The Physical Properties of a Cold H I Feature Observed Transitioning from Absorption to Emission

Large-scale surveys of neutral hydrogen (H I) 21 cm line emission from the Galactic plane at similar to 1' resolution have revealed an extensive population of H I self-absorption (HISA) features, i.e., presumably cold H I seen in absorption against warmer background H I. These observations provide previously unknown information regarding the morphology and distribution of cold H I gas in our Galaxy, but determining the physical properties of such features primarily depends on some sort of radiative transfer modeling that only loosely constrains various model parameters. We present here observations and analysis of one HISA feature that is seen to move from absorption into emission and show how observations made at the transition point allow the physical properties of the HISA feature to be tightly constrained. The derived physical properties are consistent with a diffuse, cold H I cloud with minimal molecular gas content. Identification of objects like this is currently uncommon in the interferometer-based Galactic plane surveys, which have necessarily focused on lower Galactic latitudes. As high-resolution Galactic surveys are extended to higher latitudes, we expect that many additional H I features will be seen transitioning from absorption to emission and will thus be amenable to the analysis techniques presented here.