Directional Variations of the Non-Gaussianity Parameter f_NL

TL;DR

This study examines the spatial variations of the primordial non-Gaussianity parameter f_NL in WMAP data, assessing foreground contamination effects and finding band-dependent discrepancies, especially in the Q band, with implications for non-Gaussianity analysis.

Contribution

It improves the needlet bispectrum estimate of f_NL and evaluates foreground contamination effects across different WMAP frequency bands.

Findings

No significant foreground contamination in V and W bands.

High f_NL values near the galactic plane in Q band suggest contamination.

Consistent positive f_NL estimates across the sky in V+W channel.

Abstract

We investigate local variations of the primordial non-Gaussianity parameter f_NL in the WMAP data, looking for possible influence of foreground contamination in the full-sky estimate of f_NL. We first improve the needlet bispectrum estimate in (Rudjord et al. 2009) on the full-sky to f_NL= 73 +/- 31 using the KQ75 mask on the co-added V+W channel. We find no particular values of f_NL estimates close to the galactic plane and conclude that foregrounds are unlikely to affect the estimate of f_NL in the V and W bands even for the smaller KQ85 mask. In the Q band however, we find unexpectedly high values of f_NL in local estimates close to the galactic mask, as well as significant discrepancies between Q band estimates and V/W band estimates. We therefore conclude that the Q band is too contaminated to be used for non-Gaussianity studies even with the larger KQ75 mask. We further noted that the local f_NL estimates on the V+W channel are positive on all equatorial bands from the north to the south pole. The probability for this to happen in a universe with f_NL = 0 is less than one percent.