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M.D. Faison (University of Wisconsin/NRAO)
We have used the Very Long Baseline Array (VLBA) with the Very Large Array (VLA) to image Galactic neutral hydrogen in absorption towards seven bright, extragalactic radio sources. By imaging the sources in both continuum emission and neutral hydrogen (HI) 21-cm line absorption, we have made direct images of the HI opacity along these lines of sight. In most cases, the angular resolution of our images is 10 milliarcseconds and the velocity resolution is 0.4 km s-1 per spectral channel. We are able to probe the structure of the absorbing HI up to the largest angular size of the source, from 50 milliarcseconds for the smallest source to 300 milliarcseconds for our largest source. Two of the lines of sight in our sample show very large variations in HI optical depth over very small angular scales. If the opacity variations are due to density structure in the absorbing HI, large fluctuations in HI density over small physical scales of order 10 AU are implied. Four of the lines of sight show moderate optical depth variations over the angular scales probed, and one line of sight shows no variations, consistent with uniform HI absorption across the face of the source. We have also fit the circular polarization spectra towards these sources to place limits on the line-of-sight magnetic field in the Interstellar Medium on small scales. These optical depth images will be presented, and the implications of this work for the power spectrum of cold HI structure in our Galaxy will be discussed.
The VLA and VLBA are facities of the National Science Foundation opererated under cooperative agreement by Associated Universities, Inc.
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