Comparison of Velocity and Gravity Fields: The Mark III Tully-Fisher Catalog Versus the IRAS 1.2 Jy Survey

Abstract

We consider a measure of the peculiar velocity field derived from the Mark III compilation of 2900 spiral galaxies (Willick \etal 1996b), using an analysis that is substantially free of bias (Nusser and Davis 1995). We expand the velocity field in a set of orthogonal, smooth modes, reducing the data to a set of 56 coefficients fitted to a maximum redshift of 6000 km/s, and maximum spherical harmonic of l=3. Equivalent mode coefficients can be computed for the gravity field derived from any whole-sky redshift catalog of galaxies, such as the \iras 1.2 Jy survey (Fisher \etal 1995). Detailed intercomparison shows the two independent fields to be remarkably aligned in general. There are, however, systematic discrepancies in the fields that lead to coherence in the residuals between them. These residuals take the form of a dipole field in the LG frame that grows with distance; it is not consistent with a bulk flow residual. A chi-squared analysis of the mode-mode comparison designed to determine which value of $\beta\equiv \Omega^{0.6}/b$ for the \iras gravity field best fits the Mark III velocity field shows instead that no value of $\beta$ is acceptable. This is in contrast to the simulated catalogs, and suggests that there must be some systematic discrepancy between the two fields.