The Lognormal Probability Distribution Function of the Perseus Molecular Cloud: A Comparison of HI and Dust

TL;DR

This study analyzes the probability distribution function of atomic gas in the Perseus molecular cloud, finding it is well described by a lognormal distribution and identifying the HI-H2 transition point, with implications for understanding star formation.

Contribution

It provides the first detailed comparison of the atomic gas PDF with dust and molecular gas PDFs in Perseus, highlighting the lognormal shape and the transition at the HI-H2 boundary.

Findings

Atomic gas PDF is lognormal in shape.

The HI-H2 transition occurs at the PDF peak.

Median sonic Mach number of CNM is 4.0.

Abstract

The shape of the probability distribution function (PDF) of molecular clouds is an important ingredient for modern theories of star formation and turbulence. Recently, several studies have pointed out observational difficulties with constraining the low column density (i.e. Av <1) PDF using dust tracers. In order to constrain the shape and properties of the low column density probability distribution function, we investigate the PDF of multiphase atomic gas in the Perseus molecular cloud using opacity-corrected GALFA-HI data and compare the PDF shape and properties to the total gas PDF and the N(H2) PDF. We find that the shape of the PDF in the atomic medium of Perseus is well described by a lognormal distribution, and not by a power-law or bimodal distribution. The peak of the atomic gas PDF in and around Perseus lies at the HI-H2 transition column density for this cloud, past which the N(H2) PDF takes on a powerlaw form. We find that the PDF of the atomic gas is narrow and at column densities larger than the HI-H2 transition the HI rapidly depletes, suggesting that the HI PDF may be used to find the HI-H2 transition column density. We also calculate the sonic Mach number of the atomic gas by using HI absorption line data, which yields a median value of Ms=4.0 for the CNM, while the HI emission PDF, which traces both the WNM and CNM, has a width more consistent with transonic turbulence.