Small-scale structure traced by neutral hydrogen absorption in the
  direction of multiple-component radio continuum sources
  We have studied the small scale distribution of atomic hydrogen (HI) using
21-cm absorption spectra against multiple-component background radio continuum
sources from the 21-SPONGE survey and the Millennium Arecibo Absorption Line
Survey. We have found $>5\sigma$ optical depth variations at a level of
$\sim0.03-0.5$ between 13 out of 14 adjacent sightlines separated by a few
arcseconds to a few arcminutes, suggesting the presence of neutral structures
on spatial scales from a few to thousands of AU (which we refer to as tiny
scale atomic structure, TSAS). The optical depth variations are strongest in
directions where the HI column density and the fraction of HI in the cold
neutral medium (CNM) are highest, which tend to be at low Galactic latitudes.
By measuring changes in the properties of Gaussian components fitted to the
absorption spectra, we find that changes in both the peak optical depth and the
linewidth of TSAS absorption features contribute to the observed optical depth
variations, while changes in the central velocity do not appear to strongly
impact the observed variations. Both thermal and turbulent motions contribute
appreciably to the linewidths, but the turbulence does not appear strong enough
to confine overpressured TSAS. In a majority of cases, the TSAS column
densities are sufficiently high that these structures can radiatively cool fast
enough to maintain thermal equilibrium with their surroundings, even if they
are overpressured. We also find that a majority of TSAS is associated with the
CNM. For TSAS in the direction of the Taurus molecular cloud and the local Leo
cold cloud, we estimate densities over an order of magnitude higher than
typical CNM densities.
