VLBA imaging of small-scale structure in galactic H I

We have used the Very Long Baseline Array (VLBA) and the phased Very Large Array (VLA) to image Galactic H I in absorption in the direction of three bright extragalactic objects: 3C 138, QSO 2255 + 416, and CJ1 0404 + 768. The angular resolution of these images is 20 milliarcseconds (mas) in the case of 3C 138 and 10 mas for 2255 + 416 and 0404 + 768. H I optical depth variations up to a factor of 2 are observed in the directions of 3C 138 and 2255 +416 on angular scales larger than 10-20 mas. The source 0404 + 768 shows no significant optical depth variations from 10 mas up to the angular size of the source, about 150 mas. This is the first source for which significant variations are not observed, which possibly indicates an inner scale of a few AU for the H I small-scale structure. The small-scale opacity structures seen toward 3C 138 and 2255 + 416 could be due to density variations, spin temperature variations, velocity turbulence in the atomic gas, or a combination of these effects. If the opacity variations are due to fluctuations in density, the data suggest high-density structures (similar to 10(6) cm(-3)) in the cold neutral medium on physical scales less than 10 AU. However, if the large column density variations are due to elongated structures such as filaments or sheets observed edge-on, the densities (and pressures) can be reduced by invoking an elongation factor along the line of sight. Measurements of Stokes V in the H I absorbing gas show no indication of Zeeman splitting of the absorption line, which result in 2 a upper limits for the line-of-sight magnetic fields of 40 mu G for 3C 138, 100 mu G for 2255 + 416, and 700 mu G for 0404 + 768.