3.1. Sources of Astronomical Contamination and Confusion
There are three main sources of astronomical contamination and confusion that must be considered when observing the SZE: (1) CMB primary anisotropies, (2) radio point sources, and (3) dust from the Galaxy and external galaxies. For distant clusters with angular extents of a few arcminutes or less, the CMB anisotropy is expected (Hu & White, 1997) and found to be damped considerably on these scales (Church et al. 1997; Dawson et al. 2001; Subrahmanyan et al. 2000; see also Holzapfel et al. 1997b and LaRoque et al. 2003 for CMB limits to SZE contamination). For nearby clusters, or for searches for distant clusters using beams larger than a few arcminutes, the intrinsic CMB anisotropy must be considered. The unique spectral behavior of the thermal SZE can be used to separate it from the intrinsic CMB in these cases. Note, however, that for such cases it will not be possible to separate the kinetic SZE effects from the intrinsic CMB anisotropy without relying on the very small spectral distortions of the kinetic SZE due to relativistic effects.
Historically, the major source of contamination in the measurement of the SZE has been radio point sources. It is obvious that emission from point sources located along the line of the sight to the cluster could fill in the SZE decrement, leading to an underestimate. Radio point sources can also lead to overestimates of the SZE decrement -- for example, point sources in the reference fields of single-dish observations. The radio point sources are variable and therefore must be monitored. Radio emission from the cluster member galaxies, from the central dominant (cD) galaxy in particular, is often the largest source of radio point source contamination, at least at high radio frequencies (LaRoque et al., 2003; Cooray et al., 1998).
At frequencies near the null of the thermal SZE and higher, dust emission from extragalactic sources as well as dust emission from our own Galaxy must be considered. At the angular scales and frequencies of interest for most SZE observations, contamination from diffuse Galactic dust emission will not usually be significant and is easily compensated. Consider instead the dusty extragalactic sources such as those that have been found toward massive galaxy clusters with the SCUBA bolometer array (Smail et al., 1997). Spectral indices for these sources are estimated to be ~ 1.5 - 2.5 (Fischer & Lange, 1993; Blain, 1998). Sources with 350 GHz (850 µm) flux densities greater than 8 mJy are common, and all clusters surveyed had multiple sources with flux densities greater than 5 mJy. This translates into an uncertainty in the peculiar velocity for a galaxy cluster of roughly 1000 km s-1 (see Carlstrom et al. 2002 for details).
As with SZE observations at radio frequencies, the analyses of high-frequency observations must also consider the effects of point sources and require either high dynamic angular range, large spectral coverage, or both to separate the point source emission from the SZE.