Before proceeding with a determination of the Hubble constant, we turn our attention to the question of the local flow field, recalling that H0 requires a solid knowledge of both distances and velocities. The large-scale distribution of matter in the nearby universe perturbs the local Hubble flow, causing peculiar motions. If uncorrected for, these perturbations can be a significant fraction of the measured radial velocity, particularly for the nearest galaxies. The local flow field has been modeled extensively by a number of authors (e.g., Tonry et al. 2000). In general, there is good qualitative agreement amongst different studies. On average, these peculiar motions amount to ~ 200-300 km/sec (Tonry et al.; Giovanelli et al. 1999), but the flow field is complicated locally by the presence of massive, nearby structures, most notably, the Virgo Cluster. At 3,000 km/sec, the peculiar motion for an individual object can amount to a 7-10% perturbation, whereas for Type Ia supernovae (which reach out to 30,000 km/sec), these effects drop to less than 1%, on average.
For the nearest galaxies, the effects of the local peculiar velocity field, and the resultant uncertainty in H0 can be quite large. For example, a recent study by Willick & Batra (2000) finds values of H0 = 85 ± 5 and 92 ± 5 km s-1 Mpc-1 based on applying different local velocity models to 27 Cepheid galaxies within ~20 Mpc. However, the velocity model of Han & Mould (1990) applied to 12 Cepheid distances fits best with H0 ~ 70 km s-1 Mpc-1 (Mould et al. 1996). Some of this difference reflects a difference in calibration of the surface-brightness-fluctuation method. However, the remaining large discrepancies serve to emphasize that the Key Project strategy of extending secondary distance measurements beyond 100 Mpc, where recession velocities have become large, is preferable to any local determination.
For the Key Project, we have corrected the observed galaxy velocities for the local flow field as described in Mould et al. (2000a, 2001). (19) A linear infall model composed of 3 mass concentrations (the Local Supercluster, the Great Attractor, and the Shapley concentration) is constructed with parameters estimated from existing catalogs of Tully-Fisher distances and velocities. In Section 8.6, we return to the question of whether there is evidence for a bulk (or non-converging) flow on larger scales.
19 Note that the signs in Equation A2 published in Mould et al. 2000a are wrong in the text; however, they were correct in the code used to do the calculations. Back.