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 and the Abell–Corwin–Olowin (ACO) catalogue. We interpret this as evidence for the thermal Sunyaev–Zel'dovich (SZ) effect from the clusters. Most interestingly, the signal may extend to ≈ 1 deg (≈5 h −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 < 0.2) = 7 ± 3.8 × 10 −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 of M ( r < 1.75 h −1 Mpc) ≈ 3 × 10 13 h −2 M ⊙ for both, the assumed gas temperature and SZ central decrement differences approximately cancelling. Using the space density of APM groups we then estimate Ω gas 0 ≈ 0.03 h −1 (1 keV/ kT )(θ max /20 arcmin) 0.75 . For an SZ extent of θ max = 20 arcmin, kT = 1 keV and h = 0.7 , this value of Ω gas 0 ≈ 0.04 is consistent with the standard value of Ω baryon 0 = 0.044 , but if the indications we have found for a more extended SZ effect out to θ max ≈ 60 arcmin are confirmed, then higher values of Ω gas 0 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.