2MASS constraints on the local large-scale structure: a challenge to ΛCDM?

Abstract

We investigate the large-scale structure of the local galaxy distribution using the recently completed 2 Micron All Sky Survey (2MASS) via three techniques. First, we determine the Ks-band number counts over the ≈4000 deg 2 APM survey area where evidence for a large-scale ‘local hole ’ has previously been detected and compare them to a homogeneous prediction. Considering a ΛCDM form for the 2-point angular correlation function, the observed deficiency represents a 5σ 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 3.8σ. 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 Ks-band counts over the APM survey area continue to suggest the possible presence of excess clustering over the ΛCDM prediction, we next probe the power at large scales by comparing the 2MASS and ΛCDM mock galaxy angular power spectra. We find a 3σ excess in th