A new measurement of the bulk flow of X-ray luminous clusters of
  galaxies
  We present new measurements of the large-scale bulk flows of galaxy clusters
based on 5-year WMAP data and a significantly expanded X-ray cluster catalogue.
Our method probes the flow via measurements of the kinematic Sunyaev-Zeldovich
(SZ) effect produced by the hot gas in moving clusters. It computes the dipole
in the cosmic microwave background (CMB) data at cluster pixels, which
preserves the SZ component while integrating down other contributions. Our
improved catalog of over 1,000 clusters enables us to further investigate
possible systematic effects and, thanks to a higher median cluster redshift,
allows us to measure the bulk flow to larger scales. We present a corrected
error treatment and demonstrate that the more X-ray luminous clusters, while
fewer in number, have much larger optical depth, resulting in a higher dipole
and thus a more accurate flow measurement. This results in the observed
correlation of the dipole derived at the aperture of zero monopole with the
monopole measured over the cluster central regions. This correlation is
expected if the dipole is produced by the SZ effect and cannot be caused by
unidentified systematics (or primary cosmic microwave background anisotropies).
We measure that the flow is consistent with approximately constant velocity out
to at least 800 Mpc. The significance of the measured signal peaks around 500
Mpc, most likely because the contribution from more distant clusters becomes
progressively more diluted by the WMAP beam. We can, however, at present not
rule out that these more distant clusters simply contribute less to the overall
motion.
