Small-scale H i 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 the Sloan Digital Sky Survey. We measure the equivalent widths of interstellar Na i D1 and Na i D2 absorption, and robustly conclude that together they depend nearly four times as 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.