Abundance Of Galaxy Clusters Research Articles

Measuring Omega0 using cluster evolution

The evolution of the abundance of galaxy clusters depends sensitively on the value of the cosmological density parameter, Ω0. Recent ASCA data are used to quantify this evolution as measured by the cluster X-ray temperature function. A χ2 minimization fit to the cumulative temperature function, as well as a maximum-likelihood estimate (which requires additional assumptions about cluster luminosities), leads to the estimate Ω0 ≈ 0.45 ± 0.25 (1σ statistical error). Various systematic uncertainties are considered, none of which significantly enhances the probability that Ω0 = 1. These conclusions hold for models with or without a cosmological constant, i.e., with Λ0 = 0 or Λ0 = 1 − Ω0. The statistical uncertainties are at least as large as any of the individual systematic errors that have been considered here, suggesting that additional temperature measurements of distant clusters will allow an improvement in this estimate. An alternative method that uses the highest redshift clusters to place an upper limit on Ω0 is also presented and tentatively applied, with the result that Ω0 1 can be ruled out at the 98 per cent confidence level. Whilst this method does not require a well-defined statistical sample of distant clusters, there are still modelling uncertainties that preclude a firmer conclusion at this time.

Evolution of Structure in Cold Dark Matter Universes

We present an analysis of the clustering evolution of dark matter in four cold dark matter (CDM) cosmologies. We use a suite of high resolution, 17-million particle, N-body simulations which sample volumes large enough to give clustering statistics with unprecedented accuracy. We investigate both a flat and an open model with Omega_0=0.3, and two models with Omega=1, one with the standard CDM power spectrum and the other with the same power spectrum as the Omega_0=0.3 models. The amplitude of primordial fluctuations is set so that the models reproduce the observed abundance of rich galaxy clusters by the present day. The mass 2-point correlation function and power spectrum of all the simulations differ significantly from those of the observed galaxy distribution, in both shape and amplitude. Thus, for any of these models to provide an acceptable representation of reality, the distribution of galaxies must be biased relative to the mass in a non-trivial, scale-dependent, fashion. In the Omega=1 models the required bias is always greater than unity, but in the Omega_0=0.3 models an "antibias" is required on scales smaller than \sim 5\hmpc. The mass correlation functions in the simulations are well fit by recently published analytic models. The velocity fields are remarkably similar in all the models, whether they be characterised as bulk flows, 1-particle or pairwise velocity dispersions. This similarity is a direct consequence of our adopted normalisation. The small-scale pairwise velocity dispersion of the dark matter is somewhat larger than recent determinations from galaxy redshift surveys, but the bulk-flows predicted by our models are broadly in agreement with most available data.

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 >z > 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).

Large-scale structure in COBE-normalized cold dark matter cosmogonies

We study the clustering of the mass distribution in COBE-normalized open and flat CDM models using large N-body simulations. With an age of the universe of 14 Gyr (12 Gyr) for the flat (open) models and a baryon density fixed by nucleosynthesis constraints, the observed abundance of rich galaxy clusters leads to tight constraints on the density parameter; 0.25<Omega_0<0.4 (0.4<Omega_0<0.5) for flat (open) models. The upper limits on Omega_0 can be relaxed if one lowers the H_0 and increases the age of the universe, but h<0.25 is required to allow Omega_0=1. The constraints also change if one allows tilted primordial power spectra. An acceptable Omega_0=1 model with h=0.5 can be constructed, with a tilt of n=0.8. We compare the evolved mass correlation functions of the most promising models with those of APM galaxies. The flat models have steep correlation functions at small scales and require galaxies to be antibiased on scales r<8 Mpc/h. The open models require little or no antibias on small scales and a positive bias on large scales. The lack of a positive bias on small scales is difficult to reconcile with the virial analysis of galaxy clusters which, if Omega_0>0.2, implies that galaxies are overabundant in clusters relative to the field. The tilted Omega_0=1 model, on the other hand, does require that galaxies be positively biased on all scales. We also compute the topology of isodensity contours in these models, obtaining theoretical predictions that are less sensitive to galaxy bias.

Testing Cosmological Models with a L[CLC]y[/CLC]α Forest Statistic: The High End of the Optical Depth Distribution

We pay particular attention to the high end of the $\lya$ optical depth distribution of a quasar spectrum. Based on the flux distribution (Miralda-Escud\'e et al 1996), a simple yet seemingly cosmological model -differentiating statistic, $\Delta_{\tau_0}$ -- the cumulative probability of a quasar spectrum with $\lya$ optical depth greater than a high value $\tau_0$ -- is emphasized. It is shown that two different models -- the cold dark matter model with a cosmological constant and the mixed hot and cold dark matter model, both normalized to COBE and local galaxy cluster abundance -- yield quite different values of $\Delta_{\tau_0}$: 0.13 of the former versus 0.058 of the latter for $\tau_0=3.0$ at $z=3$. Moreover, it is argued that $\Delta_{\tau_0}$ may be fairly robust to compute theoretically because it does not seem to depend sensitively on small variations of simulations parameters such as radiation field, cooling, feedback process, radiative transfer, resolution and simulation volume within the plausible ranges of the concerned quantities. Furthermore, it is illustrated that $\Delta_{\tau_0}$ can be obtained sufficiently accurately from currently available observed quasar spectra for $\tau_0\sim 3.0-4.0$, when observational noise is properly taken into account. We anticipate that analyses of observations of quasar $\lya$ absorption spectra over a range of redshift may be able to constrain the redshift evolution of the amplitude of the density fluctuations on small-to-intermediate scales, therefore providing an independent constraint on $\Omega_0$, $\Omega_{0,HDM}$ and $\Lambda_0$.

The Power Spectrum of Galaxy Clustering in the Las Campanas Redshift Survey

The Las Campanas Redshift Survey (LCRS) contains 23, 697 galaxies, with an average redshift z = 0.1, distributed over six 15 × 80° slices in the north and south galactic caps. We have computed the power spectrum P(k) for magnitude-limited samples of LCRS galaxies over wavelengths λ = 2π/k = 5–400 h−1 Mpc. The LCRS P(k) may be approximated as ∝ k−1.8±0.1 for small scales λ = 5–30 h−1 Mpc, changing to ∝k1 ± 1 for large scales λ ≈ 200-400 h−1 Mpc. The overall amplitude corresponds to σ8 = 1.0 ± 0.1 in redshift space. Comparisons to the power spectra of other redshift surveys will be presented; the LCRS results agree best with those from the combined Center for Astrophysics (CfA2) and Southern Sky redshift surveys (SSRS2). For λ 100 h−1 Mpc, the LCRS results are consistent with those of other surveys, given the large errors among all the surveys on these scales. For λ 100 h−1 Mpc, the LCRS P(k) is well determined and similar in shape to the P(k) of other surveys, but with an amplitude differing from some of the other samples, possibly because of inherent clustering differences among different types of galaxies. In particular, power spectrum measurements for volume-limited LCRS samples show that galaxies brighter than about M* − 1 appear about 50% more strongly clustered than those fainter. Also, a sample of LCRS emission galaxies shows 30% weaker clustering than the full LCRS sample. Comparisons to N-body models show that the LCRS power spectrum lies intermediate between that of a standard flat Ω0h = 0.5 cold dark matter (CDM) model and an open Ω0h = 0.2 model, both normalized to σ8 = 1 for galaxies. On large scales λ 40 h−1 Mpc, we have fit the LCRS results to various linear CDM models, and find that a number of them could meet the constraints set by the LCRS power spectrum, the Hubble constant range 0.5 h 0.8, the abundance of galaxy clusters, and the reasonable assumption that LCRS galaxies are roughly unbiased tracers of the mass, relative to the normalization provided by the 4 year COBE DMR data. The possibilities include open CDM or flat nonzero cosmological-constant CDM models with Ω0 ≈ 0.4–0.6 and shape parameter Γ ≈ Ω0h ≈ 0.2–0.3, as well as flat Ω0 = 1 models with massive neutrino density Ωv ≈ 0.2–0.3 or a spectral tilt n ≈ 0.7–0.8.

Large-Scale Structure in Mixed Dark Matter Models with a Nonthermal Volatile Component

view Abstract Citations (8) References (59) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Large-Scale Structure in Mixed Dark Matter Models with a Nonthermal Volatile Component Pierpaoli, Elena ; Coles, Peter ; Bonometto, Silvio ; Borgani, Stefano Abstract We investigate the properties of large-scale structure predicted in a class of mixed dark matter models in which the volatile component (made of particles with high rms velocity) derives from the decay of a heavier particle. Such models based on cold + volatile dark matter (CVDM) differ from the standard mixture of CDM and massive neutrinos, usually known as CHDM, in that they involve a component which has a nonthermal phase-space distribution function. As a consequence, and differently from CHDM models, the value of the redshift at which volatile particles become nonrelativistic, z_nr_, can be varied almost independently of the volatile fraction, {OMEGA}_X_. We compute transfer functions for a selection of such models, having 0.1 <= {OMEGA}_X_ <= 0.5 and different values of z_nr_. Using linear theory and assuming a scale-free primordial spectrum, we compare such models with observational constraints on large- scale galaxy clustering and bulk flows, as well as on the abundance of galaxy clusters and high-redshift damped Lyα systems. We find that these constraints enable us to discriminate between different {OMEGA}_x_ and z_nr_; within the range of the models inspected, those that can be accommodated most easily by the data correspond to the parameter choice {OMEGA}_X_ ~ 0.2 and z_nr_ ~2 x 10^4OMEGA_X_. Publication: The Astrophysical Journal Pub Date: October 1996 DOI: 10.1086/177852 arXiv: arXiv:astro-ph/9603150 Bibcode: 1996ApJ...470...92P Keywords: COSMOLOGY: LARGE-SCALE STRUCTURE OF UNIVERSE; GALAXIES: CLUSTERS: GENERAL; COSMOLOGY: DARK MATTER; Astrophysics E-Print: 20 pages LateX file, plus 4 ps-figures. Accepted for publication in ApJ full text sources arXiv | ADS |

The epoch of structure formation in blue mixed dark matter models

Recent data on the high-redshift abundance of damped Lyα systems are compared with theoretical predictions for ‘blue’ (i.e. n> 1) mixed dark matter (MDM) models. The results show that decreasing the hot component fraction Ωv and/or increasing the primordial spectral index n leads to an earlier epoch of cosmic structure formation. We also show, however, that varying Ωv and n in these ways makes the models barely consistent with the observed abundance of galaxy clusters. Therefore, requiring both the observational constraints on damped Lya systems and the cluster abundance to be satisfied simultaneously represents a challenge for the MDM class of models.

Cosmic microwave background anisotropies and the large scale structure of the universe

We discuss the implications of the COBE/DMR detection of Cosmic Microwave Background anisotropies on inflationary models for the large scale structure of the universe. We also discuss the constraints set on these models from current upper limits on the Cosmic Microwave Background anisotropies at small and intermediate angular scales, and from the local abundance of X-ray galaxy clusters.

Galaxy clustering and the dark-matter problem

Recent observational and theoretical results on galaxy clustering are reviewed. A major difficulty in relating observations to theory is that the former refer to luminous material whereas the latter is most directly concerned with the gravitationally dominant but invisible dark matter. The simple assumption that the distribution of galaxies generally follows that of the mass appears to conflict with evidence suggesting that galaxies of different kinds are clustered in different ways. If galaxies are indeed biased tracers of the mass, then dynamical estimates of the mean cosmic density, which give Ω « 0.2 may underestimate the global value of Ω. There are now several specific models for the behaviour of density fluctuations from very early times to the present epoch. The late phases of this evolution need to be followed by N -body techniques; simulations of scale-free universes and of universes dominated by various types of elementary particles are discussed. In the former case, the models evolve in a self-similar way; the resulting correlations have a steeper slope than that oberved for the galaxy distribution unless the primordial power spectral index n « 2. Universes dominated by light neutrinos acquire a large coherence length at early times. As a result, an early filamentary phase develops into a present day distribution that is more strongly clustered than observed galaxies and is dominated by a few clumps with masses larger than those of any known object. If the dark matter consists of ‘cold’ particles such as photinos or axions, then structure builds up from subgalactic scales in a roughly hierarchical way. The observed pattern of galaxy clustering can be reproduced if either Ω « 0.2 and the galaxies are distributed as the mass, or if Ω — 1, H 0 = 50 km s -1 Mpc -1 and the galaxies form only at high peaks of the smoothed linear density field. The open model, however, is marginally ruled out by the observed small-scale isotropy of the microwave background, whereas the flat one is consistent with such observations. With no further free parameters a flat cold dark-matter universe produces the correct abundance of rich galaxy clusters and of galactic halos; the latter have flat rotation curves with amplitudes spanning the observed range. Preliminary calculations indicate that the properties of voids may be consistent with the data, but the correlations of rich clusters appear to be somewhat weaker than those reported for Abell clusters.