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

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 degrees, D sin|b| > 2 kpc) as probes, we measure their dispersion measures and the neutral hydrogen component of the warm neutral medium (WNMH 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 (ss), and dust opacity (t/NH) in both phases. We find T WNMH 20 23 I = () + K, ss (WNMH I) = 1.5 +/- 0.4, and t353/NH (WNMH I) = (1.0 +/- 0.1) x 10-26 cm2. Assuming that the temperature and spectral index are the same in both the WNMH 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 353 NH WIM 0.3 0.3 10t () = (.) ' -26, which is about 3 times smaller than t353/NH (WNMH I). We are 80% confident that t353 NH (WIM) is at least 2 times smaller than t353/NH (WNMH I).