Cosmic Microwave Background filters and the Dark-Flow measurement

Recent measurements of large-scale peculiar velocities from the cumulative kinematic Sunyaev-Zel'dovich effect using WMAP data and X-ray selected clusters from ROSAT have identified a bulk flow of galaxy clusters at $\sim 600-1,000$ km s$^{-1}$ on scales of $\sim0.5-1$ Gpc, roughly aligned with the all-sky Cosmic Microwave Background (CMB) dipole. The flow is inferred from the detection of a residual dipole generated by CMB fluctuations exclusively in the direction of galaxy clusters, and measured within apertures containing zero monopole. Consistent with this interpretation, the amplitude of the dipole correlates with the X-ray luminosity of the clusters. To enable this measurement, the CMB data need to be filtered to remove the primary CMB, thereby increasing the data's signal-to-noise ratio. Filtering cannot imprint a signal with the mentioned properties at cluster positions; however, an inadequately designed filter can greatly suppress such a signal. We show here that recent studies that failed to detect a large-scale flow with various filters indeed adopted flawed implementations; when correctly implemented, these alternative filters lead to results that are in fact consistent with the Dark Flow signal. The discrepancies can be traced to the likely presence of residual dipoles caused by the thermal Sunyaev-Zel'dovich effect, to assumptions about cluster profiles incompatible with the data as well as failure to compute dipoles at the zero monopole aperture. PLANCK maps, with their large frequency coverage and a 217 GHz channel, will be instrumental to probe bulk flows, to remove spurious dipole signals and to help identify filtering schemes appropriate for this measurement.