Galaxy pairs, redshift catalogs, and the cosmic peculiar velocity

We discuss pairs of spiral galaxies with projected separations r(p) ranging from very wide (r(p) similar to 1.0 Mpc) to very small (r(p) less than or similar to 75 kpc). Single-dish H I data for the wide pairs and H I aperture synthesis as well as CCD images for the close pairs were presented in earlier papers. From pairs (regardless of isolation criteria) in redshift catalogs, we find that the kinematics of spiral-spiral pairs in low-density regions is qualitatively different from that in high-density regions. The velocity distribution function f(Delta V) in low-density regions has two components: (i) a narrow component (which is related to isolated pairs) with a very small rms value sigma(upsilon,n) of the ''one-dimensional random cosmic peculiar velocity'' delta upsilon, and (ii) a broad, slowly decreasing component (which is related to pairs in loose groups) with a large rms value of the ''one-dimensional random cosmic peculiar velocity'' sigma(upsilon,b). Thus, we confirm that the distribution of the ''one-dimensional cosmic peculiar velocity'' delta upsilon is not Gaussian and its rms value a, varies with environment. The median Delta V-med of the absolute value of the velocity difference of isolated wide pairs is similar to 30 km s(-1), considerably smaller than all estimates of sigma(upsilon). This small Delta V-med suggests that the pairs are on low total energy, almost radial orbits, and it is also a constraint on cosmological models of structure formation. We find some circumstantial evidence for a negative correlation between the peculiar velocity delta upsilon and the differential Hubble flow velocity Delta V-H which would be the case for bound orbits near the maximum separation (''turnaround'') at the current epoch. If future larger samples (e.g., the CfA2 and the SSRS2 surveys) confirm this, then a total galaxy mass similar to 10(12) M. is implied. All the six close pairs for which we have detailed H I and optical CCD images show some signs of interaction, even though the selection criteria were specifically independent of known morphological peculiarities. For three of the six pairs there is good evidence for the pairs also being on high eccentricity, low total energy orbits, and there is some evidence that the impact parameter for one pair is similar to 1 disk radius. The median velocity difference Delta V-med, of the close pairs sample is also similar to 30 km s(-1). This small value of Delta V-med is probably an indirect effect of dynamical friction.