Radiative and mechanical feedback into the molecular gas in the Large Magellanic Cloud

I. N159W^⋆,⋆⋆

¹ Laboratoire AIM, CEA/IRFU/Service d’Astrophysique, Bât. 709, 91191 Gif-sur-Yvette, France
e-mail: min-young.lee@cea.fr
² LERMA, Observatoire de Paris, École Normale Supérieure, PSL Research University, CNRS, UMR 8112, 75014 Paris, France
³ Sorbonne Universités, UPMC Univ. Paris 6, UMR 8112, LERMA, 75005 Paris, France
⁴ LERMA, Observatoire de Paris, PSL Research University, CNRS, UMR 8112, 92190 Meudon, France
⁵ International Research Fellow of the Japan Society for the Promotion of Science (JSPS), Department of Astronomy, University of Tokyo, Bunkyo-ku, 113-0033 Tokyo, Japan
⁶ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching-bei-München, Germany
⁷ Institut für Theoretische Astrophysik, Zenturm für Astronomie der Universität Heidelberg, Albert-Ueberle Str. 2, 69120 Heidelberg, Germany
⁸ ICMM, Consejo Superior de Investigaciones Cientificas, 28049 Madrid, Spain
⁹ Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, USA
¹⁰ Department of Physics, Nagoya University, Chikusa-ku, 464-8602 Nagoya, Japan
¹¹ CNRS, IRAP, 9 Av. colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
¹² Department of Astronomy, University of Virginia, PO Box 400325, Charlottesville, VA 22904-4325, USA
¹³ National Radio Astronomy Observatory, 520 Edgemont Rd., Charlottesville, VA 22903, USA
¹⁴ Sterrewacht Leiden, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
¹⁵ National Astronomical Observatory of Japan, Mitaka, 181-8588 Tokyo, Japan
¹⁶ Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218, USA
¹⁷ Department of Physics and Astronomy, The Johns Hopkins University, 366 Bloomberg Center, 3400 N Charles St., Baltimore, MD 21218, USA
¹⁸ Department of Physical Science, Graduate School of Science, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, 599-8531 Osaka, Japan
¹⁹ NASA Goddard Space Flight Center, 8800 Greenbelt Rd., Greenbelt, MD 20771, USA

Received: 9 January 2016
Accepted: 12 June 2016

Abstract

We present Herschel SPIRE Fourier Transform Spectrometer (FTS) observations of N159W, an active star-forming region in the Large Magellanic Cloud (LMC). In our observations, a number of far-infrared cooling lines, including carbon monoxide (CO) J = 4 → 3 to J = 12 → 11, [CI] 609 μm and 370 μm, and [NII] 205 μm, are clearly detected. With an aim of investigating the physical conditions and excitation processes of molecular gas, we first construct CO spectral line energy distributions (SLEDs) on ~10 pc scales by combining the FTS CO transitions with ground-based low-J CO data and analyze the observed CO SLEDs using non-LTE (local thermodynamic equilibrium) radiative transfer models. We find that the CO-traced molecular gas in N159W is warm (kinetic temperature of 153–754 K) and moderately dense (H₂ number density of (1.1−4.5) × 10³ cm^-3). To assess the impact of the energetic processes in the interstellar medium on the physical conditions of the CO-emitting gas, we then compare the observed CO line intensities with the models of photodissociation regions (PDRs) and shocks. We first constrain the properties of PDRs by modeling Herschel observations of [OI] 145 μm, [CII] 158 μm, and [CI] 370 μm fine-structure lines and find that the constrained PDR components emit very weak CO emission. X-rays and cosmic-rays are also found to provide a negligible contribution to theCO emission, essentially ruling out ionizing sources (ultraviolet photons, X-rays, and cosmic-rays) as the dominant heating source for CO in N159W. On the other hand, mechanical heating by low-velocity C-type shocks with ~10 km s^-1 appears sufficient enough to reproduce the observed warm CO.