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Register a new userConstraints on Omega_0 and Cluster Evolution Using the ROSAT LogN-LogS
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We examine the likelihoods of different cosmological models and cluster evolutionary histories by comparing semi-analytical predictions of X-ray cluster number counts to observational data from the ROSAT satellite. We model cluster abundance as a function of mass and redshift using a Press-Schechter distribution, and assume the temperature T(M,z) and bolometric luminosity L_X(M,z) scale as power laws in mass and epoch, in order to construct expected counts as a function of X-ray flux. The L_X-M scaling is fixed using the local luminosity function while the degree of evolution in the X-ray luminosity with redshift L_X propto (1+z)^s is left open, with s an interesting free parameter which we investigate. We examine open and flat cosmologies with initial, scale-free fluctuation spectra having indices n = 0, -1 and -2. An independent constraint arising from the slope of the luminosity-temperature relation strongly favors the n = -2 spectrum. The expected counts demonstrate a strong dependence on Omega_0 and s, with lesser dependence on lambda_0 and n. Comparison with the observed counts reveals a ridge of acceptable models in the Omega_0 - s plane, roughly following the relation s = 6 Omega_0 and spanning low-density models with a small degree of evolution to Omega = 1 models with strong evolution. Models with moderate evolution are revealed to have a strong lower limit of Omega_0 gtrsim 0.3, and low-evolution models imply that Omega_0 < 1 at a very high confidence level. We suggest observational tests for breaking the degeneracy along this ridge, and discuss implications for evolutionary histories of the intracluster medium.
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We use the ROSAT Deep Cluster Survey (RDCS) to trace the evolution of the cluster abundance out to $zsimeq 0.8$ and constrain cosmological models. We resort to a phenomenological prescription to convert masses into $X$-ray fluxes and apply a maximum-likelihood approach to the RDCS redshift- and luminosity-distribution. We find that, even changing the shape and the evolution on the $L_{bol}$-$T_X$ relation within the observational uncertainties, a critical density Universe is always excluded at more than $3sigma$ level. By assuming a non-evolving $X$-ray luminosity-temperature relation with shape $L_{bol}propto T_X^3$, it is $Omega_m=0.35^{+0.35}_{-0.25}$ and $sigma_8=0.76^{+0.38}_{-0.14}$ for flat models, with uncertainties corresponding to $3sigma$ confidence levels.
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 (zlesssim 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 BCS, in order to constrain the amplitude of the power spectrum, sigma_8, and the shape of the local luminosity-temperature relation. We obtain sigma_8=0.58 +/- 0.06 for Omega_0=1 for open models at 90% confidence level, almost independent of the L-T shape. The density parameter Omega_0 and the evolution of the L-T relation are constrained by the RDCS XLF at z>0 and the EMSS XLF at z=0.33, and by the RDCS n(S) and n(z) distributions. By modelling the evolution for the amplitude of the L-T relation as (1+z)^A, an Omega_0=1 model can be accommodated for the evolution of the XLF with 1<A<3 at 90% confidence level, while Omega_0=0.4^{+0.3}_{-0.2} and Omega_0<0.6 are implied by a non--evolving L-T for open and flat models, respectively.
We have constructed a large, statistically complete sample of galaxy clusters serendipitously detected as extended X-ray sources in 647 ROSAT PSPC pointed observations. The survey covers 158 square degrees with a median sample flux limit of 1.2 x 10^-13 erg cm^-2 s^-1 (0.5-2.0 keV). Our sample consists of 201 clusters of galaxies characterized by a median redshift of z=0.25 and a maximum of z=1.26. With 22 clusters at z > 0.5, the 160 Square Degree ROSAT Survey is the largest high-redshift sample of X-ray-selected clusters published to date. Here we describe the revised sample which features spectroscopic redshifts for 99.5% of the clusters and discuss the implications for evolution in the cluster abundance.
A new flux limited catalogue of low luminosity (Lx <= 10^44 erg/s) X-ray galaxy clusters and groups covering a redshift range of z~0.1 to z~0.7 has been produced from the WARPS project. We present the number counts of this low luminosity population at high redshifts (z>0.3). The results are consistent with an unevolving population which does not exhibit the evolution seen in the higher luminosity cluster population. These observations can be qualitatively described by self-similarly evolving dark matter and preheated IGM models of X-ray cluster gas, with a power law index for the spectrum of matter density fluctuations n >= -1.
We measure the luminosity profiles of 16 brightest cluster galaxies (BCGs) at $0.4 < z < 0.8$ using high resolution F160W NICMOS and F814W WFPC2 HST imaging. The heterogeneous sample is drawn from a variety of surveys: seven from clusters in the Einstein Medium Sensitivity Survey, five from the Las Campanas Distant Cluster Survey and its northern hemisphere precursor, and the remaining four from traditional optical surveys. We find that the surface brightness profiles of all but three of these BCGs are well described by a standard de Vaucouleurs ($r^{1/4}$) profile out to at least $sim2r_{e}$ and that the biweight-estimated NICMOS effective radius of our high redshift BCGs ($r_{e} = 8.3pm 1.4$ kpc for $H_{0} = 80$ km s$^{-1}$ Mpc$^{-1}$, $Omega_{m} = 0.2, Omega_Lambda = 0.0$) is $sim 2$ times smaller than that measured for a local BCG sample. If high redshift BCGs are in dynamical equilibrium and satisfy the same scaling relations as low redshift ones, this change in size would correspond to a mass growth of a factor of 2 since $z sim 0.5$. However, the biweight-estimated WFPC2 effective radius of our sample is 18 $pm $ 5.1 kpc, which is fully consistent with the local sample. While we can rule out mass accretion rates higher than a factor of 2 in our sample, the discrepancy between our NICMOS and WFPC2 results, which after various tests we describe appears to be physical, does not yet allow us to place strong constraints on accretion rates below that level.
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