The density and velocity fields of the local universe.

Abstract

We present two self-consistent non-parametric models of the local cosmic velocity field based on the density distribution in the PSCz redshift survey of IRAS galaxies. Two independent methods have been applied, both based on the assumptions of gravitational instability and linear biasing. They give remarkably similar results, with no evidence of systematic differences and an r.m.s discrepancy of only ~ 70 kms(^-1) in each Cartesian velocity component. These uncertainties are consistent with a detailed independent error analysis carried out on mock PSCz catalogues constructed from TV-body simulations. The denser sampling provided by the PSCz survey compared to previous IRAS galaxy surveys allows us to reconstruct the velocity field out to larger distances. The most striking feature of the model velocity field is a coherent large-scale streaming motion along a basehne connecting Perseus-Pisces, the Local Supercluster, the Great Attractor, and the Shapley Concentration. We find no evidence for back-infall onto the Great Attractor. Instead, material behind and around the Great Attractor is inferred to be streaming towards the Shapley Concentration, aided by the expansion of two large neighbouring un- derdense regions. The PSCi model velocities compare well with those predicted from the 1.2-Jy redshift survey of IRAS galaxies and, perhaps surprisingly, with those predicted from the distribution of Abell/ACO clusters, out to 140 h(^-1)Mpc. Comparison of the real-space density fields (or, alternatively, the peculiar velocity fields) inferred from the PSCz and cluster catalogues gives a relative (linear) bias parameter between clusters and IRAS galaxies of b(_c) = 4.4 ± 0.6. In addition, we compare the cumulative bulk flows predicted from the PSCz gravity field with those measured from the MarkIII and SFI catalogues of peculiar velocities. A conservative estimate of β = Ω(_0)(^0.6)/b, where b is the bias parameter for IRAS galaxies, gives β= 0.76 ± 0.13 (1-σ), in agreement with other recent determinations. Finally, we perform a detailed comparison of the IRAS PSCz and 1.2-Jy spherical harmonic coefficients of the density and velocity fields in redshift space. Both the monopole terms of the density and velocity fields predicted from the surveys show some inconsistencies. The mismatch in the velocity monopole terms is resolved by masking the 1.2-Jy survey with the PSCz mask and using the galaxies within the PSCz survey for fluxes larger than 1.2 Jy. Davis, Nusser and Willick (1996) have found a discrepancy between the IRAS 1.2-Jy survey gravity field and the MarkIII peculiar velocity field. We conclude that the use of the deeper IRAS PSCz catalogue cannot alone resolve this mismatch.