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We investigate the physical properties of structures seen in channel mapobservations of 21-cm neutral hydrogen (HI) emission. HI intensity maps displayprominent linear structures that are well aligned with the ambient magneticfield in the diffuse interstellar medium (ISM). Some literature hold that thesestructures are "velocity caustics", fluctuations imprinted by the turbulentvelocity field, and are not three-dimensional density structures in the ISM. Wetest this hypothesis by stacking probes of the density field -- broadband farinfrared (FIR) emission and the integrated HI column density (NHI) -- at thelocations of linear HI intensity structures. We find that the HI intensityfeatures are real density structures and not velocity caustics. We generalizethe investigation to all small-scale structure in HI channel maps, and analyzethis correlation as a function of velocity channel width, finding no measurablecontribution from velocity caustics to the HI channel map emission. Further, wefind that small-scale HI channel maps structures have elevated FIR/NHI,implying that this emission originates from a colder, denser phase of the ISMthan the surrounding material. The data are consistent with a multi-phasediffuse ISM in which small-scale structures in narrow HI channel maps arepreferentially cold neutral medium (CNM) that is anisotropically distributedand aligned with the local magnetic field. The shallow spatial power spectrum(SPS) of narrow HI channels is often attributed to velocity caustics. Weconjecture instead that the small-scale structure and narrow linewidths typicalof CNM explain the observed relationship between the SPS and channel width.

The Physical Nature of Neutral Hydrogen Intensity Structure