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 × 106 M, 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