The local hole revealed by galaxy counts and redshifts

Abstract

The redshifts of ≈ 250000 galaxies are used to study the Local Hole and its associated peculiar velocities. The sample, compiled from 6dF Galaxy Redshift Survey (6dFGS) and Sloan Digital Sky Survey (SDSS), provides wide sky coverage to a depth of ≈ 300h −1 Mpc. We have therefore examined K and r limited galaxy redshift distributions and number counts to map the local density field. Comparing observed galaxy n(z) distributions to homogeneous models in three large regions of the high latitude sky, we find evidence for under-densities ranging from ≈4-40% in these regions to depths of ≈150h −1 Mpc with the deepest under-density being over the Southern Galactic cap. Using the Galaxy and Mass Assembly (GAMA) survey we then establish the normalisation of galaxy counts at fainter magnitudes and thus confirm that the underdensity over all three fields at K < 12.5 is ≈ 15 ± 3%. Finally, we further use redshift catalogues to map sky-averaged peculiar velocities over the same areas using the average redshift -magnitude, z(m), technique of Soneira (1979). After accounting for the direct effect of large-scale structure on z(m) we can then search for peculiar velocities. Taking all three regions into consideration the data reject at the ≈ 4σ level the idea that we have recovered the CMB rest frame in the volume probed. We therefore conclude that there is some consistent evidence from both counts and Hubble diagrams for a 'Local Hole' with a ≈ 150h −1 Mpc under-density that deeper counts and redshifts in the Northern Galactic cap suggest may extend to ≈ 300h −1 Mpc.

An unbiased estimator of peculiar velocity with Gaussian distributed errors for precision cosmology

“…We see that subtracting a relatively modest outflow has resulted in an error distribution that is Gaussian and centered on zero. The existence of a coherent outflow would support recent arguments that we live in a low density region, dubbed the "local hole" (Whitbourn & Shanks 2014;Wiltshire et al 2013).…”

Galaxy and Mass Assembly (GAMA): maximum-likelihood determination of the luminosity function and its evolution

“…Perhaps a more likely explanation is that the apparent density evolution at low redshift is actually caused by a local underdensity, e.g. Keenan, Barger & Cowie (2013) and Whitbourn & Shanks (2014).…”

Observational challenges for the standard FLRW model

“…Whitbourn and Shanks confirmed this 84 by analyzing 250,000 galaxy redshifts from the six degree field galaxy survey (6dFGS) and the SDSS, using both their redshift distributions and number counts in comparison to the deeper galaxy and mass assembly (GAMA) survey. They also made peculiar velocity maps that rejected the possibility of the local 150 h −1 Mpc diameter region being at rest in the CMB frame at 4σ.…”