2012A&A...548A..91L

The Magellanic Clouds provide a nearby laboratory for metal-poor dwarf galaxies. The low dust abundance enhances the penetration of UV photons into the interstellar medium (ISM), resulting in a relatively larger filling factor of the ionized gas. Furthermore, there is very likely a molecular gas reservoir probed by the [CII] 157µm line not traced by CO(1-0), the so-called ``dark'' gas. The Herschel Space Telescope allows us to observe far-infrared (FIR) cooling lines and to examine the physical conditions in the gas phases of a low-metallicity environment to unprecedented, small spatial scales. Our objective is to interpret the origin of the diffuse emission of FIR cooling lines in the H region N11B in the Large Magellanic Cloud. We first investigate the filling factor of the ionized gas. We then constrain the origin of the [CII] line by comparing to tracers of the low-excitation ionized gas and of photodissociation regions (PDRs). We present Herschel/PACS maps of N11B in several tracers, [CII] 157µm, [OI] 63µm and 145µm, [NII] 122µm, [NIII] 57µm, and [OIII] 88µm. Optical images in Hα and [OIII] 5007Å were used as complementary data to investigate the effect of dust extinction. Observations were interpreted with photoionization models to infer the gas conditions and estimate the ionized gas contribution to the [CII] emission. PDRs were probed through polycyclic aromatic hydrocarbons (PAHs) observed with the Spitzer Space Telescope. [OIII] 88µm is dominated by extended emission from the high-excitation diffuse ionized gas. This is the brightest FIR line throughout N11B, ∼4 times brighter than [CII]. We find that about half of the emission from the ionized gas is extinguished by dust. We modeled [OIII] around each O-type star and find that the density of the ISM is ≲16cm^–3 on large scales. The extent of the [OIII] emission suggests that the medium is rather fragmented, allowing far-UV photons to permeates the ISM to scales of >30pc. Furthermore, by comparing [CII] with [NII] 122µm, we find that 95% of [CII] arises in PDRs, except toward the stellar cluster for which as much as 15% could arise in the ionized gas. We find a remarkable correlation between [CII]+[OI] and PAH emission, with [CII] dominating the cooling in diffuse PDRs and [OI] dominating in the densest PDRs. The combination of [CII] and [OI] provides a proxy for the total gas cooling. Our results suggest that PAH emission describes better the gas heating in PDRs as compared to the total infrared emission.