Dipolar modulation in number counts of WISE-2MASS sources

TL;DR

This paper investigates the statistical isotropy of the universe by analyzing WISE-2MASS extragalactic sources, detecting a dipole in number counts that aligns with large-scale structure and CMB anisotropies.

Contribution

It provides the first detailed analysis of dipolar modulation in WISE-2MASS sources, linking local structure to cosmic anisotropy signals.

Findings

Detected a dipole amplitude of ~0.05 in source counts.

Dipole direction aligns with previous large-scale structure and CMB dipole directions.

Results suggest a connection between local galaxy distribution and cosmic anisotropy signals.

Abstract

We test the statistical isotropy of the universe by analyzing the distribution of WISE extragalactic sources that were also observed by 2MASS. We pay particular attention to color cuts and foreground marginalization in order to cull a uniform sample of extragalactic objects and avoid stars. We detect a dipole gradient in the number-counts with an amplitude of $\sim$0.05, somewhat larger than expectations based on local structures corresponding to the depth and (independently measured) bias of our WISE-2MASS sources. The direction of the dipole, $(l,b)\simeq (310\,^{\circ}, -15\,^{\circ})$, is in reasonably good agreement with that found previously in the (shallower) 2MASS Extended Source Catalog alone. Interestingly, the dipole direction is not far from the direction of the dipolar modulation in the CMB found by Planck, and also fairly closely matches large-scale-structure bulk-flow directions found by various groups using galaxies and type Ia supernovae. It is difficult, however, to draw specific conclusions from the near-agreement of these directions.