A Tale of Two SZ Sources

Abstract

The recent discovery of two flux decrements in deep radio maps obtained by the VLA and the Ryle Telescope can have powerful implications for the density parameter of the Universe, $\Omo$. We outline these implications by modeling the decrements as the thermal Sunyaev-Zel'dovich (SZ) effect from two clusters assuming their properties are similar to those of the low redshift population. In this case, the absence of any optical or X-ray counterparts argues that the clusters must be at large redshifts. We highlight the difficulty this poses for a critical cosmology by a comparison with a fiducial open model with $\Omo=0.2$ ($\lamo=0$). Applying the phenomenological X-ray luminosity-temperature relation needed to explain the EMSS cluster redshift distribution, as inferred by Oukbir and Blanchard (1997), we convert the X-ray band upper limits to lower limits on the clusters' redshifts. Comparison of the counts implied by these two SZ detections with model predictions, for clusters with redshifts larger than these lower limits, illustrates quantitatively the inability of the critical cosmology to account for such high-redshift clusters. On the other hand, the open model with $\Omo=0.2$ remains consistent with the existence of the two objects; it possibly has, however, difficulties with current limits on spectral distortions and temperature fluctuations of the cosmic microwave background. The discussion demonstrates the value of SZ cluster searches for testing cosmological models and theories of structure formation.