ON THE RECOVERY OF THE LOCAL GROUP MOTION FROM GALAXY REDSHIFT SURVEYS

There is an similar to 150 km s(-1) discrepancy between the measured motion of the Local Group (LG) of galaxies with respect to the cosmic microwave background and the linear theory prediction based on the gravitational force field of the large-scale structure in full-sky redshift surveys. We perform a variety of tests which show that the LG motion cannot be recovered to better than 150-200 km s(-1) in amplitude and within similar to 10 degrees in direction. The tests rely on catalogs of mock galaxies identified in the Millennium simulation using semi-analytic galaxy formation models. We compare these results to the K-s = 11.75 Two-Mass Galaxy Redshift Survey, which provides the deepest and most complete all-sky spatial distribution of galaxies with spectroscopic redshifts available thus far. In our analysis, we use a new concise relation for deriving the LG motion and bulk flow from the true distribution of galaxies in redshift space. Our results show that the main source of uncertainty is the small effective depth of surveys like the Two-Mass Redshift Survey (2MRS), which prevents a proper sampling of the large-scale structure beyond similar to 100 h(-1) Mpc. Deeper redshift surveys are needed to reach the convergence scale of approximate to 250 h(-1) Mpc in a ACDM universe. Deeper surveys would also mitigate the impact of the Kaiser rocket which, in a survey like 2MRS, remains a significant source of uncertainty. Thanks to the quiet and moderate density environment of the LG, purely dynamical uncertainties of the linear predictions are subdominant at the level of similar to 90 km s(-1). Finally, we show that deviations from linear galaxy biasing and shot noise errors provide a minor contribution to the total error budget.