Small-Scale HI Channel Map Structure is Cold: Evidence from Na I
  Absorption at High Galactic Latitudes
  The spatial distribution of neutral hydrogen (H I) emission is a powerful
probe of interstellar medium physics. The small-scale structure in H I channel
maps is often assumed to probe the velocity field rather than real density
structures. In this work we directly test this assumption, using
high-resolution GALFA-H I observations and 50,985 quasar spectra from SDSS. We
measure the equivalent widths of interstellar Na I D$_1$ and Na I D$_2$
absorption, and robustly conclude that they depend more strongly on the column
density of small-scale structure in H I than on either the large-scale H I
structure or the total H I column. This is inconsistent with the hypothesis
that small-scale channel map structure is driven by velocity crowding. Instead,
the data favor the interpretation that this emission structure predominantly
originates in cold, dense interstellar material, consistent with a clumpy cold
neutral medium.
