H I Fine Structure in Magellanic Tidal Debris

We report on H I observations with the Australia Telescope and the Parkes telescope of one of the cores of the high-velocity cloud (HVC) WW 187 (also known as HVC 287+23+240 and HVC 287.5+22.5+240). With our H I map, this becomes the first HVC for which single-dish and interferometer data have been combined. The analysis of the H I fine structure shows that the smallest concentrations are almost resolved and may be as small as 5-15 pc. The structure of the cloud is hierarchical: smaller, denser cores are embedded in a smoother envelope. At 1' resolution, the peak brightness temperatures of the cores lie in the range 15-35 K, while the FWHMs of the individual spectra range from 3 to 9 km s^-1, with a modal value of ~5 km s^-1. This limits the kinetic temperature of the gas to lie between 35 and 500 K. The peak column densities range from 1.0×10²⁰ to 3.6×10²⁰ cm^-2. The distribution of column densities in the field is an exponential: count~exp[-N(HI)]. The volume densities in the cores could be as high as 20 cm^-3, which is sufficiently high to provide a long-time source for the molecular hydrogen in the HVC that is seen in the direction of NGC 3783. The original motivation for this observation came from the detection of interstellar UV absorption lines in the spectrum of the extragalactic background source NGC 3783 from which metal abundances for WW 187 were derived that are similar to those in the SMC. We describe in detail the derivation of the value for N(H I) used to derive these abundances. In combination with model calculations, these abundances support the contention that WW 187 is part of the leading arm of the Magellanic Stream and that the Magellanic Stream is a tidal feature, rather than being formed by ram pressure. Based on observations with the Australia Telescope.