Testing Isotropy in the Local Universe

Abstract

We test the isotropy of the local distribution of galaxies using the 2MASS extended source catalogue. By decomposing the full sky survey into distinct patches and using a combination of photometric and spectroscopic redshift data, we use both parametric and non-parametric methods to obtain the shape of the luminosity function in each patch. We use the shape of the luminosity function to test the statistical isotropy of the underlying galaxy distribution. The parametric estimator shows some evidence of a hemispherical asymmetry in the north/south Galactic plane. However the non-parametric estimator exhibits no significant anisotropy, with the galaxy distribution being consistent with the assumption of isotropy in all regions considered. The parametric asymmetry is attributed to the relatively poor fit of the functional form to the underlying data. When using the non-parametric estimator, we do find a dipole in the shape of the luminosity function, with maximal deviation from isotropy at galactic coordinate $(b,l)=(30^{\circ },315^{\circ })$. However we can ascribe no strong statistical significance to this observation.

12 pages, 8 figures, 5 tables. Paper significantly modified, multiple estimators of the LF now used and a new method of selecting maximally anisotropic direction presented and implemented. Matches final version published in JCAP

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