Molecular and Atomic Hydrogen Cloud Correlations in the Interstellar Medium of the Galaxy

A key question in Galactic astrophysics is how the cold, relatively dense clouds that form new stars are assembled from the warmer, more diffuse interstellar medium (ISM). We are investigating the relationship between neutral atomic hydrogen (HI) and molecular hydrogen (H2) gas to find where molecules may be actively forming in cooling clouds prior to star formation. We use HI 21cm-line self-absorption (HISA) to identify the coldest atomic gas that is most likely associated with forming molecules, and we use carbon monoxide (CO) 2.6mm line emission as a proxy for H2, which does not have suitable ground-state transitions for direct mapping. We have measured the fraction of HISA with CO emission and CO with HISA in a variety of different Galactic plane survey data sets. We see a stronger correspondence of HISA and CO in the inner parts of the Galaxy, and a weaker correspondence elsewhere. However, this trend may arise from chance alignments of unrelated clouds being more likely in the inner Galaxy rather than true physical correlation.