Tuesday, November 20, 2012

1211.4345 (F. Atrio-Barandela et al.)

Cosmic Microwave Background filters and the Dark-Flow measurement    [PDF]

F. Atrio-Barandela, A. Kashlinsky, H. Ebeling, D. Kocevski
Recent measurements of large-scale peculiar velocities from the cumulative kinematic Sunyaev-Zeldovich (KSZ) effect 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 dipole. The signal originates from a residual dipole in the direction of galaxy clusters, at apertures containing zero monopole. Its amplitude increases with the X-ray luminosity of the clusters. The data need to be filtered to remove the primary CMB, thereby increasing the signal-to-noise ratio. Filtering cannot imprint a signal with the mentioned properties at cluster positions, but an inadequately designed and implemented filter can greatly suppress it. We show here that recent studies that failed to detect a large-scale flow indeed used inadequate implementations. These analysis assumed cluster extents and electron-pressure profiles inconsistent with the data. We show that the results from these alternative filters are consistent (although not identical) with our measurement, when filters are normalized to the data. The discrepancies can be traced to the assumptions on cluster profile and extent that reduce the efficiency of the filter and the possible existance of thermal Sunyaev-Zeldovich residual dipoles. The upcoming PLANCK maps, with their large frequency coverage, and in particular the 217GHz channel, will be important to probe the bulk flows as well as to remove spurious dipole signals and further identify the filtering schemes appropriate for this measurement.
View original: http://arxiv.org/abs/1211.4345

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