A Measurement of Anisotropy in the Cosmic Microwave Background on 7′–22′ Scales

Abstract

We report a measurement of anisotropy in the cosmic microwave background radiation (CMBR) on 7'–22' scales. Observations of 36 fields near the North Celestial Pole (NCP) were made at 31.7 and 14.5 GHz, using the 5.5 m and 40 m telescopes at the Owens Valley Radio Observatory (OVRO) from 1993 to 1996. Multiepoch VLA observations at 8.5 and 15 GHz allow removal of discrete source contamination. After point-source subtraction, we detected significant structure, which we identify with emission from a combination of a steep-spectrum foreground and the CMBR. The foreground component is found to correlate with IRAS 100 μm dust emission. Lack of Hα emission near the NCP suggests that this foreground is either high-temperature thermal bremsstrahlung (Te 106 K), flat-spectrum synchrotron, or an exotic component of dust emission. On the basis of low-frequency maps of the NCP, we can restrict the spectral index of the foreground to β ≥ −2.2. Although the foreground signal dominates at 14.5 GHz, the extracted CMBR component contributes 88% of the variance at 31.7 GHz, yielding an rms fluctuation amplitude of 82 μK, including 4.3% calibration uncertainty and 12% sample variance (68% confidence). In terms of the angular power spectrum, Cl = |a|2, averaged over a range of multipoles l = 361-756, the detected broadband amplitude is δTle ≡ [l(l + 1)Cl/(2π)]1/2 = 59 μK. This measurement, when combined with small angular scale upper limits obtained at the OVRO, indicates that the CMBR angular power spectrum decreases between l ~ 600 and l ~ 2000 and is consistent with flat cosmological models.