The velocity field predicted by the Optical Redshift Survey

We have used the Optical Redshift Survey (ORS) of Santiago et al. to constr uct the gravity field due to fluctuations in the galaxy density held out to distances of 8000 km s(-1). At large scales where linear theory applies, t he comparison of this gravity held with the observed peculiar velocity fiel d offers a powerful cosmological probe because the predicted flow field is proportional to the parameter Omega(0.6)/b, where Omega is the matter densi ty and b is the bias of the galaxy distribution. The more densely sampled O RS gravity field, to excellent approximation, matches that of the earlier I RAS 1.2 Jy redshift survey of Fisher et al., provided beta is reduced by a factor of b(opt)/b(IRAS) approximate to 1.4. Apart from this scaling, the m ost significant difference between the ORS and IRAS fields is induced by di ffering estimates of the overdensity of the Virgo cluster. Neither of these gravity fields is consistent with the peculiar velocity field constructed from the full Mark III sample. We find that a simple but plausible nonlinea r bias algorithm for the galaxy distribution relative to the mass has a neg ligible effect on the derived fields. We conclude that the substitution of optical for IRAS catalogs cannot alone resolve the discrepancies between th e IRAS gravity held and the Mark III peculiar velocity field.