The local hole in the galaxy distribution: evidence from 2MASS

Using the bright galaxy counts from the 2 Micron All‐sky Survey (2MASS) second incremental release, two techniques for probing large‐scale structure at distances of ∼150 h −1 Mpc are investigated. First, we study the counts from two sets of six 5°×∼80° strips in the two galactic caps. In the six southern strips a deficit of ∼30 per cent was found relative to a predicted homogeneous distribution at K s ∼ 11 . These strips were also in good agreement with a model incorporating the radial density function of the southern 2dF Galaxy Redshift Survey (2dFGRS), which shows a deep underdensity between ∼90 and 180 h −1 Mpc . These results indicate a very large ‘local hole’ in the Southern Galactic Cap (SGC) to ≳150 h −1 Mpc with a linear size across the sky of ∼200 h −1 Mpc , but with a significantly lower mean underdensity of ∼30 per cent compared with that suggested by the bright APM SGC counts. The counts in the northern set of strips are low overall but indicate a more varied pattern. When all the available 2MASS data with | b | > 30° were aggregated, they indicated underdensities of ∼18 and ∼30 per cent at K s ∼ 11 for the northern and southern areas, respectively. Our second method uses the ratio of the counts between 11.38 < K s < 12.38 and 12.88 < K s < 13.38 to the ratio of the corresponding counts of the homogeneous model binned in 25 deg 2 areas; the counts from these areas provide a smoothed map over the sky of the slope in the counts between K s = 11.38 and 13.38. Visually, the resulting map shows the expected complex form of the cosmic web and picks out known rich clusters, demonstrating the usefulness of this ‘slope statistic’ as a probe of large‐scale structure at distances of ≲150 h −1 Mpc . Most interestingly, the map also shows large regions, ∼100° across, of steep counts in both hemispheres. Thus, the present 2MASS data suggest the presence of a potentially huge contiguous void stretching from south to north. Not only would this delineate further the limits for the cosmological principle but it would also show the possible presence of significant power on scales of ≳300 h −1 Mpc in the galaxy power spectrum.