THE PECULIAR VELOCITY-FIELD IN THE PERSEUS-PISCES SUPERCLUSTER

I describe the main results of a program to study peculiar motions of galaxies in the Perseus-Pisces supercluster. The data set consists of newly obtained CCD photometry and published 21 cm line widths for 320 spiral galaxies in the Perseus-Pisces region. The peculiar velocities of these objects are estimated using the Tully-Fisher relation, which has been calibrated with respect to an independent sample of cluster galaxies. Analysis of the Tully-Fisher data shows that the principal feature of the velocity field in the Perseus-Pisces region is a coherent streaming toward the Local Group. Nearly all the galaxies well in the foreground (cz≤ 2000 km s-1) have negative peculiar velocities of amplitude ∼1000 km s-1. A great majority of objects in the background of the supercluster, out to redshifts of ∼7000 km s-1, also have very large negative peculiar velocities. The supercluster ridge itself, at a redshift of ∼5000 km s-1, appears to consist of positive and negative peculiar velocity groups of galaxies, but with negative peculiar velocities predominating both in amplitude and in number of objects involved; the net effect is a peculiar velocity of ∼-450 km s-1. The data for galaxies with the highest redshifts included in the sample, cz≥8000 km s-1, are consistent with their being at rest. These streaming motions in Perseus-Pisces are similar in amplitude and direction to those of elliptical galaxies in the southern sky, which have large positive peculiar velocities to distances of at least 4000 km s-1. These have been modeled as arising from the pull of a "great attractor." If, as seems probable, the flow traced by the Perseus-Pisces spirals is physically related to that of the elliptical galaxies, the coherence length for this streaming motion is of order 100h-1 Mpc. It is unlikely that coherent motion on this scale can arise entirely from the great attractor; rather, it may be due in part to low-amplitude density fluctuations on much larger scales.