A MULTIWAVELENGTH STUDY OF 30-DORADUS - THE INTERSTELLAR-MEDIUM IN A LOW-METALLICITY GALAXY

We report maps of the 158 mu m [C II] fine-structure line, the 63 mu m and 146 mu m [OI] fine-structure lines, the 2.2 mu m H I Br gamma line, the 2.1 mu m H-2 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 Br gamma map with H alpha and H beta measurements, we find that visual to near-infrared extinction and reddening follow the standard dust extinction law and that the Br gamma extinction is small which allows for a reliable determination of the Lyman-continuum intensity. The Lyman continuum as derived from the Br gamma 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 H-2 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 H-2 line and the ionized gas traced by the Br gamma 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 H-2 molecular gas may be contained in the photodissociation region where CO has been destroyed.