Examination of Evidence for a Preferred Axis in the Cosmic Radiation Anisotropy

We examine previous claims for a preferred axis at $(b,l)\approx (60,-100)$ in the cosmic radiation anisotropy, by generalizing the concept of multipole planarity to any shape preference. Contrary to earlier claims, we find that the amount of power concentrated in planar modes for $l=2,3$ is not inconsistent with isotropy and Gaussianity. The multipoles’ alignment, however, is indeed anomalous, and extends up to $l=5$ rejecting statistical isotropy with a probability in excess of 99.9%. There is also an uncanny correlation of azimuthal phases between $l=3$ and $l=5$. We are unable to blame these effects on foreground contamination or large-scale systematic errors. This reappraisal may be crucial in identifying the theoretical model behind the anomaly.

Figure 1

The $l=5$ multipole in galactic coordinates (top) and aligned with $(b,l)\approx (50,-91)$ (middle), and the $l=3$ multipole in its preferred frame. We have superposed the multipole vectors, and the chain linking them.

Figure 2

$m$-preference results: we seek the direction ${\mathbf{n}}_l$ (galactic coordinates in the bottom two plots), where a given $m$ (second plot from the top) receives the highest proportion of the power (ratio $r_l$ in the top panel). We have also plotted the Monte Carlo inferred 68% region for $r_l$ (light shading). We plotted results for cleaned (solid line), Wiener filtered (dashed line), and internal linear combination (dotted line) maps.

Figure 3

$m$-preference results up to $l=20$ for the $V$ band of galactic templates. We have included free-free (solid line), synchrotron (dot-dashed line), and dust (long-dashed line) maps; for reference we have reproduced results for the cleaned maps (dotted line).