Probing Galatic ISM with halo HI clouds: the first results

Several neutral clouds in B lower Galactic halo were observed in 21 cm emission with both the GBT and the VLA D array. Because of their isolation the halo clouds give basic information on interstellar processes not available from the highly blended spectra typical of the rest of the ISM. The GBT spectra of most clouds have two line components and the VLA data show compact clumps in a more diffuse background, possibly indicating two coexisting thermal phases. As halo clouds have kinematics dominated by Galactic rotation, a reasonably accurate distance can be obtained for an individual cloud thus allowing to estimate its size and, consequently, pressure. One of the clouds lies in front of a 5.2 Jy radio continuum source (1819-096), strong enough to produce detectable absorption in the 21 cm HI line. The absorption spectrum allows to measure the cloud's opacity and spin temperature, which were found to be 0.07 and 150 K, respectively. The line also has weak wings suggesting absorption from warm HI. A ratio of observed to thermal line widths is close to unity, which indicates that the cloud may contain little turbulence.

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