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 degree by ~80 degree strips in the two galactic caps. In the six southern strips a deficit of ~30% was found relative to a predicted homogeneous distribution at K~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% than 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% at K~11 for the northern and southern areas respectively. Our second method uses the ratio of the counts with 11.38<K<12.38 to 12.88<K<13.38 binned in 25 degree squared areas; the counts from these areas provide a smoothed map over the sky of the slope in the counts between K=11.38 and 13.38. The resulting map picks out known rich clusters, demonstrating usefulness as a probe of LSS at distances of <150 h-1 Mpc. Interestingly, the map also shows large regions, \~100 degrees 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, indicating the possible presence of significant power on scales of >300 h-1 Mpc in the galaxy power spectrum.