First measurement of the bulk flow of nearby galaxies using the cosmic microwave background

Guilhem Lavaux, Niayesh Afshordi, Michael J. Hudson

Published 2012-07-06, updated 2013-01-13Version 2

Peculiar velocities in the nearby Universe can be measured via the kinetic Sunyaev-Zel'dovich (kSZ) effect. Using a statistical method based on an optimised cross-correlation with nearby galaxies, we extract the kSZ signal generated by plasma halo of galaxies from the Cosmic Microwave Background (CMB) temperature anisotropies observed by the Wilkinson Microwave Anisotropy Probe (WMAP). Marginalising over the thermal Sunyaev-Zel'dovich contribution from clusters of galaxies, possible unresolved point source contamination, and Galactic foregrounds (dust, synchrotron and free-free emission), we report a kSZ bulk flow signal present at the 90% confidence level in the seven-year WMAP data. When only galaxies within 50 Mpc/h are included in the kSZ template we find a bulk flow in the CMB frame of |V|=533 +/- 263 km/s, in the direction l=324 +/- 27, b=-7 +/- 17, consistent with bulk flow measurements on a similar scale using classical distance indicators. We show how this comparison constrains the (ionised) baryonic budget in the local universe. On very large (~ 500 Mpc/h) scales, we find a 95% upper limit of 470 km/s, inconsistent with some analyses of bulk flow of clusters from the kSZ. We estimate that the significance of the bulk flow signal may increase to 3-5 sigma using data from the PLANCK probe.