Spectral energy distribution of NGC 4418 and Arp 220 from mid-IR to millimeter wavelengths. Both the Herschel/PACS and the Spitzer/IRS spectra are shown. Data points at (sub)millimeter wavelengths for NGC 4418 are from (Yang & Phillips 2007, 350 μm), (Roche & Chandler 1993, 450, 800, and 1100 μm), (Dunne et al. 2000; Lisenfeld et al. 2000, 850 μm), and (Albrecht et al. 2007, 1300 μm). For Arp 220, they are taken from (Eales et al. 1989, 450 μm); (Rigopoulou et al. 1996, 350, 800, and 1100 μm); and (Sakamoto et al. 1999, 2008, 1300 μm). Models discussed in Sect. 3 are included. For NGC 4418, the yellow, blue, light-blue, and solid-green curves show the models for the hot, core, warm, and extended components, respectively, with parameters given in Table 1. An additional cold component, shown with the dashed-green curve, is included to better fit the SED at long wavelengths. For Arp 220, the yellow, blue, light-blue, and solid-green curves show the models for the hot, western nucleus, eastern nucleus, and extended components, respectively (Table 1).