Peculiar velocity reconstruction with the fast action method: tests on mock redshift surveys

We present extensive tests of the fast action method (FAM) for recovering the past orbits of mass tracers in an expanding universe from their redshift‐space coordinates at the present epoch. The tests focus on the reconstruction of present‐day peculiar velocities using mock catalogues extracted from high‐resolution N ‐body simulations. The method allows for a self‐consistent treatment of redshift‐space distortions by direct minimization of a modified action for a cosmological gravitating system. When applied to ideal, volume‐limited catalogues, FAM recovers unbiased peculiar velocities with a one‐dimensional, 1σ error of ∼220 km s −1 , if velocities are smoothed on a scale of 5 h −1 Mpc. Alternatively, when no smoothing is applied, FAM predicts nearly unbiased velocities for objects residing outside the highest density regions. In this second case the 1σ error decreases to a level of ∼150 km s −1 . The correlation properties of the peculiar velocity fields are also correctly recovered on scales larger than 5 h −1 Mpc. Similar results are obtained when FAM is applied to flux‐limited catalogues mimicking the IRAS PSC z survey. In this case FAM reconstructs peculiar velocities with similar intrinsic random errors, while velocity–velocity correlation properties are well reproduced beyond scales of ∼8 h −1 Mpc. We also show that FAM provides better velocity predictions than other, competing methods based on linear theory or the Zel'dovich approximation. These results indicate that FAM can be successfully applied to presently available galaxy redshift surveys such as IRAS PSC z .