Neutral Hydrogen Structures Trace Dust Polarization Angle: Implications for Cosmic Microwave Background Foregrounds

Using high-resolution data from the Galactic Arecibo L-Band Feed Array HI (GALFA-Hi) survey, we show that linear structure in Galactic neutral hydrogen (Hi) correlates with the magnetic field orientation implied by Planck 353 GHz polarized dust emission. The structure of the neutral interstellar medium is more tightly coupled to the magnetic field than previously known. At high Galactic latitudes, where the Planck data are noise dominated, the Hi data provide an independent constraint on the Galactic magnetic field orientation, and hence the local dust polarization angle. We detect strong cross-correlations between template maps constructed from estimates of dust intensity combined with either Hi-derived angles, starlight polarization angles, or Planck 353 GHz angles. The Hi data thus provide a new tool in the search for inflationary gravitational wave $B$-mode polarization in the cosmic microwave background, which is currently limited by dust foreground contamination.

Figure 1

Plane-of-sky magnetic field orientation as predicted by ${\theta }_{\mathrm{RHT}}$ (top) and ${\theta }_{353}$ (bottom). Color maps are integrated HI column density from $v=-61.5\mathrm{km}{\mathrm{s}}^{-1}$ to $+61.5\mathrm{km}{\mathrm{s}}^{-1}$ ($N_{\mathrm{HI}}[{\mathrm{cm}}^{-2}]$), and dust opacity (${\tau }_{353}$). Planck and RHT $Q$ and $U$ values are smoothed with a $\mathrm{FWHM}=1°$ Gaussian kernel, then used to construct ${\theta }_{353}$ and ${\theta }_{\mathrm{RHT}}$, which are visualized using line integral convolution (LIC; [24]). The high latitude ($b\gtrsim 70$) behavior of the ${\theta }_{353}$ LIC pattern is due to Planck noise. White pseudovectors represent starlight polarization angles. Galactic latitude lines lie at $b=30°$, 50°, 70°, from left to right. Galactic longitude lines lie at $l=80°$, 50°, 20°, from top to bottom.

Figure 2

Normalized histograms of $\delta \theta ={\theta }_{353}-{\theta }_{\mathrm{RHT}}$ in 1° bins at resolutions of $\mathrm{FWHM}=5^{\prime }$, $15^{\prime }$, and $30^{\prime }$. The Gaussian fit to the $\mathrm{FWHM}=15^{\prime }$ histogram shown has a standard deviation $\sigma =19.4°$.

Figure 3

Cross-power spectra of polarization template maps constructed from $I_{353}$ and either Planck (${\psi }_{353}$), RHT (${\psi }_{\mathrm{RHT}}$), or starlight polarization (${\psi }_{*}$) data [Eq. (2)]. Shown are $E$-mode (circles) and $B$-mode (squares) components. Significant ($40–70\sigma$) cross-correlations are detected in all cases.

Figure 4

Angle uncertainties averaged over 2° Galactic latitude bins, normalized by their respective median values (dashed line).