A multiwavelength study of 30 Doradus: The interstellar medium in a low-metallicity galaxy

We report maps of the 158 μm [C II] fine-structure line, the 63 μm and 146 μm [O I] fine-structure lines, the 2.2 μm H I Brγ line, the 2.1 μm H₂ 1-0 S(1) ro-vibrational line, and the 2.6 mm CO (1-0) rotational line toward the 30 Doradus complex in the Large Magellanic Cloud. Comparing our Brγ map with Hα and Hβ measurements, we find that visual to near-infrared extinction and reddening follow the standard dust extinction law and that the Brγ extinction is small, which allows for a reliable determination of the Lyman-continuum intensity. The Lyman continuum as derived from the Brγ emission and the far-UV derived from the far-infrared continuum match the average spectrum of the exciting stars in the 30 Doradus cluster. The observed H₂ line intensity may be produced in dense clumps exposed to the stellar radiation fields. The maps of all tracers emphasize a shell-like structure of the 30 Doradus region, which is seen approximately edge-on. The warm molecular gas traced by the H₂ line and the ionized gas traced by the Brγ line are intermixed, while the cold molecular gas as traced by CO (1-0) and the photodissociated gas as traced by [C II] are coextensive over tens of parsecs. This distribution can be explained only by a highly fragmented structure of the interstellar medium that allows UV radiation to penetrate deep into the molecular cloud. Clumpiness is also the key to understanding the extremely high [C II]/CO line intensity ratio. Depending on cloud geometry and physical conditions, the relative beam-filling factors of the partly atomic, partly molecular photodissociated gas as seen in the FIR tracers, and of the purely molecular gas traced by CO, can differ substantially in a clumpy, low-metallicity environment. This effect also leads to a greatly increased H₂/CO conversion factor because a major part of the H₂ molecular gas may be contained in the photodissociation region where CO has been destroyed.