The clustering of galaxies on large scales

Abstract

We investigate the local large-scale structure of the Universe, addressing various possible issues confronting the ACDM paradigm. Primarily, we investigate the clustering statistics of the newly-completed 2 Micron All Sky Survey (2MASS), the largest all sky galaxy survey to date.The 2MASS galaxy number counts over the ͌ 4000 deg(^2) APM survey area are found to be low compared to predictions but are in good agreement with previous optical results. Surprisingly, the number counts over almost the entire sky ([b] >20 ͌ 27000 deg(^2) ) are also deficient compared to our predictions. These results do not appear to be significantly affected by systematic errors. Assuming a ACDM cosmology, the observed deficiencies in the APM survey area and for [b] >20 represent ͌ 2.5σ and ͌ 4.0σ fluctuations in the local galaxy distribution respectively. These results are therefore potentially at odds with the form of clustering expected on large scales. We examine the form of galaxy clustering to Ṯ < 1000 h (^-1) Mpc scales using the 2MASS angular power spectrum. We find a 3σ excess over mock ACDM results; however this is not enough to account for the observed number counts mentioned above. We determine the implied cosmological constraints; the 2MASS galaxy angular power spectrum is, in fact, in strong support of ACDM, with a measured power spectrum shape of Γ (_eff) = 0.14±0.02. In addition, we determine a K(_8)-band galaxy bias of b(_K) = 1.39 ± 0.12.We determine high-order correlation functions of the 2MASS galaxy sample to extremely large scales (Ṯ < 1000 h (^-1)). The results are in strong support of Gaussian initial conditions and hierarchical clustering; we reject primordial strong non-Gaussianity at the ͌ 2.5σ confidence level. Unlike all previous such analyses, our results are relatively robust to the removal of large superclusters from the sample. We also measure a K(_8)-band quadratic galaxy bias of c(_2) = 0.57 ± 0.33. This result differs significantly from previous negative constraints; we discuss a possible explanation for this apparent discrepancy. Finally, we examine the extent of possible Sunyaev-Zeldovich contamination in the first year Wilkinson Microwave Anisotropy Probe (WMAP) data using various foreground galaxy cluster catalogues. We find evidence suggesting that the associated temperature decrements extend to > 1 scales. Such a result would indicate a much higher baryon density than the concordance value; in addition, CMB power spectrum fits and the associated cosmological constraints would also be compromised.