CO-Dark Molecular Gas and the Origin of [CII] Emission in Metal-Poor Galaxies: the Case Study of Velocity-Resolved [CII] in LMC-N11
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Vianney Leboutellier
University of Paris-Saclay
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The ambiguous origin of the [CII] 158μm line in the interstellar medium complicates its use for diagnostics on the star-formation rate and physical conditions in photodissociation regions. I will present a study of the giant HII region N11 in the Large Magellanic Cloud with SOFIA/GREAT using an innovative spectral decomposition method that allows statistical trends to be derived for the molecular gas content using CO, [CII], and HI profiles. The [CII] line is resolved in velocity and compared to HI and CO, using a Bayesian approach to decompose the line profiles. We find that the profile of [CII] most closely resembles that of CO, but the integrated [CII] line width lies between that of CO and that of HI. Using various methods, we find that [CII] mostly originates from the neutral gas. We show that [CII] mostly traces the CO-dark H2 gas but there is evidence of a weak contribution from neutral atomic gas preferentially in the faintest components (as opposed to components with low [CII]/CO or low CO column density). Most of the molecular gas is CO-dark. The CO-dark H2 gas, whose density is typically a few 100s cm-3 and thermal pressure in the range 10^3.5-5 K cm-3, is not always in pressure equilibrium with the neutral atomic gas. The fraction of CO-dark H2 gas decreases with increasing CO column density, with a slope that seems to depend on the impinging radiation field from nearby massive stars. Finally we extend previous measurements of the photoelectric effect heating efficiency, which we find is constant across regions probed with Herschel, with [CII] and [OI] being the main coolants in faint and diffuse, and bright and compact regions, respectively, and with polycyclic aromatic hydrocarbon emission tracing the CO-dark H2 gas heating where [CII] and [OI] emit.

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