IONIZED GAS IN THE SMITH CLOUD

We present Wisconsin H alpha 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 H alpha map shows the brightest H alpha emission, 0.43 +/- 0.04 R, coincident with the brightest Hi, while slightly fainter H alpha emission (0.25 +/- 0.02 R) is observed in a region with Hi intensities < 0.1 times as bright as the brightest Hi. We derive an ionized mass of greater than or similar to 3 x 10(6) M(circle dot), comparable to the Hi mass, with the H(+) mass spread over a considerably larger area than that of Hi. An estimated Galactic extinction correction could adjust these values upward by 40%. H alpha 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] gamma 6583 in the same direction with a line ratio [N II]/H alpha = 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.