Measurements of Arcminute Scale Anisotropy with the BIMA Array

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Observations and Results

BIMA Observations

All anisotropy observations were made using the BIMA array in Hat Creek, CA. Nine 6.1 m telescopes were equipped for operation at 28.5 GHz, providing a 6.6' FWHM field of view. All field observations were bracketed by observations of bright radio point sources in order to track the phase fluctuations caused by the instrument and the atmosphere.

The survey consists of 10 fields of fairly uniform depth. The average field was observed for 50-60 hours, achieving a noise level of ~150 uJy/beam on short baselines. The noise level corresponds to a temperature sensitivity of ~15 uK for a 2' synthesized beam. Each of these fields was then analyzed for signal that exceeds what is to be expected from instrumental noise. This excess power is thought to be caused by signal from CMB anisotropy.

VLA Observations

Emission from radio point sources at 28.5 GHz is expected to contribute significantly to excess power in the BIMA observations. The compact configuration has limited sensitivity to discriminate between extended emission and point sources. To help constrain the contribution from point sources to the anisotropy measurements, we used the Very Large Array (VLA) at a frequency of 4.8 GHz to observe each field in the survey. With 1.5 hr per field, we identifed the positions of all point sources with flux greater than 6 times the noise level in the map at 4.8 GHz. Assuming the flux of a radio point source decreases or remains relatively flat with increasing frequency, the VLA observations should be deep enough to identify all point sources that lie near the noise level in the 28.5 GHz BIMA maps. The contribution of point soures to estimates of anisotropy can then be removed knowing only the positions. Below is an example of an image from the VLA observations. The inner dashed circle represents the primary beam of the BIMA telescopes while the outer dotted circle represents the primary beam of the VLA telescopes.


A significant level of excess power was found in the anisotropy survey. Combining the sample of ten fields, we find a level of excess power that exceeds the expectation from instrumental noise by better than 95% confidence. To the left is a figure that compares the results of this work with the results from the Cosmic Background Imager (CBI) and theoretical models. A description of the data analysis and a detailed explanation of the results of the experiment can be found in our most recent paper. As described by Komatsu and Seljak, the results favor a σ8=1 cosmology. Keep posted for results from our last summer of observation with which we doubled the size of the survey.

Planned Follow-up Observations

The SZ effect is believed to be the most likely candidate for the excess power detected in the BIMA fields. However, independent observations with an optical telescope are required in order to confirm that the signal is indeed caused by clusters of galaxies. We have applied for observing time with the Apache Point Observatory (APO) to observe the 10 fields in the anisotropy survey. With observations in the V band to a limiting magnitude of M=25, and observations in the I band to a limiting magnitude of M=24, we hope to identify the galaxies associated with a cluster. A cluster would appear as an overdensity of objects of the same color. Correlating the optical data with the BIMA data should produce a sensitivity to low mass clusters that would not otherwise be identified in a single measurement.

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