A milestone toward understanding PDR properties in the extreme environment of LMC-30 Doradus

Abstract

Context. More complete knowledge of galaxy evolution requires understanding the process of star formation and the interaction between the interstellar radiation field and interstellar medium (ISM) in galactic environments traversing a wide range of physical parameter space. We focus on the impact of massive star formation on the surrounding low metallicity ISM in 30 Doradus in the Large Magellanic Cloud (LMC). A low metal abundance, which can characterizes some galaxies of the early Universe, results in less ultraviolet (UV) shielding for the formation of the molecular gas necessary for star formation to proceed. The half-solar metallicity gas in this region is strongly irradiated by the super star cluster R136, making it an ideal laboratory to study the structure of the ISM in an extreme environment. Aims. Our goal is to construct a comprehensive, self-consistent picture of the density, radiation field, and ISM structure in the most active star-forming region in the LMC, 30 Doradus. Our spatially resolved study investigates the gas heating and cooling mechanisms, particularly in the photodissociation regions (PDR) where the chemistry and thermal balance are regulated by far-UV photons (6 eV < hν < 13.6 eV). Methods. We present Herschel observations of far-infrared (FIR) fine-structure lines obtained with PACS and SPIRE/FTS. We combined atomic fine-structure lines from Herschel and Spitzer observations with ground-based CO data to provide diagnostics on the properties and structure of the gas by modeling it with the Meudon PDR code. For each tracer we estimate the possible contamination from the ionized gas to isolate the PDR component. We derive the spatial distribution of the radiation field, the pressure, the size, and the filling factor of the photodissociated gas and molecular clouds. Results. We find a range of pressure of ∼105-1.7 × 106 cm-3 K and a range of incident radiation field GUV∼102-2.5 × 104 through PDR modeling. Assuming a plane-parallel geometry and a uniform medium, we find a total extinction AVmax of 1-3 mag, which corresponds to a PDR cloud size of 0.2 to 3pc with small CO depth scale of 0.06 to 0.5 pc. At least 90% of the [C ii] originates in PDRs in this region, while a significant fraction of the LFIR (up to 70% in some places) can be associated with an ionized gas component. The high [O iii]/[C ii] ratio (2 to 60) throughout the observed map, correlated with the filling factor, reveals the porosity of the ISM in this region, which is traversed by hard UV photons surrounding small PDR clumps. We also determine the three-dimensional structure of the gas, showing that the clouds are distributed 20 to 80 pc away from the main ionizing cluster, R136.