2MASS Constraints on the Local Large-Scale Structure: A Challenge to LCDM?

Abstract

We investigate the large-scale structure of the local galaxy distribution using the recently completed 2 Micron All Sky Survey (2MASS). First, we determine the K-band number counts over the 4000 sq.deg. APM survey area where evidence for a large-scale `local hole' has previously been detected and compare them to a homogeneous prediction. Considering a LCDM form for the 2-point angular correlation function, the observed deficiency represents a 5 sigma fluctuation in the galaxy distribution. We check the model normalisation using faint K-band data compiled from the literature; the normalisation used in this paper is in excellent agreement, and the observed counts over the APM survey area would require the model to be lowered by 3.8 sigma. However, the issue is complicated by the b>20 and b<-20 2MASS counts which lie below the best-fit model normalisation. Second, since the K-band counts over the APM survey area continue to suggest the possible presence of excess clustering over the LCDM prediction, we next probe the power at large scales by comparing the 2MASS and LCDM mock galaxy angular power spectra. We find a 3 sigma excess in the 2MASS catalogue over the LCDM prediction at large scales (l<30). However, this excess is not enough to account for the low counts over the APM survey area. Finally, we apply a counts in cells analysis to the 2MASS data and mock catalogues; on the assumption that the 2MASS catalogue at |b|>20 is representative, we find excellent agreement between the biased LCDM mocks and the 2MASS catalogue to 30 deg. The crux of the interpretation of these results appears to be whether the 2MASS volume is yet big enough to constitute a fair sample of the Universe. (abridged)