A limit of the anisotropy of the microwave background radiation on arc minute scales

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Abstract

Observations at the Owens Valley Radio Observatory at a frequency of 20 GHz give an upper limit on the anisotropy of the microwave background radiation of σT/T < 1.7 x 10^-5^ (95% confidence) for uncorrelated patches of sky that are uniform on a 2' scale. This limit is more than a factor of 2 lower than previous limits on comparable angular scales. For the more realistic case of fluctuations with a Gaussian autocorrelation function with coherence angle φ_c_, the corresponding upper limits are 9.4 x 10^-5^ (φ_c_ = 12"), 1.9 x 10^-5^ ({phi_c_ = 2.6'), and 3.0 x 10^-4^ (φ_c_ = 25'). These results place useful constraints on models of galaxy formation based on adiabatic or isocurvature fluctuations in baryonic matter, provided that any reionization of the intergalactic medium occurred at z < 40. Adiabatic models are ruled out with greater than 95% confidence, and isocurvature models with {OMEGA} < 0.8 are inconsistent with our limits. Theories of galaxy formation that invoke nonbaryonic matter, biased galaxy formation, or a significant fraction of ionized hydrogen at z > 40 predict levels of anisotropy a factor of 2 or 3 (and in extreme cases a factor of 10) below the present limit. In the case of nonstandard recombination our limits may provide useful constraints on possible reionization processes. The predictions of most popular contending theories of galaxy formation are within reach of the techniques used in this study.

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