Results of the PDR modeling of the Planck’s dusty GEMS using line luminosity ratios of CO, [C I], and [C II] (red points, see further details in the text), with error bars showing the 1σ uncertainties on the best-fitting values. The average PDR densities, n_PDR, correspond to number densities of hydrogen nuclei and the FUV radiation fields, G₀, are given in Habing units (1.6 × 10⁻³ erg cm⁻² s⁻¹). The sources without [C I](1–0) detection are plotted in light red. We compare with other SMGs in the literature (blue points, Cox et al. 2011; Danielson et al. 2011; Valtchanov et al. 2011; Alaghband-Zadeh et al. 2013; Huynh et al. 2014; Rawle et al. 2014; Bothwell et al. 2017), with the regimes of local normal star-forming galaxies (green points, Malhotra et al. 2001), and local ULIRGs (orange region, Davies et al. 2003), and with the average position of nearby galaxy populations and Galactic regions inferred in Stacey et al. (1991) using [C II]/CO(1–0) line ratios (hatched regions). The physical conditions in the GEMS closely resemble those of local ULIRGs and starbursts, with very similar densities in the range n_PDR = 10⁴–10⁵ cm⁻³. The PDR models also suggest that some Planck’s dusty GEMS are illuminated by lower FUV radiation fields, more typical of local spiral galaxies and molecular clouds in the Milky Way.