Position of the five observed fields overploted on the DSS-Red image. Yellow isocontours are the inner and the outer (dashed) PDRs boundaries as defined by Luisi et al. (2016) and delineated here from the HERSCHEL−70 μm image. The blue isocontours plot the column density (from 1.6 10²¹ to 1.2 10²⁴ cm⁻²) constructed from Hi-GAL survey maps (Marsh et al. 2017). In red are indicated the main infrared sources and the particular features named ‘Ark’ and ‘Rim’.