12.4.2. Deviations from Hubble Flow
Despite the current uncertainties in its application, the velocity width-magnitude relation has been shown to be a promising tool in the study of deviations from the smooth Hubble expansion. Once the relation is established for a certain sample of galaxies, a predicted redshift, Vp, can be estimated for each galaxy. Vp can then be compared with the observed redshift, V0. Systematic variations over the celestial sphere of the residuals V0 - Vp imply a large-scale bulk flow. Hart and Davies (1982) have used HI observations of nearby Sbc galaxies to determine the motion of the Sun and the Local Group relative to the more distant galaxies. Their method uses the HI mass as the standard candle for galaxies in this single Hubble type. Further extension of this technique to larger distances will investigate the scale over which peculiar motions can be measured beyond the boundaries of the Local Supercluster.
The velocity width-infrared magnitude method has also been used to derive relative distances to nearby clusters of galaxies. Since the galaxies in an individual cluster can be assumed to be at the same distance, a determination of the relation for galaxies in separate clusters yields a measure of the Hubble ratio (velocity/distance) for each cluster. Aaronson et al. (1986) have derived Hubble ratios for ten nearby clusters; the scatter in their results again implies significant deviation of the Local Group motion from what would be expected if the local Hubble flow were simply a smooth expansion.
It is clear that future refinement of this technique will provide a critical link in our understanding of the magnitude and direction of the peculiar motion and the scale size of inhomogeneity in the local universe.