The Structure of the Cold Neutral Interstellar Medium on 10–100 AU Scales

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Abstract
We have used the Very Long Baseline Array (VLBA) and the Very Large Array (VLA) to image Galactic neutral hydrogen in absorption toward four compact extragalactic radio sources with a resolution of 10 mas. Previous VLBA data published by Faison et al. have shown the existence of prominent structures in the direction of the extragalactic source 3C 138 with scale sizes of 10–20 AU and changes in H I optical depth in excess of 0.8 ± 0.1. We confirm the small-scale H I optical depth variations toward 3C 147 suggested earlier at a level up to 20% ± 5%. The sources 3C 119, CJ1 2352+495, and CJ1 0831+557 show no significant change in H I optical depth across the sources with 1 σ limits of 30%, 50%, and 100%. Of the seven sources recently investigated with the VLBA and VLA, only 3C 138 and 3C 147 show statistically significant variations in H I opacities. Deshpande and collaborators have attempted to explain the observed small-scale structures as extensions of the observed power spectra of structures on parsec-size scales. The predictions by Deshpande are consistent with the VLBA H I data observed in the directions of a number of sources, including 3C 147, but they are not consistent with our previous observations of the H I opacity structure toward 3C 138.
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