Left: comparison of ¹²CO SLEDs (normalized to the ¹²CO(1–0) flux of each specific source) obtained from different sources. Starburst, AGN, and AGN–SB sources have been considered. A ¹²CO SLED of a SMG at high-z analyzed by Cañameras et al. (2018) is also shown. Different colors identify the galaxies while different symbols indicate the types of objects. In each group, the following galaxies have been selected: M82 and NGC 253 as SB, NGC 6240 and NGC 4945 as AGN–SB composite galaxies, and NGC 1068 and Mrk 231 as AGN-dominated galaxies. The galaxy G244.8+54.9 has been chosen as representative of SMG at high-z, for which the authors derived log n_gas ∼ 5.1 cm⁻³ and log G₀ ∼ 3.7 (Habing units). Right: results of the ¹²CO SLED of NGC 4945 from J = 1–0 through 20–19 applying the Kazandjian et al. (2015) mechanical heating models. The black points are the observed ¹²CO fluxes obtained from ground-based observations (Hitschfeld et al. 2008) while in green are the data from this work. The fit was constrained to three PDR models, all of them with some contribution of mechanical heating (mPDR), displayed in green, magenta, and blue. Dashed gray lines show the other best-fit results for the three PDRs. The dashed blue line is also shown to represent a model with the same n_gas and G₀ parameters as those characterizing the PDRIII model but with no mechanical heating contribution (α = 0). The red line represents the best fit with the minimum (∼0.8) while the light red area shows the combination of other best fits with slightly higher χ² (<1.4). The H₂ density (i.e., n_gas in log cm⁻³), G₀ (log Habing flux), and percentage of mechanical heating α values are shown in the legend for the three best-fit components.