Submillimeter line emission from LMC 30 Doradus: The impact of a starburst on a low-metallicity environment

¹ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109-8099, USA
e-mail: Jorge.Pineda@jpl.nasa.gov
² ALMA-J Project Office, National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, 181-8588 Tokyo, Japan
³ KOSMA, I. Physikalisches Institut, Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany
⁴ Instituto Radioastronomía Milimétrica, Av. Divina Pastora 7, Nucleo Central, 18012 Granada, Spain
⁵ Argelander-Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany
⁶ Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
⁷ Department of Physics, Faculty of Science, Hokkaido University, N10W8, Kita-ku, 060-0810 Sapporo, Japan
⁸ Institute of Astronomy, ETH Zürich, 8093 Zürich, Switzerland
⁹ School of Physics, UNSW, Sydney, NSW 2052, Australia
¹⁰ Department of Astrophysics, Nagoya University, Chikusa-ku, 464-8602 Nagoya, Japan
¹¹ Seoul National University, Seoul 151-742, Korea
¹² Department of Astrophysics, Osaka Prefecture University, 1-1 Gakuen-cho, 599-8531 Osaka, Japan

Received: 22 October 2011
Accepted: 15 June 2012

Abstract

Context. The 30 Dor region in the Large Magellanic Cloud (LMC) is the most vigorous star-forming region in the Local Group. Star formation in this region is taking place in low-metallicity molecular gas that is exposed to an extreme far-ultraviolet (FUV) radiation field powered by the massive compact star cluster R136. 30 Dor is therefore ideally suited to study the conditions in which stars formed at earlier cosmological times.

Aims. Observations of (sub)mm and far-infrared (FIR) spectral lines of the main carbon-carrying species, CO, [C i] and [C ii], which originate in the surface layers of molecular clouds illuminated by the FUV radiation of young stars, can be used to constrain the physical and chemical state of the star-forming ISM.

Methods. We used the NANTEN2 telescope to obtain high-angular resolution observations of the ¹²CO J = 4 → 3, J = 7 → 6, and ¹³CO J = 4 → 3 rotational lines and [C i] ³P₁- ³P₀ and ³P₂- ³P₁ fine-structure submillimeter transitions in 30 Dor-10, the brightest CO and FIR-emitting cloud at the center of the 30 Dor region. We derived the physical and chemical properties of the low-metallicity molecular gas using an excitation/radiative transfer code and found a self-consistent solution of the chemistry and thermal balance of the gas in the framework of a clumpy cloud PDR model. We compared the derived properties with those in the N159W region, which is exposed to a more moderate far-ultraviolet radiation field compared with 30 Dor-10, but has similar metallicity. We also combined our CO detections with previously observed low-J CO transitions to derive the CO spectral-line energy distribution in 30 Dor-10 and N159W.

Results. The separate excitation analysis of the submm CO lines and the neutral carbon fine structure lines shows that the mid-J CO and [C i]-emitting gas in the 30 Dor-10 region has a temperature of about 160 K and a H₂ density of about 10⁴ cm^-3. We find that the molecular gas in 30 Dor-10 is warmer and has a lower beam filling factor compared to that of N159W, which might be a result of the effect of a strong FUV radiation field heating and disrupting the low-metallicity molecular gas. We use a clumpy PDR model (including the [C ii] line intensity reported in the literature) to constrain the FUV intensity to about χ₀ ≈ 3100 and an average total H density of the clump ensemble of about 10⁵ cm^-3 in 30 Dor-10.