Annu. Rev. Astron. Astrophys. 1992. 30: 653-703
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3.2.2 ANISOTROPY OBSERVATIONS AT THE OWENS VALLEY RADIO OBSERVATORY Intrinsic anisotropy observations have been carried out on the 40 meter telescope at the Owens Valley Radio Observatory since January 1984. The receiver was a reflected-wave ruby maser similar to the one used by Uson and Wilkinson (1984a, b, c). The one major difference between this system and that of Uson and Wilkinson is that it is a symmetric dual-feed system which has two beams separated on the sky in azimuth by 7'.15. The operating frequency for these observations was 20 GHz and the bandwidth was 400 MHz. The beamwidth was 108" (FWHM) and the beam efficiency was 47%. The input to the maser receiver was switched at 10 Hz between the feeds and the ``main'' and ``reference'' beams were alternated on the field of interest - i.e. this is a double differencing scheme similar to that employed by Uson and Wilkinson. The telescope has an altazimuth mount, which is well suited to this kind of work since it is possible to observe fields arbitrarily close to the north celestial pole, and the beam switching in azimuth minimizes the effects of differential ground spillover. The second order ground spillover terms are therefore much smaller than in the case of the Uson and Wilkinson observations described in the previous section, and it was not necessary to subtract offsets or drift terms from these observations (see below).

Two different observational programs were carried out on this telescope, designated the ``North Celestial Pole (NCP) experiment'' and the ``RING experiment''. These are described separately below.

The NCP program The object of this program was to obtain the maximum sensitivity on a small number of fields (Readhead et al 1989). Twelve fields equally spaced in hour angle at delta = 89° were selected, of which eight were observed for two hours centered on upper or lower culmination each day. The observations were made during five observing sessions. No adjustments to the data for variations in differential ground spillover or other systematic effects were necessary. The mean value of DeltaT found by averaging all of the data except that for NCP 7 (see below) was found to be

Equation 8a (8a)

which is consistent with zero - the expected mean value. It is remarkable that this result has been obtained from the raw data without any subtraction of means, allowances for drifts, etc. This gives great confidence that systematic errors have been successfully removed down to about 10 µK.

The results of these observations are shown in Figure 1b. It was clear from the first series of observations that one of the fields (NCP 7) was badly contaminated by a confusing discrete source, but observations were continued on this field since they gave some confidence that the system was working correctly. The confusing source in this field has been identified with observations at the VLA. For the purposes of estimating the intrinsic anisotropy this field was discarded and only the remaining seven fields were used. Readhead et al carried out a number of tests to ascertain the reliability of their data, and concluded that there were no serious problems with the data set. As the dataset has chi2nu approx 1, statistical analyses based on Bayesian methods and on the likelihood ratio test yield similar upper limits, which are given in Table 3. These are the most stringent limits obtained to date on angular scales of a few arc minutes. They definitively rule out adiabatic fluctuations in baryonic matter as the primary agents of galaxy formation with standard recombination. They also place interesting limits on many theories of galaxy formation (Readhead et al 1989, Vittorio & Muciaccia 1991, Bond & Myers 1991a).

The RING program The object of this program was to search for possible non-Gaussian fluctuations in the microwave background radiation (Myers 1990, Myers et al 1991, Myers et al 1992). Ninety-six fields were selected equally spaced in hour angle at delta = 88° 10' 42". The declination was chosen such that the fields are separated by an angle equal to the separation of the two beams on the sky, i.e. 7'.15. Thus, with the double switching scheme, any sky signal would appear in three adjacent fields as a main lobe straddled by two negative sidelobes whose strength depends on the exact location of the source within the beam. For a source in any position within the beam, the sum of the power in the two negative sidelobes is equal to the power in the main (positive) lobe, so that the sum of the signal over all three lobes for any sky source is zero.

The 96 fields were observed with the same setup as the NCP program, except that here the individual fields were observed for twelve minutes centered on upper culmination each day. The results of the observations on these 96 fields are shown in Figure 1c. A number of fields stand out. Convolution with a matched filter, which affords the maximum signal-to-noise-ratio for detecting signals with a given signature in noise, results in seven fields which differ from zero by more than 3 sigma. VLA observations at 1.4 GHz and 8.5 GHz revealed that four of these fields are contaminated by discrete sources. The remaining three fields contain no known sources which could be responsible for the observed signal. It is possible, however, that these fields are contaminated by sources with flat or inverted spectra, or variable sources. A detailed analysis of the level of the ``excess variance'' in the fields shows that it is unlikely, but by no means impossible, that this is entirely due to contamination by discrete sources (Myers et al 1991, 1992). For this reason the RING results are interpreted as upper limits rather than detections of intrinsic anisotropy. There is clearly a danger, in this approach, of misidentifying real intrinsic fluctuations as due to foreground objects, and the fields in which significant ``signals'' were detected will be studied further to ascertain the origin of these signals.

The upper limit derived from the RING data after subtraction of the effects of the identified confusing sources is given in Table 3. This is about a factor 2.5 higher than the limit derived from the NCP program for Gaussian fluctuations, but it provides more stringent limits on models with non-Gaussian fluctuations.

The design of the interlocking fields in the RING program was chosen specifically to provide an independent test of the experimental procedure. Whereas in the case of the NCP program we expect the average of a sufficient number of fields to approach zero in the case of the RING program the true sky mean must be zero. Thus a stringent test of the observing procedure is provided by averaging all 96 fields. The result of this average is:

Equation 8b (8b)

The fact that the value of < DeltaT/T > for the raw data lies within 1 sigma of zero when no zero levels or drifts have been subtracted is a powerful demonstration of freedom from systematic errors.

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