The Diffuse Ionized Gas Halo of the Small Magellanic Cloud

Abstract: Observations with the Wisconsin H-alpha Mapper (WHAM) reveal a large, diffuse ionized halo that surrounds the Small Magellanic Cloud (SMC). We present the first kinematic H-alpha survey of an extended region around the galaxy, from (l,b) = (289.5,-35.0) to (315.1,-5.3) and covering +90 <= vLSR <= +210 km s-1. The ionized gas emission extends far beyond the central stellar component of the galaxy, reaching similar distances to that of the diffuse neutral halo traced by 21 cm observations. H-alpha emission extends several degrees beyond the sensitivity of current H I surveys toward smaller Galactic longitudes and more negative Galactic latitudes. The velocity field of the ionized gas near the SMC appears similar to to the neutral halo of the galaxy. Using the observed emission measure as a guide, we estimate the mass of this newly revealed ionized component to be roughly (0.8 - 1.0) x 10^9 M_sun, which is comparable to the total neutral mass in the same region of (0.9 - 1.1) x 10^9 M_sun. We find ratios of the total ionized gas mass divided by the total neutral plus ionized gas mass in three distinct subregions to be: (1) 46%-54% throughout the SMC and its extended halo, (2) 12%-32% in the SMC Tail that extends toward the Magellanic Bridge, and (3) 65%-79% in a filament that extends away from the SMC toward the Magellanic Stream. This newly discovered, coherent H-alpha filament does not appear to have a well-structured neutral component and is also not coincident with two previously identified filaments traced by 21 cm emission within the Stream.