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.
