Implications for Galactic Electron Density Structure from Pulsar Sightlines Intersecting HII Regions

Recent radio surveys have revealed pulsars with dispersion and scattering delays induced by ionized gas that are larger than the rest of the observed pulsar population, in some cases with electron column densities (or dispersion measures; DMs) larger than the maximum predictions of Galactic electron density models. By cross-matching the observed pulsar population against HII region catalogs, we show that the majority of pulsars with $\rm DM > 600$ pc cm$^{-3}$ and scattering delays $\tau(1\ {\rm GHz}) > 10$ ms lie behind HII regions, and that HII region intersections may be relevant to as much as a third of the observed pulsar population. The fraction of the full pulsar population with sightlines intersecting HII regions is likely larger. Accounting for HII regions resolves apparent discrepancies where Galactic electron density models place high-DM pulsars beyond the Galactic disk. By comparing emission measures (EMs) inferred from recombination line observations to pulsar DMs, we show that HII regions can contribute tens to hundreds of pc cm$^{-3}$ in electron column density along a pulsar LOS. We find that nearly all pulsars with significant excess (and deficit) scattering from the mean $\tau$-DM relation are spatially coincident with known discrete ionized gas structures, including HII regions. Accounting for HII regions is critical to the interpretation of radio dispersion and scattering measurements as electron density tracers, both in the Milky Way and in other galaxies.