# Characterizing the transition from diffuse atomic to dense molecular clouds in the Magellanic clouds with [CII], [CI], and CO

## TL;DR

This study uses Herschel/HIFI observations of [CII], [CI], and CO lines to analyze the transition from atomic to molecular gas in the low-metallicity environments of the Magellanic Clouds, revealing the prevalence of CO-dark H2 and variations in X_CO.

## Contribution

It provides detailed measurements of gas-phase carbon species and molecular fractions, offering new insights into the atomic-to-molecular transition in low-metallicity galaxies.

## Key findings

- Most molecular gas is CO-dark H2 (77-89%) in the Magellanic Clouds.
- The mean X_CO conversion factors are higher than in the Milky Way but closer after correction.
- [CII] accounts for about 1% of the total far-infrared emission in the observed lines of sight.

## Abstract

We present and analyze deep Herschel/HIFI observations of the [CII] 158um, [CI] 609um, and [CI] 370um lines towards 54 lines-of-sight (LOS) in the Large and Small Magellanic clouds. These observations are used to determine the physical conditions of the line--emitting gas, which we use to study the transition from atomic to molecular gas and from C^+ to C^0 to CO in their low metallicity environments. We trace gas with molecular fractions in the range 0.1<f(H2)<1, between those in the diffuse H2 gas detected by UV absorption (f(H2)<0.2) and well shielded regions in which hydrogen is essentially completely molecular. The C^0 and CO column densities are only measurable in regions with molecular fractions f(H2)>0.45 in both the LMC and SMC. Ionized carbon is the dominant gas-phase form of this element that is associated with molecular gas, with C^0 and CO representing a small fraction, implying that most (89% in the LMC and 77% in the SMC) of the molecular gas in our sample is CO-dark H2. The mean X_CO conversion factors in our LMC and SMC sample are larger than the value typically found in the Milky Way. When applying a correction based on the filling factor of the CO emission, we find that the values of X_CO in the LMC and SMC are closer to that in the Milky Way. The observed [CII] intensity in our sample represents about 1% of the total far-infrared intensity from the LOSs observed in both Magellanic Clouds.