A New Constraint on the Relative Disorder of Magnetic Fields between Neutral Interstellar Medium Phases

Abstract

Utilizing Planck polarized dust emission maps at 353 GHz and large-area maps of the neutral hydrogen (H i ) cold neutral medium (CNM) fraction ( f _CNM ), we investigate the relationship between dust polarization fraction ( p _353 ) and f _CNM in the diffuse high latitude ( $\left|b\right|\gt 30^\circ $ ) sky. We find that the correlation between p _353 and f _CNM is qualitatively distinct from the p _353 –H i column density ( N _H _i ) relationship. At low column densities ( N _H _i < 4 × 10 ^20 cm ^−2 ) where p _353 and N _H _i are uncorrelated, there is a strong positive p _353 – f _CNM correlation. We fit the p _353 – f _CNM correlation with data-driven models to constrain the degree of magnetic field disorder between phases along the line of sight. We argue that an increased magnetic field disorder in the warm neutral medium (WNM) relative to the CNM best explains the positive p _353 – f _CNM correlation in diffuse regions. Modeling the CNM-associated dust column as being maximally polarized, with a polarization fraction p _CNM ∼ 0.2, we find that the best-fit mean polarization fraction in the WNM-associated dust column is 0.22 p _CNM . The model further suggests that a significant f _CNM -correlated fraction of the non-CNM column (an additional 18.4% of the H i mass on average) is also more magnetically ordered, and we speculate that the additional column is associated with the unstable medium. Our results constitute a new large-area constraint on the average relative disorder of magnetic fields between the neutral phases of the interstellar medium, and are consistent with the physical picture of a more magnetically aligned CNM column forming out of a disordered WNM.

Keywords

• Interstellar medium
• Cold neutral medium
• Interstellar magnetic fields
• Interstellar dust