WESTERBORK OBSERVATIONS OF HIGH-VELOCITY CLOUDS - DISCUSSION

Six high-velocity cloud fields were observed with 1' and 1 km s-1 resolution, using the Westerbork Synthesis Radio Telescope. The structures seen in earlier observations at 10' resolution break up into a disorderly collection of concentrations. The presence of much substructure has important implications for determinations of distances and interstellar abundances, as the neutral hydrogen column density shows a large contrast (about a factor 5) on small angular scales (a few arcminutes). In the HVCs only about 30% of the total single-dish flux is recovered, just as for low-velocity H I. This shows that for high-velocity gas, too, there is an extended, warm or hot, background present.The linewidths as measured at the present resolution generally are below 5 km s-1, though there is a large spread of widths in any one field. About 1 km s-1 of this width can be attributed to beam-smearing within the 1' beam. Temperatures in the concentrations are generally between 30 and 300 K, and pressures appear to be of the same order of magnitude as in the extended background gas (except maybe in chain A). The volume density in the concentrations is of the order of 75/D particles per cubic centimeter for an assumed distance of D kpc.