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We present the measured Sunyaev-Zel'dovich (SZ) flux from 474optically-selected MaxBCG clusters that fall within the Atacama CosmologyTelescope (ACT) Equatorial survey region. The ACT Equatorial region used inthis analysis covers 510 square degrees and overlaps Stripe 82 of the SloanDigital Sky Survey. We also present the measured SZ flux stacked on 52X-ray-selected MCXC clusters that fall within the ACT Equatorial region and anACT Southern survey region covering 455 square degrees. We find that themeasured SZ flux from the X-ray-selected clusters is consistent withexpectations. However, we find that the measured SZ flux from theoptically-selected clusters is both significantly lower than expectations andlower than the recovered SZ flux measured by the Planck satellite. Since wefind a lower recovered SZ signal than Planck, we investigate the possibilitythat there is a significant offset between the optically-selected brightestcluster galaxies (BCGs) and the SZ centers, to which ACT is more sensitive dueto its finer resolution. Such offsets can arise due to either an intrinsicphysical separation between the BCG and the center of the gas concentration orfrom misidentification of the cluster BCG. We find that the entire discrepancyfor both ACT and Planck can be explained by assuming that the BCGs are offsetfrom the SZ maxima with a uniform random distribution between 0 and 1.5 Mpc.Such large offsets between gas peaks and BCGs for optically-selected clustersamples seem unlikely given that we find the physical separation between BCGsand X-ray peaks for an X-ray-selected subsample of MaxBCG clusters to have amuch narrower distribution that peaks within 0.2 Mpc. It is possible that othereffects are lowering the ACT and Planck signals by the same amount, withoffsets between BCGs and SZ peaks explaining the remaining difference betweenmeasurements. (Abridged)

The Atacama Cosmology Telescope: Relation Between Galaxy Cluster Optical Richness and Sunyaev-Zel'dovich Effect