A measurement of large-scale peculiar velocities of clusters of galaxies: results and cosmological implications

Peculiar velocities of clusters of galaxies can be measured by studying the fluctuations in the cosmic microwave background (CMB) generated by the scattering of the microwave photons by the hot X-ray emitting gas inside clusters. While for individual clusters such measurements result in large errors, a large statistical sample of clusters allows one to study cumulative quantities dominated by the overall bulk flow of the sample with the statistical errors integrating down. We present results from such a measurement using the largest all-sky X-ray cluster catalog combined to date and the 3-year WMAP CMB data. We find a strong and coherent bulk flow on scales out to at least >∼300h −1Mpc, the limit of our catalog. This flow is difficult to explain by gravitational evolution within the framework of the concordance ΛCDM model and may be indicative of the tilt exerted across the entire current horizon by far-away pre-inflationary inhomogeneities. Subject headings: cosmology: observations cosmic microwave background early Universe large-scale structure of universe In the gravitational-instability picture peculiar velocities probe directly the peculiar gravitational potential [e.g. Kashlinsky & Jones 1991, Strauss & Willick 1995]. Inflationbased theories, such as the concordance ΛCDM model, predict that, on scales outside the horizon during the radiation-dominated era, the peculiar density field remained in the Harrison-Zeldovich regime set during inflationary epoch and on these scales, the peculiar bulk velocity due to gravitational instability should decrease as Vrms ∝ r−1 and be quite small. Peculiar velocities can be obtained from the kinematic SZ (KSZ) effect on the CMB SSAI and Observational Cosmology Laboratory, Code 665, Goddard Space Flight Center, Greenbelt MD 20771; e–mail: [email protected] Fisica Teorica, University of Salamanca, 37008 Salamanca, Spain Department of Physics, University of California at Davis, 1 Shields Avenue, Davis, CA 95616 Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822