Optical galaxies within 8000 km s-1 - II. The peculiar velocity of the Local Group

Optical galaxies within 8000 km s^–1 are used to predict the peculiar velocity of the Local Group. Our sample is drawn from the UGC and ESO catalogues. The predicted motion is $${28}^{\circ}-{32}^{\circ}$$ away from the motion of the Local Group in the CMB frame, and the predicted velocity is $$760 \pm 160 {\beta }_{\circ} \text {km}\, \text {s}^{-1}, \text {where}\enspace {\beta}_{\circ} \equiv {\Omega }^{0.6} b_{\circ}^{-1}$$. The direction of the optical dipole agrees with the directions of the IRAS dipoles, and is misaligned from the CMB dipole in the same sense. If it is assumed that the galaxies within 8000 km s^–1 are responsible for all of the 606 km s^–1 velocity of the Local Group, then $${\beta }_{\circ} = 0.08_{-0.13}^{+0.21} (1 \sigma \text {errors})$$. The density of mass clustered with optical galaxies is then $$\Omega = 0.69_{-0.18}^{+0.33}$$, if mass traces optical light (b_° = 1). Alternatively, if Ω = 1 then optical galaxies are mildly biased with b_° = 1.25 ± 0.26. For cosmological models that fit the large-scale galaxy counts, however, we do not expect the mass within 8000 km s¹ to account for all of the motion of the Local Group, and a better estimate is $${\beta }_{\circ} = 0.72_{-0.18}^{+0.37}$$. For the same model, but using only the galactic Z-component of the dipole, we obtain $${\beta }_{\circ} = 0.49_{-0.11}^{+0.18}$$. Then, if mass traces light, $$\Omega \gt{} 0.14$$.14 at the 95 per cent confidence level.