Optical galaxies within Formula - IV. The peculiar velocity field

The density field of optically selected galaxies within |8000\, \ km \ s^-1| is used to predict the peculiar velocities of |D ₍-| and IRTF galaxies employing linear gravitational instability theory. To obtain acceptable fits to the observed peculiar velocities in the CMB frame, it is necessary to fit two parameters: | ₎ ^0. 6 b ₎^-1| (where |b ₎| is a biasing parameter for optical galaxies), which scales the predictions of the optical density field within |8000\, \ km \ s^-1|⁠; and a residual bulk motion due to sources outside the volume. The fits yield | ₎=0. 50 0. 06 (1 | random errors) for both the IRTF and the |D ₍-| samples, and a residual bulk motion of |405 45\, \ km \ s^-1| towards |l=292^, b=7^| for the IRTF field galaxies. There is no evidence for a mass concentration in the zone of avoidance within |8000 \ km \ s^-1| whose effect on the Local Group is more than |100 \ km \ s^-1|⁠, so the residual bulk motion must be due primarily to sources beyond |8000 \ km \ s^-1|⁠. The result for | ₎| corresponds to |=0. 32 0. 06| if mass traces light | (b ₎=1) |⁠, or to |b ₎=2. 0 0. 24| if |=1|⁠. If biasing is independent of scale, then taken together these results are inconsistent with the predictions of the standard cold dark matter cosmology for any choice of the biasing parameter, but are compatible with cosmological models that fit the galaxy number counts on large scales.