arXiv subjects

Gilbert P. Holder

Publications and source records attributed to Gilbert P. Holder.

45 records · Page 3Linked to original sources

arXiv2000

Galaxy Cluster Gas Mass Fractions from Sunyaev-Zel'dovich Effect Measurements: Constraints on Omega_M

Using sensitive centimeter-wave receivers mounted on the Owens Valley Radio Observatory and Berkeley-Illinois-Maryland-Association millimeter arrays, we have obtained interferometric measurements of the Sunyaev-Zel'dovich (SZ) effect toward massive galaxy clusters. We use the SZ data to determine the pressure distribution of the cluster gas and, in combination with published X-ray temperatures, to infer the gas mass and total gravitational mass of 18 clusters. The gas mass fraction, f_g, is calculated for each cluster, and is extrapolated to the fiducial radius r_{500} using the results of numerical simulations. The mean f_g within r_{500} is 0.081+0.009 -0.011/(h_{100} (statistical uncertainty at 68% confidence level, assuming OmegaM=0.3, OmegaL=0.7). We discuss possible sources of systematic errors in the mean f_g measurement. We derive an upper limit for OmegaM from this sample under the assumption that the mass composition of clusters within r_{500} reflects the universal mass composition: Omega_M h < Omega_B/f_g. The gas mass fractions depend on cosmology through the angular diameter distance and the r_{500} correction factors. For a flat universe (OmegaL = 1 - OmegaM) and h=0.7, we find the measured gas mass fractions are consistent with Omegam less than 0.40, at 68% confidence. Including estimates of the baryons contained in galaxies and the baryons which failed to become bound during the cluster formation process, we find OmegaM \~0.25.

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arXiv2000

Constraints on Cosmological Parameters from Future Galaxy Cluster Surveys

We study the expected redshift evolution of galaxy cluster abundance between 0 < z < 3 in different cosmologies, including the effects of the cosmic equation of state parameter w=p/rho. Using the halo mass function obtained in recent large scale numerical simulations, we model the expected cluster yields in a 12 deg^2 Sunyaev-Zeldovich Effect (SZE) survey and a deep 10^4 deg^2 X-ray survey over a wide range of cosmological parameters. We quantify the statistical differences among cosmologies using both the total number and redshift distribution of clusters. Provided that the local cluster abundance is known to a few percent accuracy, we find only mild degeneracies between w and either Omega_m or h. As a result, both surveys will provide improved constraints on Omega_m and w. The Omega_m-w degeneracy from both surveys is complementary to those found either in studies of CMB anisotropies or of high-redshift Supernovae (SNe). As a result, combining these surveys together with either CMB or SNe studies can reduce the statistical uncertainty on both w and Omega_m to levels below what could be obtained by combining only the latter two data sets. Our results indicate a formal statistical uncertainty of about 3% (68% confidence) on both Omega_m and w when the SZE survey is combined with either the CMB or SN data; the large number of clusters in the X-ray survey further suppresses the degeneracy between w and both Omega_m and h. Systematics and internal evolution of cluster structure at the present pose uncertainties above these levels. We briefly discuss and quantify the relevant systematic errors. By focusing on clusters with measured temperatures in the X-ray survey, we reduce our sensitivity to systematics such as non-standard evolution of internal cluster structure.

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arXiv2000

Galaxy Cluster Baryon Fractions, Cluster Surveys and Cosmology

The properties of nearby galaxy clusters limit the range of cosmological parameters consistent with our universe. We describe the limits which arise from studies of the intracluster medium (ICM) mass fraction fICM and consideration of the possible sources of systematic error: Omega_M<0.44h_{50}^{-1/2} at 95% confidence. We emphasize that independent of Type Ia supernovae (SNe Ia) observations, this cluster study, taken together with published cosmic microwave background (CMB) anisotropy studies, indicates a non-zero quintessence or dark energy component Omega_Q>0. We then discuss future galaxy cluster surveys which will probe the abundance of galaxy clusters to intermediate and high redshift. We investigate the sensitivity of these surveys to the cosmological density parameter Omega_M and the equation of state parameter w of any quintessence component. In particular, we show that cluster survey constraints from a proposed large solid angle X-ray survey are comparable in precision and complementary in nature to constraints expected from future CMB anisotropy and SNe Ia studies.

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arXiv2000

The Sunyaev-Zel'dovich Effect in Abell 370

We present interferometric measurements of the Sunyaev-Zel'dovich (SZ) effect towards the galaxy cluster Abell 370. These measurements, which directly probe the pressure of the cluster's gas, show the gas distribution to be strongly aspherical, as do the x-ray and gravitational lensing observations. We calculate the cluster's gas mass fraction in two ways. We first compare the gas mass derived from the SZ measurements to the lensing-derived gravitational mass near the critical lensing radius. We also calculate the gas mass fraction from the SZ data by deprojecting the three-dimensional gas density distribution and deriving the total mass under the assumption that the gas is in hydrostatic equilibrium (HSE). We test the assumptions in the HSE method by comparing the total cluster mass implied by the two methods and find that they agree within the errors of the measurement. We discuss the possible systematic errors in the gas mass fraction measurement and the constraints it places on the matter density parameter, OmegaM.

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arXiv1999

The Sunyaev-Zeldovich Effect as Microwave Foreground and Probe of Cosmology

The Sunyaev-Zel'dovich (SZ) effect from clusters of galaxies should yield a significant signal in cosmic microwave background(CMB) experiments at small angular scales ($\ell \ga 1000$). Experiments with sufficient frequency coverage should be able to remove much of this signal in order to recover the primary anisotropy. The SZ signal is interesting in its own right; the amplitude and angular dependence are sensitive to both cosmology and the evolution of the gas. Combining CMB measurements with planned non-targeted SZ surveys could isolate the cosmological effects, providing CMB experiments with a low-redshift test of cosmology as a consistency check. Improvements in the determination of the angular diameter distance as a function of redshift from SZ and X-ray observations of a large sample of clusters will also provide a probe of cosmology.

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arXiv1999

Cluster-Cluster Strong Lensing: Expectations and Detection Methods

We calculate the all-sky number of galaxy clusters that are expected to be gravitationally lensed by foreground massive clusters. We describe the redshift and number distributions of clusters using a Press-Schechter analysis, and model the foreground lensing clusters as singular isothermal spheres. If Omega_m=0.3 and Omega_Lambda=0.7, we expect ~ 30 cluster-cluster strong lensing events that involve foreground X-ray luminous clusters with total mass greater than 7.5 x 10^14 h^-1 M_sun, or X-ray luminosity L_x (2-10 keV) 8 x 10^44 h^-2 ergs s^-1, and background clusters with total mass greater than 10^14 h^-1 M_sun. The number expected in an open universe with Omega_m = 0.3 is less than \~ 4. Because of uncertainty in sigma_8, the root-mean-square density fluctuations in spheres of radius 8 h^-1 Mpc, the exact number of such lensing events is uncertain by a factor of about 5. We examine methods to detect cluster-cluster lensing events based on optical, X-ray, and Sunyaev-Zel'dovich effect observations.

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arXiv1997

No Need for MACHOS in the Halo

A simple interpretation of the more than dozen microlensing events seen in the direction of the LMC is a halo population of MACHOs which accounts for about half of the mass of the Galaxy. Such an interpretation is not without its problems, and we show that current microlensing data can, with some advantage, be explained by dark components of the disk and spheroid, whose total mass is only about 10% of the mass of the Galaxy.

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arXiv1997

Globular Cluster Microlensing: Globular Clusters as Microlensing Targets

We investigate the possibility of using globular clusters as targets for microlensing searches. Such searches will be challenging and require more powerful telescopes than now employed, but are feasible in the 0 future. Although expected event rates are low, we show that the wide variety of lines of sight to globular clusters greatly enhances the ability to distinguish between halo models using microlensing observations as compared to LMC/SMC observations alone.

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arXiv1995

Microlensing By a Prolate All-Macho Halo

It is widely believed that dark matter halos are flattened, that is closer to oblate than prolate. The evidence cited is based largely on observations of galaxies which do not look anything like our own and on numerical simulations which use ad hoc initial conditions. Given what we believe to be a ``reasonable doubt'' concerning the shape of dark Galactic halo we calculate the optical depth and event rate for microlensing of stars in the LMC assuming a wide range of models that include both prolate and oblate halos. We find, in agreement with previous analysis, that the optical depth for a spherical (E0) halo and for an oblate (E6) halo are roughly the same, essentially because two competing effects cancel approximately. However the optical depth for an E6 prolate halo is reduced by ~35%. This means that an all-Macho prolate halo with reasonable parameters for the Galaxy is consistent with the published microlensing event rate.

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