Testing Isotropy in the Local Universe

TL;DR

This study tests the isotropy of the local galaxy distribution using the 2MASS catalogue, finding no significant anisotropy with non-parametric methods but some hemispherical asymmetry with parametric methods, likely due to model fit issues.

Contribution

The paper introduces a combined parametric and non-parametric approach to test galaxy distribution isotropy using luminosity function shapes across the sky.

Findings

Non-parametric estimator shows no significant anisotropy.

Parametric estimator indicates hemispherical asymmetry, likely due to poor model fit.

A dipole in luminosity function shape observed but not statistically significant.

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.