Isotropy-Violation Diagnostics for $B$-mode Polarization Foregrounds to the Cosmic Microwave Background

Abstract

Isotropy-violation statistics can highlight polarized galactic foregrounds that contaminate primordial $B$-modes in the Cosmic Microwave Background (CMB). We propose a particular isotropy-violation test and apply it to polarized Planck 353 GHz data, constructing an map that indicates $B$-mode foreground dust power over the sky. We build our main isotropy test in harmonic space via the bipolar spherical harmonic basis, and our method helps us to identify the least-contaminated directions. By this measure, there are regions of low foreground in and around the BICEP field, near the South Galactic Pole, and in the Northern Galactic Hemisphere. There is also a possible foreground feature in the BICEP field. We compare our results to those based on the local power spectrum, which is computed on discs using a version of the method of Planck Int.~XXX (2016). The discs method is closely related to our isotropy-violation diagnostic. We pay special care to the treatment of noise, including chance correlations with the foregrounds. Currently we use our isotropy tool to assess the cleanest portions of the sky, but in the future such methods will allow isotropy-based null tests for foreground contamination in maps purported to measure primordial $B$-modes, particularly in cases of limited frequency coverage.

Published in JCAP. Added appendix and updated text to match the accepted version

References in corpus (12)

• A Joint Analysis of BICEP2/Keck Array and Planck Data
• BICEP2 / Keck Array VI: Improved Constraints On Cosmology and Foregrounds When Adding 95 GHz Data From Keck Array
• Planck 2015 results. X. Diffuse component separation: Foreground maps
• CLASS: The Cosmology Large Angular Scale Surveyor
• Neutral hydrogen structures trace dust polarization angle: Implications for cosmic microwave background foregrounds
• BICEP2 / Keck Array V: Measurements of B-mode Polarization at Degree Angular Scales and 150 GHz by the Keck Array
• Characterization of foreground emission at degree angular scale for CMB B-modes observations. Thermal Dust and Synchrotron signal from Planck and WMAP data
• Statistical Isotropy of CMB Polarization Maps
• CMB E/B decomposition of incomplete sky: a pixel space approach
• Statistical diagnostics to identify Galactic foregrounds in B-mode maps
• A LEKID-based CMB instrument design for large-scale observations in Greenland
• Weak lensing in non-statistically isotropic universes

Cited by in corpus (7)

• The Power Spectra of Polarized, Dusty Filaments
• The bispectrum of polarized galactic foregrounds
• Characterizing bias on large scale CMB $B$-modes after galactic foregrounds cleaning
• Full-sky, arcminute-scale, 3D models of Galactic microwave foreground dust emission based on filaments
• Joint constraints on reionization: a framework for combining the global 21cm signal and the kinetic Sunyaev-Zel'dovich effect
• Detection and Removal of B-mode Dust Foregrounds with Signatures of Statistical Anisotropy
• Impact of Galactic non-Gaussian foregrounds on CMB lensing measurements