Galaxy Distances in the Nearby Universe: Corrections for Peculiar Motions

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Abstract
By correcting the redshift-dependent distances for peculiar motions through some peculiar velocity field models, we recover the true distances of an extensive, all-sky sample of nearby galaxies (~6400 galaxies with recession velocities cz < 5500 km s-1) that is complete up to the apparent limiting blue magnitude B = 14 mag. Relying on available catalogs of galaxy groups, we treat ~2700 objects as members of galaxy groups, and the remaining objects as field galaxies. We invert the derived redshift-distance relations to estimate distances for field galaxies and groups, and we overcome the ambiguity inherent in the triple-valued zones by using Tully-Fisher relations calibrated on suitably defined samples of galaxies that have distances predicted by peculiar velocity models. We use two independent approaches to model the peculiar velocity field: (1) a cluster dipole reconstruction scheme that we modify with the inclusion of a local model of Virgocentric infall and (2) a multiattractor model fitted to the Mark II and Mark III catalogs of galaxy peculiar velocities. In the multiattractor model, we assume that the velocity field is generated by a few prominent gravitational sources (the Virgo cluster, the Great Attractor, the Perseus-Pisces supercluster, and the Shapley supercluster). We discuss differences in the results from different velocity models and from different Mark II and Mark III data subsets. In particular, according to Mark III data, the Great Attractor appears to have a smaller influence on local dynamics than previously believed, whereas the Perseus-Pisces and Shapley superclusters acquire a specific dynamical role. Remarkably, the Shapley structure, which is found to account for nearly half the peculiar motion of the Local Group, is placed by Mark III data closer to the zone of avoidance with respect to its optical position. On the other hand, the modified cluster dipole model is characterized by relatively small flows toward the Great Attractor and the Shapley supercluster, together with a large Virgocentric infall. Our multiattractor model based on Mark III data favors a cosmological density parameter Ω0 ~ 0.5 (irrespective of a biasing factor of order unity). The use of different peculiar velocity field models allows us to check to what extent differences in current views on cosmic flows affect the recovery of galaxy distances. We find that differences among distance estimates are less pronounced in the ~2000-4000 km s-1 distance range than at larger or smaller distances. In the last regions, these differences have a serious impact on the three-dimensional maps of the galaxy distribution and on the local galaxy density (on small scales, <1 Mpc), which is a crucial parameter being used in statistical studies of environmental effects on the properties of nearby galaxies.
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