Probing the Cold Neutral Medium through H I Emission Morphology with the Scattering Transform

Neutral hydrogen (H I) emission exhibits complex morphology that encodes rich information about the physics of the interstellar medium. We apply the scattering transform (ST) to characterize the H I emission structure via a set of compact and interpretable coefficients, and find a connection between the H I emission morphology and H I cold neutral medium (CNM) phase content. Where H I absorption measurements are unavailable, the H I phase structure is typically estimated from the emission via spectral line decomposition. Here, we present a new probe of the CNM content using measures that are solely derived from H I emission spatial information. We apply the ST to GALFAH I data at high Galactic latitudes (|b| > 30.), and compare the resulting coefficients to CNM fraction measurements derived from archival H I emission and absorption spectra. We quantify the correlation between the ST coefficients and the measured CNM fraction ( fCNM), finding that the H I emission morphology encodes substantial fCNM-correlating information and that ST-based metrics for small-scale linearity are particularly predictive of fCNM. This is further corroborated by the enhancement of the I857/NHI ratio with larger ST measures of small-scale linearity. These results are consistent with the picture of regions with higher CNM content being more populated by small-scale filamentary H I structures. Our work illustrates a physical connection between the H I morphology and phase content, and suggests that future phase decomposition methods can be improved by making use of both H I spectral and spatial information.