IONIZED GAS IN THE SMITH CLOUD

We present Wisconsin Hα Mapper observations of ionized gas in the Smith Cloud, a high-velocity cloud which Lockman et al. have recently suggested is interacting with the Galactic disk. Our Hα map shows the brightest Hα emission, 0.43 ± 0.04 R, coincident with the brightest H I, while slightly fainter Hα emission (0.25 ± 0.02 R) is observed in a region with H I intensities <0.1 times as bright as the brightest H I. We derive an ionized mass of ∼>3 x 106 M sun, comparable to the H I mass, with the H+ mass spread over a considerably larger area than that of H I. An estimated Galactic extinction correction could adjust these values upward by 40%. Hα and [S II] line widths toward the region of brightest emission constrain the electron temperature of the gas to be between 8000 K and 23,000 K. A detection of [N II] λ6583 in the same direction with a line ratio [N II]/Hα = 0.32 ± 0.05 constrains the metallicity of the cloud: for typical photoionization temperatures of 8000-12,000 K, the nitrogen abundance is 0.15-0.44 times solar. These results lend further support to the claim that the Smith Cloud is new material accreting onto the Galaxy.