Scheme of our two-sector model. In a galaxy, starbust regions (blue stars) emit radiation that passes an equivalent HII region (blue zone) that is partially covered by clouds (red regions). They are modeled as a photodissociation region and a molecular region. We do not consider nebular emission from old stars (red stars). One part of the emission line flux directly moves from the HII region to the observer and is attenuated by galactic dust. A second part is attenuated within the PDR, and a third part is created inside the PDR. This is not the actual geometry used in Cloudy simulations, which are performed for a slab of gas that is illuminated by a starburst-region radiation field, as described in Sect. 2.2.2.