2014ApJ...795..174K

The ¹²CO J = 4 ⟶ 3 to J = 13 ⟶ 12 lines of the interstellar medium from nearby galaxies, newly observable with the Herschel SPIRE Fourier transform spectrometer, offer an opportunity to study warmer, more luminous molecular gas than that traced by ¹²CO J = 1 ⟶ 0. Here we present a survey of 17 nearby infrared-luminous galaxy systems (21 pointings). In addition to photometric modeling of dust, we modeled full ¹²CO spectral line energy distributions from J = 1 ⟶ 0 to J = 13 ⟶ 12 with two components of warm and cool CO gas, and included LTE analysis of [C I], [C II], [N II], and H₂lines. CO is emitted from a low-pressure/high-mass component traced by the low-J lines and a high-pressure/low-mass component that dominates the luminosity. We found that, on average, the ratios of the warm/cool pressure, mass, and ¹²CO luminosity are 60±30, 0.11±0.02, and 15.6±2.7. The gas-to-dust-mass ratios are <120 throughout the sample. The ¹²CO luminosity is dominated by the high-J lines and is 4x10^–4 L_FIR on average. We discuss systematic effects of single-component and multi-component CO modeling (e.g., single-component J ≤ 3 models overestimate gas pressure by ∼0.5 dex), as well as compare to Galactic star-forming regions. With this comparison, we show the molecular interstellar medium of starburst galaxies is not simply an ensemble of Galactic-type giant molecular clouds. The warm gas emission is likely dominated by regions resembling the warm extended cloud of Sgr B2.