Comparing ionized gas with dense gas and dust in the Horsehead Nebula. In Hα (top left), we see the filamentary photo-evaporation flow that streams out from the PDR towards the west. The Horsehead sits in projection against the vertically oriented PDR, where we see increased [S II]/Hα and [N II]/Hα (top center and left) in the ionization front. Hα contours (red) mirror the dust features unveiled in PAH-rich WISE 12 μm imaging (bottom left), atomic/ionized gas highlighted by [C II] imaging (bottom center, Pabst et al. 2017) and molecular gas seen in ¹²CO(1–0) (bottom right; Pety et al. 2017).