Measurements of Arcminute Scale Anisotropy with the BIMA Array

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Structure Formation

Optical redshift surveys have shown that galaxies are not distributed randomly in the universe, but are instead grouped into clusters and voids as shown in the figure below.

The clustering of galaxies is the largest scale at which matter is gravitationally bound in the universe, a galaxy cluster often contains thousands of galaxies. Studies of galaxy clusters can therefore act as a probe for studying the cosmological parameters that determine the growth of structure in the universe.

Clusters of galaxies are typically studied through optical observations, X-ray observations, and in the last ten years, Sunyaev-Zel'dovich (SZ) observations. However, most methods are subject to a bias due to cosmological redshift. In other words, the more distant clusters are more difficult to detect. Only the SZ effect can be used to identify clusters of galaxies independent of redshift. This property should allow an SZ survey to identify galaxy clusters that are too distant to be discovered in other surveys, producing a more complete catalog of clusters.

SZ Effect

The thermal SZ effect arises when low energy (2.73 K) Cosmic Microwave Background (CMB) photons scatter off high energy charged particles (100,000,000 K) in a galaxy cluster. These charged particles were originally released when stars started burning. A certain fraction of the starlight was absorbed by neutral atoms, thereby releasing the electrons. The free electrons and ionized nuclei fell into the gravitational well of the neighborhood galaxy cluster and gained large amounts of energy (several keV) in the process. This energy is transferred from the hot intracluster plasma to the CMB photons through inverse Compton scattering. This process is referred to as the thermal SZ effect. A similar process is the kinetic SZ effect which arises from a Doppler shift of CMB photons that scatter from the core of a cluster that is travelling with some velocity with respect to the CMB. The signal of the kinetic SZ effect is significantly smaller than that from the thermal SZ effect and is yet to be observed.

The average energy exchange in the scattering process is kBTe/mec2, where kB is Boltzmann's constant, Te is the temperature of the cluster gas core, me is the mass of the electron, and c is the speed of light. About one out of every hundred CMB photons is scattered by the particles in the cluster core. Since these photons typically gain energy in the process, the result of the scattering is that the spectrum of the CMB photons is distorted in the direction of a galaxy cluster when observed from Earth (right). For more information on the SZ effect and cosmology, look here.

The SZ Power Spectrum

The evolution of galaxy clusters is a sensitive probe of cosmology. Measurements of cluster masses and number density as a function of redshift can be used to constrain the matter density, ΩM of the universe, and the equation of state of the cosmological constant. Perhaps the most powerful use of the SZ effect will be to probe the high redshift universe for clusters of galaxies in a blind survey. Deep surveys that cover large areas of sky will be sensitive to many high redshift clusters that would not otherwise be observable. Correlating observations from an SZ survey with optical observations will provide not only cluster detections, but will hopefully also provide redshift information which is essential for constraining these cosmological parameters.

However, the available telescopes are not sensitive enough to cover the field of view required to identify these large numbers of galaxy clusters. Instead, it is possible to choose a smaller region of sky and search for CMB anisotropy on the arcminute angular scales that are matched to the size of a typical galaxy cluster. From this sample, the data can be analyzed to estimate an angular power spectrum. The angular power spectrum is a measure of the signal that occurs as a function of angular scale. A power spectrum analysis provides sensitivity to objects that lie near the noise level of an image by comparing the observed data to the signal that is expected from instrumental noise. A low mass cluster of galaxies can still contribute to the power spectrum by altering the overall statistics of the data. We have used the BIMA array to estimate the power spectrum on angular scales that are expected to be dominated by the thermal SZ effect.

Copyright Holzapfel Group, 2002 Page last modified December 16, 2002