2022A&A...666A.112L

Context. The [O III]λ88 µm line is observed in many galaxies including our neighboring Magellanic Clouds and is a well-known tracer of H II regions, while the 24 µm continuum emission has often been used to trace warm dust in the ionized phases of galaxies. The association of both the [O III]λ88 µm line and 24 µm in galaxies to star formation motivates this study to determine their observational relation.
Aims. This study explores the link between the [O III]λ88 µm and 24 µm continuum in star-forming regions in the Magellanic Clouds. We also explore the local conditions driving the relation between those tracers.
Methods. We compared observations with 1D Cloudy models consisting of an H II region plus a photodissociation region (PDR) component, varying the stellar age, the initial density (at the illuminated edge of the cloud), and the ionization parameter. We introduced a new parameter, c_PDR, to quantify the proportion of emission arising from PDRs and that with an origin in H II regions along each line of sight. We used the ratio ([C II]+[O I])/[O III] as a proxy for the ratio of PDR versus H II region emission, and compared it to the [O III]/24 µm ratio. The use of [O III]/24 µm and [O III]/70 µm together allowed us to constrain the models most efficiently.
Results. We find a correlation over at least 3 orders of magnitude in [O III]λ88 µm and 24 µm continuum. This correlation is seen for spatially resolved maps of the Magellanic Cloud regions as well as unresolved galaxy-wide low metallicity galaxies of the Dwarf Galaxy Survey. We also find that most of the regions have low proportions of PDRs along the lines of sight (<12%), while a limited area of some of the mapped regions can reach 30-50%. For most lines of sight within the star-forming regions we have studied in the Magellanic Clouds, H II regions are the dominant phase.
Conclusions. We propose the use of the correlation between the [O III]λ88 µm and 24 µm continuum as a new predictive tool to estimate, for example, the [O III]λ88 µm when the 24 µm continuum is available or inversely. This can be especially useful to prepare for Atacama Large Milimeter Array (ALMA) observations of [O III]λ88 µm in high-z galaxies. The simple and novel method we developed may also provides a way to disentangle different phases along the line of sight, when other 3D information is not available.