摘要：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.
