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T. H. Troland (Physics & Astronomy Dept., Univ. of Kentucky), C. Heiles (Berkeley Astronomy Dept.)
We have used the upgraded Arecibo telescope to make extensive observations of HI emission and absorption toward extra-galactic continuum sources. We use new observational and analysis techniques to derive spin temperatures for the HI absorption line components and upper limits to the kinetic temperatures for the associated emission lines. Absorption and emission lines sample the CNM and the WNM, respectively. We find CNM at temperatures below 20 K. Such temperatures imply an absence of grain heating in the gas. We find WNM in the thermally-unstable range of 500-5000 K. This result is unexplained by equilibrium ISM models. Our data are also sensitive to the Zeeman effect in the CNM. Line-of-sight field strengths are often less than 5 microgauss. Evidently, the magnetic field in the CNM is comparable to that in the ISM at large despite the fact that the density of the CNM is two orders of magnitude greater than the average ISM density. Unless densities in the CNM are very low, these results suggest that super-Alfvenic turbulence is common in this gas. Both authors acknowledge NSF support for this research.