2023ApJ...947...74L

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 GALFA-H I data at high Galactic latitudes ($\left|b\right|\gt 30^\circ $), 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 (f_CNM), finding that the H I emission morphology encodes substantial f_CNM-correlating information and that ST-based metrics for small-scale linearity are particularly predictive of f_CNM. This is further corroborated by the enhancement of the I₈₅₇/N_HI 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.