3.2. Single-dish Observations
The first measurements of the SZE were made with single-dish radio telescopes at centimeter wavelengths. Advances in detector technology made the measurements possible, although early observations appear to have been plagued by systematic errors that led to irreproducible and inconsistent results. Eventually, successful detections using beam-switching techniques were obtained. During this period, the pioneering work of Birkinshaw and collaborators with the OVRO 40 m telescope stands out for its production of results that served to build confidence in the technique (Birkinshaw et al., 1978b; Birkinshaw et al., 1991; Birkinshaw et al., 1978a). Figure 4 shows the OVRO 40 m scan through the galaxy cluster Cl 0016 + 16. More recently, leading and trailing beam-switching techniques have been used successfully with the OVRO 5 m telescope at 32 GHz to produce reliable detections of the SZE in several intermediate-redshift clusters (Mason et al., 2001; Myers et al., 1997; Herbig et al., 1995). The SEST 15 m and IRAM 30 m telescopes have been used with bolometric detectors at 140 GHz and chopping mirrors to make significant detections of the SZE in several clusters (Andreani et al., 1999; Pointecouteau et al., 2001; Désert et al., 1998; Pointecouteau et al., 1999; Andreani et al., 1996). The Nobeyama 45 m telescope has also been been used at 21, 43, and 150 GHz to detect and map the SZE (Komatsu et al., 2001; Komatsu et al., 1999).
Figure 4. Measurement of the SZE profile across the galaxy cluster Cl 0016 + 16 obtained with the OVRO 40 m telescope (Hughes & Birkinshaw, 1998). The observed profile provided confidence in the reliability of SZE detections.
The Sunyaev-Zel'dovich Infrared Experiment (SuZIE) uses its six-element 140 GHz bolometer array to observe in a drift-scanning mode, where the telescope is fixed and the rotation of the Earth moves the beams across the sky. Using this drift-scanning technique, the SuZIE experiment has produced high signal-to-noise ratio strip maps of the SZE emission in several clusters (Holzapfel et al., 1997a; Mauskopf et al., 2000).
Because of the high sensitivity of bolometric detectors at millimeter wavelengths, single-dish experiments are ideally suited for the measurement of the SZE spectrum. By observing at several millimeter frequencies, these instruments should be able to separate the thermal and kinetic SZEs from atmospheric fluctuations and sources of astrophysical confusion.
The measured SZE spectrum of Abell 2163, spanning the decrement and increment with data obtained from different telescopes and techniques, is shown in Figure 3 (LaRoque et al., 2003; Holzapfel et al., 1997a; Désert et al., 1998). The SZE spectrum is a good fit to the data, demonstrating the consistency and robustness of modern SZE measurements.
Single-dish observations of the SZE are just beginning to reach their potential, and the future is very promising. The development of large-format, millimeter-wavelength bolometer arrays will increase the mapping speed of current SZE experiments by orders of magnitude. To the extent that atmospheric fluctuations are common across a bolometric array, it will be possible to realize the intrinsic sensitivity of the detectors. Operating from high astronomical sites with stable atmospheres and exceptionally low precipitable water vapor, future large-format bolometer arrays have the potential to produce high signal-to-noise ratio SZE images and search for distant SZE clusters with unprecedented speed.