Physical conditions in the gas phases of the giant H II region LMC-N 11

II. Origin of [C II] and fraction of CO-dark gas^★

¹ AIM, CEA, CNRS, Université Paris-Saclay, Université Paris Diderot, Sorbonne Paris Cité, 91191 Gif-sur-Yvette, France
e-mail: vianney.lebouteiller@cea.fr
² Department of Physics and Astronomy, University of North Carolina, 290 Phillips Hall CB 3255, Chapel Hill, NC 27599, USA
³ Institut für theoretische Astrophysik, Zentrum für Astronomie der Universität Heidelberg, Albert-Ueberle Str. 2, 69120 Heidelberg, Germany
⁴ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstraße 12-14, 69120 Heidelberg, Germany
⁵ Korea Astronomy and Space Science Institute, 776 Daedeokdae-ro, 34055 Daejeon, Republic of Korea
⁶ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁷ Laboratoire d’Astrophysique de Bordeaux, University of Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France
⁸ LERMA, Observatoire de Paris, PSL Research Univiversity, CNRS, Sorbonne Université, 75014 Paris, France
⁹ Department of Astronomy, University of Maryland, College Park, ND 20742-2421, USA
¹⁰ Department of Astronomy, University of Virginia, Charlottesville, VA 22904, USA
¹¹ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
¹² IRAP, Université de Toulouse, CNRS, UPS, CNES, 31400 Toulouse, France
¹³ University of Cincinnati, Clermont College, 4200 Clermont College Drive, Batavia, OH 45103, USA

Received: 12 July 2019
Accepted: 17 October 2019

Abstract

Context. The ambiguous origin of the [C II] 158μm line in the interstellar medium complicates its use for diagnostics concerning the star-formation rate and physical conditions in photodissociation regions.

Aims. We investigate the origin of [C II] in order to measure the total molecular gas content, the fraction of CO-dark H₂ gas, and how these parameters are impacted by environmental effects such as stellar feedback.

Methods. We observed the giant H II region N 11 in the Large Magellanic Cloud with SOFIA/GREAT. The [C II] line is resolved in velocity and compared to H I and CO, using a Bayesian approach to decompose the line profiles. A simple model accounting for collisions in the neutral atomic and molecular gas was used in order to derive the H₂ column density traced by C⁺.

Results. The profile of [C II] most closely resembles that of CO, but the integrated [C II] line width lies between that of CO and that of H I. Using various methods, we find that [C II] mostly originates from the neutral gas. We show that [C II] 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 [C II]/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 [C II] and [O I] 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 [C II] and [O I] emit.

Conclusions. We present an innovative spectral decomposition method that allows statistical trends to be derived for the molecular gas content using CO, [C II], and H I profiles. Our study highlights the importance of velocity-resolved photodissociation region (PDR) diagnostics and higher spatial resolution for H I observations as future steps.