Gilbert P. Holder

The Reionization History at High Redshifts. II. Estimating the Optical Depth to Thomson Scattering from Cosmic Microwave Background Polarization

In light of the recent inference of a high optical depth τ to Thomson scattering from the Wilkinson Microwave Anisotropy Probe (WMAP) data, we investigate the effects of extended periods of partial ionization and ask if the value of τ inferred by assuming a single sharp transition is an unbiased estimate. We construct and consider several representative ionization models and evaluate their signatures in the cosmic microwave background (CMB). If τ is estimated with a single sharp transition, we show that there can be a significant bias in the derived value (and, therefore, a bias in σ8 as well). For WMAP noise levels, the bias in τ is smaller than the statistical uncertainty, but for Planck or a cosmic variance limited experiment the τ bias could be much larger than the statistical uncertainties. This bias can be reduced in the ionization models we consider by fitting a slightly more complicated ionization history, such as a two-step ionization process. Assuming this two-step process, we find that the Planck satellite can simultaneously determine the initial redshift of reionization to ±2 and τ to ±0.01. Uncertainty about the ionization history appears to provide a limit of ~0.005 on how well τ can be estimated from CMB polarization data, much better than expected from WMAP but significantly worse than expected from cosmic variance limits.

Effects of submillimeter and radio point sources on the recovery of Sunyaev-Zel'dovich galaxy cluster parameters

Observations of clusters in the 30-350 GHz range can in principle be used to determine a galaxy clusters Comptonization parameter y, peculiar velocity v, and gas temperature Te via the dependence of the kinetic and thermal Sunyaev-Zeldovich effects on these parameters. Despite the significant contamination expected from thermal emission by dust in high-redshift galaxies, we find that the simultaneous determination of ?, v, and Te is possible from observations with sensitivity of a few ?K in three or more bands with arcminute resolution. After allowing for realistic levels of contamination by dusty galaxies and primary CMB anisotropy, we find that simultaneous determinations of velocities to an accuracy of better than 200 km s-1 and temperatures to roughly keV accuracy should be possible in the near future. We study how errors change as a function of cluster properties (angular core radius and gas temperature) and experimental parameters (observing time, angular resolution, and observing frequencies). Contaminating synchrotron emission from cluster galaxies will probably not be a major contaminant of peculiar velocity measurements.

Cluster Sunyaev-Zeldovich Effect Scaling Relations

X-ray observations of an entropy floor in nearby groups and clusters of galaxies offer evidence that important nongravitational processes, such as radiative cooling and/or preheating, have strongly influenced the evolution of the intracluster medium (ICM). We examine how the presence of an entropy floor modifies the thermal Sunyaev-Zeldovich (SZ) effect. A detailed analysis of scaling relations between X-ray and SZ effect observables and also between the two primary SZ effect observables is presented. We find that relationships between the central Compton parameter and the temperature or mass of a cluster are extremely sensitive to the presence of an entropy floor. The same is true for correlations between the integrated Compton parameter and the X-ray luminosity or the central Compton parameter. In fact, if the entropy floor is as high as inferred in recent analyses of X-ray data, a comparison of these correlations with both current and future SZ effect observations should show a clear signature of this excess entropy. Moreover, because the SZ effect is redshift independent, the relations can potentially be used to track the evolution of the cluster gas and possibly discriminate between the possible sources of the excess entropy. To facilitate comparisons with observations, we provide analytic fits to these scaling relations.

The Sunyaev-Zeldovich Effect Signature of Excess Entropy in Distant, Massive Clusters

Studies of cluster X-ray scaling relations have led to suggestions that nongravitational processes, e.g., radiative cooling and/or preheating, have significantly modified the entropy of the intracluster medium (ICM). For the first time, we test this hypothesis through a comparison of predicted thermal Sunyaev-Zeldovich (SZ) effect scaling relations with available data from the literature. One of the relations that we explore, in principle, depends solely on SZ effect observations, thus offering an X-ray-independent probe of the ICM. A detailed comparison of the theoretical relations with the largest compilation of high-z SZ effect data to date indicates that the presence of an entropy floor is favored by the data. Furthermore, the inferred level of that floor, K0 300 keV cm2, is comparable to that found in studies of X-ray scaling relations of nearby massive clusters. Thus, we find no evidence for significant evolution of the entropy floor out to z ~ 0.7. We further demonstrate that the high-quality data to be obtained from the upcoming Sunyaev-Zeldovich Array (SZA) and the (soon to be) upgraded Owens Valley Radio Observatory (OVRO) array will open powerful new windows into the properties of the ICM. Specifically, the new measurements will allow for accurate measurements of the ICM entropy for even the most distant galaxy clusters.

MEASURING CLUSTER PECULIAR VELOCITIES AND TEMPERATURES AT CENTIMETER AND MILLIMETER WAVELENGTHS

I present a detailed investigation of issues related to the measurement of peculiar velocities and temperatures using Sunyaev-Zeldovich (SZ) effects and estimate the accuracy to which peculiar velocities and gas temperatures of distant galaxy clusters could be measured. With μK sensitivity on arcminute scales at several frequencies it will be possible to measure peculiar velocities to an accuracy of ~130 km s-1 and gas temperatures to better than 1 keV. The limiting factor for the accuracy of vpec is the presence of bulk motions within the galaxy cluster, even for apparently relaxed clusters. The accuracy of the temperature is mainly limited by noise. These results are independent of redshift, provided the clusters of interest are distant (z 0.15). Using only three frequencies, the optimal strategy is to place one observing frequency in the Rayleigh-Jeans region (ν < 40 GHz), one near 150 GHz, and the third at 300 GHz or higher. Measurements at the null of the thermal SZ effect are of marginal utility, other than as a foreground/background monitor.

Radio Point Sources and the Thermal Sunyaev-Zeldovich Power Spectrum

Radio point sources are strongly correlated with clusters of galaxies, so a significant fraction of the thermal Sunyaev-Zeldovich (SZ) effect signal could be affected by point-source contamination. Based on empirical estimates of the radio galaxy population, it is shown that the rms temperature fluctuations of the thermal SZ effect could be underestimated by as much as 30% at an observing frequency of 30 GHz at l 1000. The effect is larger at higher multipoles. If the recent report of excess power at small angular scales is to be explained by the thermal SZ effect, then radio point sources at an observing frequency of 30 GHz must be a surprisingly weak contaminant of the SZ effect for low-mass clusters.

The Cosmic Microwave Background Quadrupole in a Polarized Light

The low quadrupole of the cosmic microwave background (CMB), measured by COBE and confirmed by WMAP, has generated much discussion recently. We point out that the well-known correlation between temperature and polarization anisotropies of the CMB further constrains the low multipole anisotropy data. This correlation originates from the fact that the low-multipole polarization signal is sourced by the CMB quadrupole as seen by free electrons during the relatively recent cosmic history. Consequently, the large-angle temperature anisotropy data make restrictive predictions for the large-angle polarization anisotropy, which depend primarily on the optical depth for electron scattering after cosmological recombination, tau. We show that if current cosmological models for the generation of large angle anisotropy are correct and the COBE/WMAP data are not significantly contaminated by non-CMB signals, then the observed C_te amplitude on the largest scales is discrepant at the 99.8% level with the observed C_tt for the concordance LCDM model with tau=0.10. Using tau=0.17, the preferred WMAP model-independent value, the discrepancy is at the level of 98.5%.The low quadrupole of the cosmic microwave background (CMB), measured by the Cosmic Background Explorer (COBE) and confirmed by the Wilkinson Microwave Anisotropy Probe (WMAP), has generated much discussion recently. We point out that the well-known correlation between temperature and polarization anisotropies of the CMB further constrains the low-multipole anisotropy data. This correlation originates from the fact that the low-multipole polarization signal is sourced by the CMB quadrupole as seen by free electrons during the relatively recent cosmic history. Consequently, the large-angle temperature anisotropy data make restrictive predictions for the large-angle polarization anisotropy, which depend primarily on the optical depth for electron scattering after cosmological recombination, τ. We show that if current cosmological models for the generation of large-angle anisotropy are correct and the COBE/WMAP data are not significantly contaminated by non-CMB signals, then the observed C amplitude on the largest scales is discrepant at the ~99.8% level with the observed C for the concordant ΛCDM model with τ = 0.10. Using τ = 0.17, the preferred WMAP model-independent value, the discrepancy is at the level of 98.5%.