Boundary Kinematic Diagnostics of Compact H I Clouds in GALFA–H I

Compact neutral hydrogen (H I) clouds in the Galactic disk–halo system span a wide range of velocities, morphologies, and inferred environments. While many such clouds are known to be non-self-gravitating, the mechanisms by which they are confined and interact with their surroundings remain an active observational question. In this work, we introduce and apply a boundary kinematic diagnostic designed to quantify differences between the internal kinematics of compact H I clouds and those of their immediate surroundings. The diagnostic compares velocity dispersion measured within a cloud’s dense interior to that measured in an annular boundary region defined purely from integrated H I emission. Because the diagnostic is based on relative quantities measured within a single data cube, it is insensitive to absolute calibration uncertainties, distance assumptions, and global velocity offsets. We apply this diagnostic uniformly to a sample of compact clouds drawn from the GALFA–H I survey, spanning high-velocity clouds (HVCs), galaxy-candidate clouds (GCCs), and cold low-velocity clouds (CLVCs). This paper documents the data processing, definitions, and empirical results of that analysis. Physical interpretation of the observed trends is deferred to a companion discussion paper. All GALFA–HI data products, cloud identifications, and velocity selections used throughout this work are described in Section 2. Appendix figures reuse the same data sources and analysis pipeline unless explicitly stated otherwise.