Local Hole revisited: evidence for bulk motions and self-consistent outflow

We revisit our mapping of the ‘Local Hole’, a large underdensity in the local galaxy redshift distribution that extends out to redshift, z ≈ 0.05 and a potential source of outflows that may perturb the global expansion rate and thus help mitigate the present ‘H0 tension’. First, we compare local peculiar velocities measured via the galaxy average redshift-magnitude Hubble diagram, z¯¯¯(m)⁠, with a simple dynamical outflow model based on the average underdensity in the Local Hole. We find that this outflow model is in good agreement with our peculiar velocity measurements from z¯¯¯(m) and not significantly inconsistent with SNIa peculiar velocity measurements from at least the largest previous survey. This outflow could cause an ≈2 − 3% increase in the local value of Hubble’s constant. Second, considering anisotropic motions, we find that the addition of the outflow model may improve the z¯¯¯(m) fit of a bulk flow where galaxies are otherwise at rest in the Local Group frame. We conclude that the Local Hole plus neighbouring overdensities such as the Shapley Supercluster may cause outflow and bulk motions out to ≈150h−1Mpc that are cosmologically significant and that need to be taken into account in estimating Hubble’s constant.