|Annu. Rev. Astron. Astrophys. 1992. 30:
Copyright © 1992 by Annual Reviews. All rights reserved
4.1 Peculiar Velocities and Streaming Motions
The detailed mapping of the cosmic velocity field and the reliable measurement of deviations from a pure Hubble flow are prerequisites for our understanding of the large-scale structure of the universe. The determination of peculiar velocities requires only relative distances. The absolute peculiar velocity (in the radial direction as seen by the observer) between any two standard candles can be determined from their apparent magnitudes and recession velocities. The error of the resulting peculiar velocities depends only on how good the standard candles are. If SNe Ia have a luminosity scatter of only 0.2 mag, peculiar velocities can be measured to 10% of the recession velocity. For instance, at 2000 km s-1 the error in the peculiar motion will amount to 200 km s-1. If n SNe Ia are available in a galaxy or cluster the error will decrease by n1/2.
A first attempt to test the reality of large-scale streaming motions using SNe Ia has been made by Miller & Branch (1992). They find that the adoption of a Virgocentric infall model strikingly reduces the luminosity scatter of their sample of SNe Ia in field galaxies from B = 0.63 to 0.32 mag. Additional allowance for the influence of a Great Attractor or IRAS clusters does not give a smaller scatter. This is not surprising because the photometry, especially of the older SNe Ia, is too inaccurate and because the sample of 34 SNe Ia has a median velocity of only 1500 km s-1 - too low to be sensitive to streaming motions beyond the Virgo complex. The result is in accordance with that of Jerjen & Tammann (1992), who did not find the local microwave dipole motion to be reflected in the data of an independently defined sample of SNe Ia. Again, the photometry is insufficient and the sample is too near to provide a sensitive test.
These mainly negative results will drastically change, no doubt, when several dozen well observed SNe Ia with recession velocities in the range 4000-6000 km s-1 become available.