A new, large-scale map of interstellar reddening derived from HI emission

We present a new map of interstellar reddening, covering the 39\% of the sky with low {\rm HI} column densities ($N_{\rm HI} < 4\times10^{20}\,\rm cm^{-2}$ or $E(B-V)\approx 45\rm\, mmag$) at $16\overset{'}{.}1$ resolution, based on all-sky observations of Galactic HI emission by the HI4PI Survey. In this low column density regime, we derive a characteristic value of $N_{\rm HI}/E(B-V) = 8.8\times10^{21}\, \rm\, cm^{2}\, mag^{-1}$ for gas with $|v_{\rm LSR}| < 90\,\rm km\, s^{-1}$ and find no significant reddening associated with gas at higher velocities. We compare our HI-based reddening map with the Schlegel, Finkbeiner, and Davis (1998, SFD) reddening map and find them consistent to within a scatter of $\simeq 5\,\rm mmag$. Further, the differences between our map and the SFD map are in excellent agreement with the low resolution ($4\overset{\circ}{.}5$) corrections to the SFD map derived by Peek and Graves (2010) based on observed reddening toward passive galaxies. We therefore argue that our HI-based map provides the most accurate interstellar reddening estimates in the low column density regime to date. Our reddening map is made publicly available (http://dx.doi.org/10.7910/DVN/AFJNWJ).Comment: Re-submitted to ApJ. The reddening map is available at http://dx.doi.org/10.7910/DVN/AFJNW

An Imprint of the Galactic Magnetic Field in the Diffuse Unpolarized Dust Emission

It is well known that aligned, aspherical dust grains emit polarized radiation and that the degree of polarization depends on the angle ψ between the interstellar magnetic field and the line of sight (LOS). However, anisotropy of the dust absorption cross sections also modulates the total intensity of the radiation as the viewing geometry changes. We report a detection of this effect in the high Galactic latitude Planck data, finding that the 353 GHz dust intensity per N_(H I) is smaller when the Galactic magnetic field is mostly in the plane of the sky and larger when the field is mostly along the LOS. These variations are of opposite sign and roughly equal magnitude as the changes in polarized intensity per N_(H I) with ψ, as predicted. In principle, the variation in intensity can be used in conjunction with the dust polarization angle to constrain the full 3D orientation of the Galactic magnetic field