Evidence for an extended Sunyaev--Zel'dovich effect in WMAP data

We have cross-correlated the WMAP data with several surveys of extragalactic sources and find evidence for temperature decrements associated with galaxy clusters and groups detected in the APM Galaxy Survey survey and the ACO catalogue. We interpret this as evidence for the thermal Sunyaev-Zeldovich (SZ) effect from the clusters. Most interestingly, the signal may extend to �1 deg (� 5h −1 Mpc) around both groups and clusters and we suggest that this may be due to hot ‘supercluster’ gas. We have further cross-correlated the WMAP data with clusters identified in the 2MASS galaxy catalogue and also find evidence for temperature decrements there. From the APM group data we estimate the mean Compton parameter as y(z 10 and then could seriously affect the WMAP cosmological fits.