  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
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<0.2)=7x10^(-7). We have further estimated the gas mass
associated with the galaxy group and cluster haloes. Assuming temperatures of 5
keV for ACO clusters and 1 keV for APM groups and clusters, we derive average
gas masses. Using the space density of APM groups we then estimate Omega_gas.
For an SZ extent of theta_max=20', kT=1 keV and h=0.7, this value of
Omega_gas=0.04 is consistent with the standard value of Omega_baryon=0.044 but
if the indications we have found for a more extended SZ effect out to theta=60'
are confirmed, then higher values of Omega_gas will be implied. Finally, the
contribution to the WMAP temperature power spectrum from the extended SZ effect
around the z<0.2 APM+ACO groups and clusters is 1-2 orders of magnitude lower
than the l=220 first acoustic peak. But if a similar SZ effect arises from more
distant clusters then this contribution could increase by a factor >10 and then
could seriously affect the WMAP cosmological fits.
