ar X iv : a st ro - p h / 02 08 02 7 v 1 1 A ug 2 00 2 July 31 , 2002 - in preparation COSMIC MICROWAVE BACKGROUND TEMPERATURE AT GALAXY CLUSTERS

We have deduced the cosmic microwave background (CMB) temperature in the Coma cluster (Abell 1656, z = 0.0231), and in Abell 2163 (z = 0.203) from spectral measurements of the Sunyaev-Zel’dovich (SZ) effect over four passbands at radio and microwave frequencies. The resulting temperatures at these redshifts are TComa = 2.750 +0.043 −0.032 K and TA2163 = 3.335 +0.065 −0.066 K, respectively. These values are in good agreement with the basic relation T (z) = T0(1 + z), where T0 = (2.725± 0.002) K as measured by the COBE/FIRAS experiment. Alternative scaling relations that are conjectured in non-standard cosmologies can be constrained by the data; for example, if T (z) = T0(1+z) 1−a or T (z) = T0[1+(1+d)z], then a = −0.07 −0.11 and d = 0.07± 0.12. We briefly discuss future prospects for more precise SZ measurements of T (z) at higher redshifts. Subject headings: cosmology:CMB, the Sunyaev-Zeldovich effect, observations, galaxies:clusters