BICEP/Keck. XVI. Characterizing Dust Polarization through Correlations with Neutral Hydrogen

Abstract

We characterize Galactic dust filaments by correlating BICEP/Keck and Planck data with polarization templates based on neutral hydrogen (H i) observations. Dust polarization is important for both our understanding of astrophysical processes in the interstellar medium (ISM) and the search for primordial gravitational waves in the cosmic microwave background (CMB). In the diffuse ISM, H i is strongly correlated with the dust and partly organized into filaments that are aligned with the local magnetic field. We analyze the deep BICEP/Keck data at 95, 150, and 220 GHz, over the low-column-density region of sky where BICEP/Keck has set the best limits on primordial gravitational waves. We separate the H i emission into distinct velocity components and detect dust polarization correlated with the local Galactic H i but not with the H i associated with Magellanic Stream i. We present a robust, multifrequency detection of polarized dust emission correlated with the filamentary H i morphology template down to 95 GHz. For assessing its utility for foreground cleaning, we report that the Hi morphology template correlates in B modes at a ∼10%–65% level over the multipole range 20 < ℓ < 200 with the BICEP/Keck maps, which contain contributions from dust, CMB, and noise components. We measure the spectral index of the filamentary dust component spectral energy distribution to be β = 1.54 ± 0.13. We find no evidence for decorrelation in this region between the filaments and the rest of the dust field or from the inclusion of dust associated with the intermediate velocity H i. Finally, we explore the morphological parameter space in the H i-based filamentary model.

Probing the Cold Neutral Medium through H I Emission Morphology with the Scattering Transform

“…In recent years, with the increasing availability of H I emission observations, including some largescale high-spatial resolution observational surveys, there has been growing interest in deriving the properties of the ISM from the H I spatial morphology, with fruitful results. The physics probed by the H I morphology includes H 2 formation (Barriault et al 2010), galaxy dynamics (Soler et al 2020(Soler et al , 2022, high-velocity cloud instabilities (Barger et al 2020), galactic outflows (McClure-Griffiths et al 2018;Di Teodoro et al 2020), star formation (Hacar et al 2022;Yu et al 2022), and the structure of the interstellar magnetic field, with applications to cosmological foregrounds (Clark et al 2014(Clark et al , 2015Kalberla & Kerp 2016;Clark 2018;Clark & Hensley 2019;Ade et al 2023). In particular, the study of H I intensity maps using machine vision algorithms, like the Rolling Hough Transform (RHT; Clark et al 2014), has revealed remarkably linear filamentary features that align with magnetic field directions, as traced by dust and starlight polarization (Clark et al 2014(Clark et al , 2015Kalberla & Kerp 2016;Clark & Hensley 2019).…”