Cosmology with Clusters of Galaxies

I show that three independent methods utilizing clusters of galaxies - cluster dynamics and mass-to-light ratio, baryon fraction in clusters, and cluster evolution - all indicate the same robust result: the mass-density of the universe is low, Omega ~ 0.2, and the mass approximately traces light on large scales.Comment: Invited talk at Nobel98, ``Particle Physics and the Universe,''8/1998, 15 pages, 4 figure

CLUSTERING AND LARGE SCALE STRUCTURE WITH THE SDSS

The Sloan Digital Sky Survey (SDSS) will provide a complete imaging and spectroscopic survey of the high-latitude northern sky. The 2D survey will image the sky in five colors and will contain nearly 5 x 107 galaxies to g ~ 23m. The spectroscopic survey will obtain spectra of the brightest 106 galaxies, 105 quasars, and 103.5 rich clusters of galaxies (to g~18.3-19.3m, respectively). I summarize some of the science opportunities that will be made possible by this survey for studying the clustering and large-scale structure of the universe. The survey will identify a complete sample of several thousand rich clusters of galaxies, both in 2D and 3D - the largest automated sample yet available. The extensive cluster sample can be used to determine critical clustering properties such as the luminosity-function, velocity-function, and mass-function of clusters of galaxies (a critical test for cosmological models), detailed cluster dynamics and W(dyn), the cluster correlation function and its dependence on richness, cluster evolution, superclustering and voids to the largest scales yet observed, the motions of clusters and their large-scale peculiar velocity field, as well as detailed correlations between x-ray and optical properties of clusters, the density-morphology relation, and cluster-quasar associations. The large redshift survey, reaching to a depth of 600h-1 Mpc, will accurately map the largest scales yet observed, determine the power-spectrum and correlation function on these large scales for different type galaxies, and study the clustering of quasars to high redshifts (z 4). The implications of the survey for cosmological models, the dark matter, and W are also discussed.Comment: compressed PostScript, invited talk presented at the AAS meeting, Minneapolis, June 1994, to appear in PASP 1995; for the figures contact [email protected]

Present Status of the Theoretical Predictions for the ^(37)Cl Solar-Neutrino Experiment

The theoretical predictions for the ^(37)Cl solar-neutrino experiment are summarized and compared with the experimental results of Davis, Harmer, and Hoffman. Three important conclusions about the sun are shown to follow

Tracing mass and light in the Universe: where is the dark matter?

How is mass distributed in the Universe? How does it compare with the distribution of light and stars? We address these questions by examining the distribution of mass, determined from weak lensing observations, and starlight, around $>10^5$ SDSS MaxBCG groups and clusters as a function of environment and scale, from deep inside clusters to large cosmic scales of $22 h^{-1}$ Mpc. The observed cumulative mass-to-light profile, $M/L (< r)$, rises on small scales, reflecting the increasing $M/L$ of the central bright galaxy of the cluster, then flattens to a nearly constant ratio on scales above $\sim 300 h^{-1}$ kpc, where light follows mass on all scales and in all environments. A trend of slightly decreasing $M/L (r)$ with scale is shown to be consistent with the varying stellar population following the morphology-density relation. This suggests that stars trace mass remarkably well even though they represent only a few percent of the total mass. We determine the stellar mass fraction and find it to be nearly constant on all scales above $\sim 300 h^{-1}$ kpc, with $M_{*}/M_{tot} \simeq 1.0\pm0.4\%$. We further suggest that most of the dark matter in the Universe is located in the large halos of individual galaxies ($\sim 300$ kpc for $L^{*}$ galaxies); we show that the entire $M/L (r)$ profile -- from groups and clusters to large-scale structure -- can be accounted for by the aggregate masses of the individual galaxies (whose halos may be stripped off but still remain in the clusters), plus gas. We use the observed mass-to-light ratio on large scales to determine the mass density of the Universe: $\Omega_{m} = 0.24 \pm 0.02 \times b_{M/L}^{2} = 0.26 \pm 0.02.$Comment: 12 pages, 9 figures; version accepted to MNRA

COSMOLOGY WITH CLUSTERS OF GALAXIES

Rich clusters of galaxies, the largest virialized systems known, provide a powerful tool for the study of cosmology. Some of the fundamental questions that can be addressed with clusters of galaxies include: how did galaxies and large-scale structure form and evolve? What is the amount, composition and distribution of matter in the universe? I review some of the studies utilizing clusters of galaxies to investigare, among others: - The dark matter on clusters scale and the mean mass-density of the universe; - The large-scale structure of the universe; - The peculiar velocity field on large scales; - The mass-function of groups and clusters of galaxies; - The constraints placed on specific cosmological models using the cluster data.Comment: compressed PostScript, to appear in the Proceedings of 11th Potsdam Cosmology Workshop on "Large Scale Structrure in the Universe", 1994; for the figures contact [email protected]

Velocity Correlations of Galaxy Clusters

We determine the velocity correlation function, pairwise peculiar velocity difference, and root-mean-square pairwise peculiar velocity dispersion of rich clusters of galaxies, as a function of pair separation, for three cosmological models: Omega=1 and Omega=0.3 CDM, and Omega=0.3 PBI models (all flat and COBE-normalized). We find that close cluster pairs, with separation r<10Mpc/h, exhibit strong attractive peculiar velocities in all models; the cluster pairwise velocities depend sensitively on the model. The mean pairwise attractive velocity of clusters on 5Mpc/h scale ranges from 1700 km/s for Omega=1 CDM, to 1000 km/s for PBI, to 700 km/s for Omega=0.3 CDM. The small-scale pairwise velocities depend also on cluster mass: richer, more massive clusters exhibit stronger attractive velocities than less massive clusters. On large scales, from 20 to 200Mpc/h, the cluster peculiar velocities are increasingly dominated by bulk and random motions; they are independent of cluster mass. The cluster velocity correlation function, which reflects the bulk motion minus the relative motion of pairs, is negative on small scales for Omega=1 and Omega=0.3 CDM, and positive for PBI; this indicates stronger pairwise motion than bulk motion on small scales for CDM, and relatively larger bulk motions for PBI. The cluster velocity correlation function is positive on very large scales, from 10 to 200Mpc/h, for all models. These positive correlations, which decrease monotonically with scale, indicate significant bulk motions of clusters up to 200Mpc/h. The strong dependence of the cluster velocity functions on models, especially at small separations, makes them useful tools in constraining cosmological models when compared with observations.Comment: 12p postscript file, in press of The Astrophysical Journal Letters Local report# 94915,email: [email protected]

The Motions of Clusters and Groups of Galaxies

The distributions of peculiar velocities of rich clusters and of groups of galaxies are investigated for different cosmological models and are compared with observations. Four cosmological models are studied: standard ($\Omega=1$) CDM, low-density CDM, HDM ($\Omega=1$), and PBI. We find that rich clusters of galaxies exhibit a Maxwellian distribution of peculiar velocities in all models, as expected from a Gaussian initial density fluctuation field. The cluster 3-D velocity distribution is generally similar in the models: it peaks at $v \sim 500$ km s$^{-1}$, and extends to high cluster velocities of $v \sim 1500$ km s$^{-1}$. Approximately 10\% of all model rich clusters move with high peculiar velocities of $v \ge 10^3$ km s$^{-1}$. The highest velocity clusters frequently originate in dense superclusters. The group velocity distribution is, in general, similar to the velocity distribution of the rich clusters. In all but the low-density CDM model, the mass exhibits a longer tail of high velocities than do the clusters. This high-velocity tail originates mostly from the high velocities that exist within rich clusters. The model velocity distributions of groups and clusters of galaxies are compared with observations. The data are generally consistent with the models, but exhibit a somewhat larger high-velocity tail, to $v_r \sim 3000$ km s$^{-1}$. While this high-velocity tail is similar to the HDM model predictions, the data are consistent with the other models studied, including the low-density CDM model, which best fits most other large-scale structure observations. The observed velocityComment: 25p plaintex submitted to The Astrophysical Journa