ROSAT Deep Cluster Survey Research Articles

X‐Ray‐selected Intermediate‐Redshift Groups of Galaxies

We present spectroscopic confirmation of nine moderate-redshift galaxy groups and poor clusters selected from the ROSAT Deep Cluster Survey. The groups span the redshift range z ~ 0.23-0.59 and have between 4 and 20 confirmed members. The velocity dispersions of these groups range from ~125 to 650 km s-1. Similar to X-ray groups at low redshift, these systems contain a significant number of early-type galaxies. Therefore, the trend for X-ray-luminous groups to have high early-type fractions is already in place by at least z ~ 0.5. In four of the nine groups, the X-ray emission is clearly peaked on the most luminous early-type galaxy in the group. However, in several cases the central galaxy is composed of multiple luminous nuclei, suggesting that the brightest group galaxy may still be undergoing major mergers. In at least three (and possibly five) of the groups in our sample, a dominant early-type galaxy is not found at the center of the group potential. This suggests that many of our groups are not dynamically evolved despite their high X-ray luminosities. While similar systems have been identified at low redshift, the X-ray luminosities of the intermediate-redshift examples are 1-3 orders of magnitude higher than those of their low-redshift counterparts. We suggest that this may be evidence for group downsizing: while massive groups are still in the process of collapsing and virializing at intermediate redshifts, only low-mass groups are in the process of forming at the present day.

ChandraandXMM-NewtonObservations of RDCS 1252.9-2927, A Massive Cluster atz=1.24

We present deep Chandra and XMM-Newton observations of the galaxy cluster RDCS1252.9-2927, which was selected from the ROSAT Deep Cluster Survey (RDCS) and confirmed by extensive spectroscopy with the VLT at redshift z = 1.237. With the Chandra data, the X-ray emission from the intra-cluster medium is well resolved and traced out to 500 kpc, thus allowing a measurement of the physical properties of the gas with unprecedented accuracy at this redshift. We detect a clear 6.7 keV Iron K line in the Chandra spectrum providing a redshift within 1% of the spectroscopic one. By augmenting our spectroscopic analysis with the XMM-Newton data (MOS detectors only), we significantly narrow down the 1{sigma} error bar to 10% for the temperature and 30% for the metallicity, with best fit values kT = 6.0{sup +0.7}{sub -0.5} keV, Z = 0.36{sup +0.12}{sup -0.10}Z{sub {circle_dot}}. In the likely hypothesis of hydrostatic equilibrium, we measure a total mass of M{sub 500} = (1.9{+-}0.3)10{sup 14}h{sup -1}{sub 70} M{sub {circle_dot}} within R{sub {Delta}=500} {approx} 536 kpc. Overall, these observations imply that RDCS1252.9-2927 is the most X-ray luminous and likely the most massive bona-fide cluster discovered to date at z > 1. When combined with current samples of distantmore » clusters, these data lend further support to a mild evolution of the cluster scaling relations, as well the metallicity of the intra-cluster gas. Inspection of the cluster mass function in the current cosmological concordance model (h,{Omega}{sub m},{Omega}{sub {Lambda}}) = (0.7,0.3,0.7) and {sigma}{sub 8} = 0.7-0.8 shows that RDCS1252.9-2927 is an M* cluster at z = 1.24, in keeping with number density expectations in the RDCS survey volume.« less

Massive Star Formation in a Gravitationally Lensed HiiGalaxy atz= 3.357

The Lynx arc, with a redshift of 3.357, was discovered during spectroscopic follow-up of the z=0.570 cluster RX J0848+4456 from the ROSAT Deep Cluster Survey. The arc is characterized by a very red R - K color and strong, narrow emission lines.

Moderate-Temperature Clusters of Galaxies from the RDCS and the High-Redshift Luminosity-Temperature Relation

We present our discovery observations and analysis of RDCS 1317+2911, z = 0.805, and RDCS 1350+6007, z = 0.804, two clusters of galaxies identified through X-ray emission in the ROSAT Deep Cluster Survey (RDCS). RDCS 1317+2911 has an unusual morphology in our Chandra observations, with an asymmetric surface brightness profile and a bend in the distribution of X-ray emission. In contrast, RDCS 1350+6007 appears to be more like low-redshift clusters, with ? = 0.49 ? 0.06 and rcore = 165 ? 5 kpc (?m = 0.3, ?? = 0.7, H0 = 65 km s-1 Mpc-1), though it also has an elliptical, slightly asymmetric surface brightness profile. We find a temperature of 3.7 keV and a bolometric luminosity of 8.2 ? 1043 ergs s-1 for RDCS 1317+2911, and a temperature of 4.9 keV and a bolometric luminosity of 4.1 ? 1044 ergs s-1 for RDCS 1350+6007. Our weak-lensing analysis of RDCS 1350+6007 confirms the general shape of the inner density profile but predicts twice the mass of the model based on the X-ray profile. There are two possibilities for this discrepancy: either there is a significant amount of mass near the redshift of the cluster that has not yet fallen into the potential well and shock-heated the gas, or, as we only see the X-ray emission from the core of the cluster, our ? model fails to describe the true shape of the underlying potential. We combine the X-ray luminosities and temperatures for RDCS clusters of galaxies with such measurements of other clusters at high redshift (z > 0.7) and fit the luminosity-temperature relation. We find no statistically significant evolution in the slope or zero point of this relation at zmedian = 0.83. This result is in agreement with models of intracluster medium evolution with significant preheating or high initial entropy values. Quantifying the bolometric luminosity-temperature relation as L = L6(1 + z)A(T/6 keV)?, we find ? = 2.9 ? 0.4, L6 = 8.7 ? 0.9 ? 1044 ergs s-1 and A = 0.3 ? 0.2, or A = 0.4 ? 0.2, depending on which low-redshift luminosity-temperature relation we compare with. With this result, we rule out at the 5 ? level the self-similar scaling model of intracluster medium evolution. We discuss how low-temperature, high-redshift clusters of galaxies will allow us to improve on this result, and we announce the discovery of two such objects, CXOU J0910.1+5419 and CXOU J1316.9+2914.

An X-Ray–Selected Galaxy Cluster at [ITAL][CLC]z[/CLC][/ITAL] = 1.11 in the [ITAL]ROSAT[/ITAL] Deep Cluster Survey

We report the discovery of an X-ray–luminous galaxy cluster at z = 1.11. RDCS J0910+5422 was selected as an X-ray cluster candidate in the ROSAT Deep Cluster Survey on the basis of its spatial extent in a ROSAT Position Sensitive Proportional Counter image. Deep optical and near-IR imaging reveal a red galaxy overdensity around the peak of the X-ray emission, with a significant excess of objects with J-K and I-K colors typical of elliptical galaxies at z ~ 1. Spectroscopic observations at the Keck II telescope secured nine galaxy redshifts in the range 1.095 1.

Measuring Ωmwith theROSATDeep Cluster Survey

We analyze the ROSAT Deep Cluster Survey (RDCS) to derive cosmological constraints from the evolution of the cluster X-ray luminosity distribution. The sample contains 103 galaxy clusters out to z 0.85 and flux limit Flim = 3 × 10-14 ergs s-1 cm-2 (RDCS-3) in the [0.5-2.0] keV energy band, with a high-redshift extension containing four clusters at 0.90 ≤ z ≤ 1.26 and brighter than Flim = 1 × 10-14 ergs s-1 cm-2 (RDCS-1). We assume cosmological models to be specified by the matter density parameter Ωm, the rms fluctuation amplitude at the 8 h-1 Mpc scale σ8, and the shape parameter for the cold dark matter-like power spectrum Γ. Model predictions for the cluster mass function are converted into the X-ray luminosity function in two steps. First, we convert mass into intracluster gas temperature by assuming hydrostatic equilibrium. Then, temperature is converted into X-ray luminosity by using the most recent data on the LX-TX relation for nearby and distant clusters. These include the Chandra data for six distant clusters at 0.57 ≤ z ≤ 1.27. From RDCS-3 we find Ωm = 0.35 and σ8 = 0.66 for a spatially flat universe with a cosmological constant, with no significant constraint on Γ (errors correspond to 1 σ confidence levels for three fitting parameters). Even accounting for both theoretical and observational uncertainties in the mass-X-ray luminosity conversion, an Einstein-de Sitter model is always excluded at far more than the 3 σ level. We also show that the number of X-ray-bright clusters in RDCS-1 at z > 0.9 is expected from the evolution inferred at z < 0.9 data.

RX J0848+4456: Disentangling a Moderate Redshift Cluster

We present a multi-wavelength study of RX J0848+4456, a cluster of galaxies discovered through X-ray emission in the ROSAT Deep Cluster Survey. Our observations consist of WFPC2 imaging, optical spectra, and X-ray data collected with the Chandra observatory. We find that RX J0848+4456 consists of an X-ray emitting cluster of galaxies at a redshift of z=0.570 and a group at slightly lower redshift, z=0.543, with little X-ray emission. This lower redshift system, however, is a gravitational lens, with the lensed galaxy an unusual AGN or star-forming system at z=3.356. The cluster has an X-ray temperature of kT = 3.6 +/- 0.4 keV, a bolometric luminosity of 1.0e44 +/- 0.3e44 erg/s and a velocity dispersion of 670 +/- 50 km/s. These values all agree with the low redshift correlations for clusters of galaxies, implying a relaxed system with the ICM in equilibrium with the dark matter potential. The lower redshift group of galaxies at z=0.543 has, at most 1/5, more likely 1/10, of the X-ray luminosity of RX J0848+4456. Despite being a gravitational lens, this is a low mass system, with an X-ray temperature of kT = 2.3 +0.5 -0.4 keV and a velocity dispersion of only 430 +/- 20 km/s. Our observations show the importance of detailed studies of clusters of galaxies when using them as probes of cosmological mass functions.

Cluster Evolution in the [ITAL]ROSAT[/ITAL] North Ecliptic Pole Survey

The deepest region of the ROSAT All-Sky Survey, at the north ecliptic pole, has been studied to produce a complete and unbiased X-ray-selected sample of clusters of galaxies. This sample is used to investigate the nature of cluster evolution and to explore the potential implications for large-scale structure models. The survey is 99.6% optically identified. Spectroscopic redshifts have been measured for all the extragalactic identifications. In this Letter, first results on cluster evolution are presented based on a comparison between the number of the observed clusters in the north ecliptic pole survey and the number of expected clusters assuming no-evolution models. At z > 0.3, there is a deficit of clusters with respect to the local universe that is significant at greater than 4.7 σ. The evolution appears to commence at L0.5-2.0 keV > 1.8 × 1044 ergs s-1 in our data. The negative evolution goes in the same direction as the original Extended Medium-Sensitivity Survey result, the results from the 160 deg2 survey, and the recent results from the ROSAT Deep Cluster Survey. At lower redshifts, there is no evidence for evolution, a result in agreement with these and other cluster surveys.

An X-Ray–Selected Galaxy Cluster at [CLC][ITAL]z[/ITAL][/CLC] = 1.26

We report the discovery of an X-ray–luminous galaxy cluster at z = 1.26. RX J0848.9+4452 was selected as an X-ray cluster candidate in the ROSAT Deep Cluster Survey on the basis of its spatial extent. Deep optical and near-IR imaging have revealed a galaxy overdensity around the peak of the X-ray emission, with a significant excess of red objects with J-K colors typical of elliptical galaxies at z > 1. Spectroscopic observations at the Keck II Telescope have secured six galaxy redshifts in the range 1.257 1.

Cosmological Constraints from theROSATDeep Cluster Survey

The ROSAT Deep Cluster Survey (RDCS) has provided a new large deep sample of X-ray selected galaxy clusters. Observables such as the flux number counts n(S), the redshift distribution n(z), and the X-ray luminosity function (XLF) over a large redshift baseline (z≲0.8) are used here in order to constrain cosmological models. Our analysis is based on the Press-Schechter approach, whose reliability is tested against N-body simulations. Following a phenomenological approach, no assumption is made a priori on the relation between cluster masses and observed X-ray luminosities. As a first step, we use the local XLF from RDCS, along with the high-luminosity extension provided by the XLF from the Brightest Cluster Survey, in order to constrain the amplitude of the power spectrum, σ8, and the shape of the local luminosity-temperature, Lbol-T, relation. We obtain σ8=(0.58 ± 0.06) × Ω0-0.47+0.16Ω for flat models (ΩΛ=1-Ω0) and σ8=(0.58 ± 0.06) × Ω0-0.53+0.27Ω for open models (ΩΛ=0) at a 90% confidence level, almost independent of the Lbol-T shape. The density parameter Ω0 and the evolution of the Lbol-T relation are constrained by the RDCS XLF at z>0 and the EMSS XLF at =0.33, and by the RDCS n(S) and n(z) distributions. By modeling the evolution for the amplitude of the Lbol-T relation as (1+z)A, an Ω0=1 model can be accommodated for the evolution of the XLF with 1≲A≲3 at a 90% confidence level, while Ω0=0.4+0.3−0.2 and Ω0≲0.6 are implied by a nonevolving Lbol-T (A=0) for open and flat models, respectively.

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.