For this paper: postscript 1 Introduction... Abstract... 3 Calculation of ...

2 CL 0016+16  

Although CL 0016+16 was first characterised based on its optical appearance (Koo (1981)), it was quickly shown to be a strong X-ray source (White et al. (1981)) and Sunyaev-Zel'dovich effect cluster (Birkinshaw et al. (1981)). It also appears as an X-ray selected object in the Einstein Medium Sensitivity Survey (Gioia et al. (1990)), and is a member of several X-ray selected distant cluster samples. A mass map of the cluster has recently been derived from the gravitational lensing shear field that it generates (Smail et al. (1996)).

Recent X-ray imaging by ROSAT, and X-ray spectroscopy by ASCA, allow a more detailed examination of the cluster's gas content than the Einstein data. It is found that the cluster is strongly elliptical in its X-ray isophotes (Hughes & Birkinshaw (1997a)), with a centre at J2000 coordinates , . A fit to an ellipsoidal version of the standard isothermal beta model leads to structural parameter , core radius arcmin, and a major to minor axis ratio of , with the major axis in position angle degrees. The central X-ray surface brightness of the cluster is ROSAT PSPC .

The field of the cluster is also found to contain other clusters at similar redshift. Two adjacent X-ray emitting clusters have been identified by Hughes et al. (1995) and Hughes & Birkinshaw (1997b): these are likely to be bound to CL 0016+16 itself, and may be part of a large-scale structure including also the QSO Q 0015+162 (Margon et al. (1983)) which appears as a bright X-ray source near CL 0016+16. The redshifts of the three clusters have been measured to be for CL 0016+16 itself, for RX J0018.8+1602, and for RX J0018.3+1618. Q 0015+164 has redshift . Connolly et al. (1997) provide evidence that the supercluster may extend a degree or more on the sky.

The ASCA spectrum of the cluster, first discussed by Yamashita (1994), has been reanalysed by Hughes & Birkinshaw (1997a), who found consistent results -- the spectrum is that of an isothermal gas at , and a metallicity about 0.1 solar.

Finally, a strong Sunyaev-Zel'dovich effect is associated with the cluster. This has been detected by the single-dish telescopes (Uson (1986), Birkinshaw et al. (1997)) and mapped using two interferometers (Carlstrom et al. (1996), Grainge (1996)). The OVRO 40-m telescope data for the cluster are shown in Figure 1, and are a good match to a model for the cluster gas based on the X-ray image. An intrinsic central SZ effect of can be deduced for the cluster: less than half this effect is detectable with the OVRO 40-m telescope because of beamwidth and beam-switching effects.

 
Figure 1:  The Sunyaev-Zel'dovich effect for CL 0016+16 as seen by the OVRO 40-m telescope. A model for the effect expected from a simple isothermal atmosphere consistent with the X-ray image is superimposed.