-ray absorption intrinsic to the sources. Both this result and the spectral characteristics of the spatial fluctuations are consistent with relatively low-luminosity active galaxies (i.e. L_X < 1E44 erg/s) dominating the 2-10 keV source counts at intermediate flux levels. We also use the `excess variance' of the fluctuations to constrain possible clustering of the underlying discrete sources.