Schematic representation of the modeled sources, showing the approximate spatial scales of the different far-IR components (see Fig. 1 and Tables 1 and 2). The C_hot components in both galaxies (that account for the mid-IR spectra) are most probably associated with the nuclear regions and are not included here. The C_halo component in Arp 220 has no associated continuum, and is responsible for the absorption of the nuclear continuum (C_east and C_west) in the ground-state lines of H₂O, OH, and O i. Although, due to its apparent low excitation, C_halo is represented here as the most extended component, its size is not constrained. The plot is an oversimplification of the actual models, where the different components are modeled separately to account approximately for non-spherical symmetry.