The size-density relation of extragalactic H II regions

Aims: We investigate the size-density relation in extragalactic H ii regions, with the aim of understanding the role of dust and different physical conditions in the ionized medium.
Methods: First, we compiled several observational data sets for Galactic and extragalactic H ii regions and confirm that extragalactic H ii regions follow the same size (D)-density (n) relation as Galactic ones (n propto D^-1), rather than a relation with constant luminosity (n propto D^-1.5). Motivated by the inability of static models to explain this, we then modelled the evolution of the size-density relation of H ii regions by considering their star formation history, the effects of dust, and pressure-driven expansion. The results are compared with our sample data whose size and density span roughly six orders of magnitude.
Results: The extragalactic samples cannot be understood as an evolutionary sequence with a single initial condition. Thus, the size-density relation does not result from an evolutionary sequence of H ii regions but rather reflects a sequence with different initial gas densities (“density hierarchy”). We also find that the size of many H ii regions is limited by dust absorption of ionizing photons, rather than consumption by ionizing neutral hydrogen. Dust extinction of ionizing photons is particularly severe over the entire lifetime of compact H ii regions with typical gas densities of ⪆10³ cm^-3. Hence, as long as the number of ionizing photons is used to trace massive star formation, much star-formation activity could be missed. Such compact dense environments, the ones most profoundly obscured by dust, have properties similar to “maximum-intensity starbursts”. This implies that submillimeter and infrared wavelengths may be necessary to accurately assess star formation in these extreme conditions both locally and at high redshift.