All We Are Is Dust in the WIM: Constraints on Dust Properties in the Milky Way’s Warm Ionized Medium

Abstract

We present a comparison of the presence and properties of dust in two distinct phases of the Milky Way’s interstellar medium: the warm neutral medium (WNM) and the warm ionized medium (WIM). Using distant pulsars at high Galactic latitudes and vertical distance (∣ b ∣ > 40°, $D\sin | b| \gt 2\,\mathrm{kpc}$ ) as probes, we measure their dispersion measures and the neutral hydrogen component of the warm neutral medium (WNM _H I ) using H i column density. Together with dust intensity along these same sightlines, we separate the respective dust contributions of each ISM phase in order to determine whether the ionized component contributes to the dust signal. We measure the temperature ( T ), spectral index ( β ), and dust opacity ( τ / N _H ) in both phases. We find $T\,({\mathrm{WNM}}_{{\rm{H}}\,{\rm\small{I}}})={20}_{-2}^{+3}$ K, β (WNM _H I ) = 1.5 ± 0.4, and τ _353 / N _H (WNM _H I ) = (1.0 ± 0.1) × 10 ^−26 cm ^2 . Assuming that the temperature and spectral index are the same in both the WNM _H I and WIM, and given our simple model that widely separated lines of sight can be fit together, we find evidence that there is a dust signal associated with the ionized gas and ${\tau }_{353}/{N}_{{\rm{H}}}\,(\mathrm{WIM})=(0.3\pm 0.3)\times {10}^{-26}$ , which is about 3 times smaller than τ _353 / N _H (WNM _H I ). We are 80% confident that ${\tau }_{353}/{N}_{{\rm{H}}}\,(\mathrm{WIM})$ is at least 2 times smaller than τ _353 / N _H (WNM _H I ).

Keywords

• Gas-to-dust ratio
• Intercloud medium