Spatial Variations in Galactic H I Structure on AU-Scales Toward 3C 147
  Observed with the Very Long Baseline Array
  This paper reports dual-epoch, Very Long Baseline Array observations of H I
absorption toward 3C 147. One of these epochs (2005) represents new
observations while one (1998) represents the reprocessing of previous
observations to obtain higher signal-to-noise results. Significant H I opacity
and column density variations, both spatially and temporally, are observed with
typical variations at the level of \Delta\tau ~ 0.20 and in some cases as large
as \Delta\tau ~ 0.70, corresponding to column density fluctuations of order 5 x
10^{19} cm^{-2} for an assumed 50 K spin temperature. The typical angular scale
is 15 mas; while the distance to the absorbing gas is highly uncertain, the
equivalent linear scale is likely to be about 10 AU. Approximately 10% of the
face of the source is covered by these opacity variations, probably implying a
volume filling factor for the small-scale absorbing gas of no more than about
1%. Comparing our results with earlier results toward 3C 138 (Brogan et al.),
we find numerous similarities, and we conclude that small-scale absorbing gas
is a ubiquitous phenomenon, albeit with a low probability of intercept on any
given line of sight. Further, we compare the volumes sampled by the line of
sight through the Galaxy between our two epochs and conclude that, on the basis
of the motion of the Sun alone, these two volumes are likely to be
substantially different. In order to place more significant constraints on the
various models for the origin of these small-scale structures, more frequent
sampling is required in any future observations.
