Deviations from Hubble Flow in the Local Universe

Beginning with the pioneering efforts of Rubin et al. (1976), the study of large–scale deviations from Hubble flow has achieved undeniable observational successes (see reviews by Dekel 1994, Strauss and Willick 1995). In the spirit of providing fertile ground for discussion and further developments, this presentation will concentrate on few of the areas where a measure of variance of opinion exists, rather than on presenting a survey of results. The determination of the peculiar velocity of galaxies over volumes of cosmological interest is an exercise fraught with large amplitude uncertainties. These arise from errors in the measurements, poorly understood corrections applied to the observed parameters, “cosmic” sources related to the formation history of each galaxy and to its individual peculiarities, and statistical corrections related to the variations in the galaxy density field. The most commonly applied methods, which use the Tully–Fisher (1977;TF) relation for spirals and the Dn–σ (Dressler et al. 1987) or the Fundamental Plane (FP; Djorgovski and Davis 1987) relations for spheroidals, lack a well developed physical basis, and an understanding of the impact of individual sources of scatter has been hazy at best. It is of paramount importance that the characteristics of the error budget of peculiar velocity measurement techniques be well understood, in order both to correctly infer the predictive power of the method and to derive reliable statistical corrections. In section 2, the error budget of the TF relation will be analyzed in detail. The TF technique for spirals or the FP method for spheroidals requires a calibrated template relation. Offsets with respect to it are converted to peculiar velocities by means of exponential laws. It is well known that the estimate of peculiar velocities is independent on the assumed value of the