Cosmic Microwave Background Temperature at Galaxy Clusters

We have deduced the cosmic microwave background (CMB) temperature in the Comacluster (A1656, $z=0.0231$), and in A2163 ($z=0.203$) from spectralmeasurements of the Sunyaev-Zel'dovich (SZ) effect over four passbands at radioand microwave frequencies. The resulting temperatures at these redshifts are$T_{Coma} = 2.789^{+0.080}_{-0.065}$ K and $T_{A2163} =3.377^{+0.101}_{-0.102}$ K, respectively. These values confirm the expectedrelation $T(z)=T_{0}(1+z)$, where $T_{0}= 2.725 \pm 0.002$ K is the valuemeasured by the COBE/FIRAS experiment. Alternative scaling relations that areconjectured in non-standard cosmologies can be constrained by the data; forexample, if $T(z) = T_{0}(1+z)^{1-a}$ or $T(z)=T_{0}[1+(1+d)z]$, then$a=-0.16^{+0.34}_{-0.32}$ and $d = 0.17 \pm 0.36$ (at 95% confidence). Webriefly discuss future prospects for more precise SZ measurements of $T(z)$ athigher redshifts.Comment: 13 pages, 1 figure, ApJL accepted for publicatio