Motions of galaxies in the nearby universe with 2MASS

Abstract

I present the first model of a flow-field in the nearby Universe (cz < 12, 000 km s-') constructed from groups of galaxies identified in an all-sky flux-limited survey. The Two Micron All-Sky Redshift Survey (2MRS), upon which the model is based, represents the most complete survey of its class and, with near-IR fluxes, provides the optimal method for tracing baryonic matter in the nearby Universe. Peculiar velocities are reconstructed self-consistently with a density-field based upon groups identified in the 2MRS KS < 11.75 catalog. The model predicts infall toward Virgo, Perseus-Pisces, Hydra-Centaurus, Norma, Coma, Shapley and Hercules, and most notably predicts backside-infall into the Norma Cluster. I discuss the application of the model as a predictor of galaxy distances using only angular position and redshift measurements. By calibrating the model using measured distances to galaxies inside 3000 km s-1, I show that, for a randomly-sampled 2MRS galaxy, improvement in the estimated distance over the application of Hubble's law is expected to be - 30%, and considerably better in the proximity of clusters. I test the model using distance estimates from the SFI++ sample, and find evidence for improvement over the application of Hubble's law to galaxies inside 4000 km s-1, although the performance varies depending on the location of the target. I compute the peculiar velocity of the Local Group, predicted from the density-field, and find that less than 70% of the expected magnitude can be accounted for; the discrepancy between the predicted direction and the dipole in the Cosmic Microwave Background is significant at the 90%-confidence level. I demonstrate that a bulk flow of over 300 km s-1 in a direction close to the galactic plane is necessary to account for the remaining motion. The results suggest that one or more massive structures beyond 120/h Mpc are likely to be key contributors to the local dynamics. Although, with the direction of the bulk flow coincident with the Zone of Avoidance, incomplete sampling behind the galactic plane may be a factor in the discrepancy.