THE H i PROBABILITY DISTRIBUTION FUNCTION AND THE ATOMIC-TO-MOLECULAR TRANSITION IN MOLECULAR CLOUDS

Abstract

ABSTRACT We characterize the column-density probability distribution functions (PDFs) of the atomic hydrogen gas, H i, associated with seven Galactic molecular clouds (MCs). We use 21 cm observations from the Leiden/Argentine/Bonn Galactic H i Survey to derive column-density maps and PDFs. We find that the peaks of the H i PDFs occur at column densities in the range ∼1–2 × 1021 (equivalently, ∼0.5–1 mag). The PDFs are uniformly narrow, with a mean dispersion of (∼0.1 mag). We also investigate the H i-to-H2 transition toward the cloud complexes and estimate H i surface densities ranging from 7 to 16 at the transition. We propose that the H i PDF is a fitting tool for identifying the H i-to-H2 transition column in Galactic MCs.