Ionized Gas in the Smith Cloud

Abstract

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 \pm 0.04 R, coincident with the brightest H I, while slightly fainter H-alpha emission (0.25 \pm 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 \gtrsim 3 \times 10^6 M_\odot, comparable to the H I mass, with the H^+ mass spread over a considerably larger area than the H I. An estimated Galactic extinction correction could adjust these values upwards by 40 %. H-alpha and [S II] line widths towards the region of brightest emission constrain the electron temperature of the gas to be between 8000 K and 23000 K. A detection of [N II] \lambda 6583 in the same direction with a line ratio [N II] / H-alpha = 0.32 \pm 0.05 constrains the metallicity of the cloud: for typical photoionization temperatures of 8000-12000 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.