Abstract
Abstract We test the statistical isotropy of the Universe by analysing the distribution of WISE extragalactic sources that were also observed by 2MASS. We pay particular attention to colour 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 ∼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) ≃ (310°, −15°), 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 cosmic microwave background 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.
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