A SURVEY OF THE MOLECULAR ISM PROPERTIES OF NEARBY GALAXIES USING THE HERSCHEL FTS

The (CO)-C-12 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 (CO)-C-12 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 (CO)-C-12 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 H-2 lines. 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 (CO)-C-12 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 (CO)-C-12 luminosity is dominated by the high-J lines and is 4 x 10(-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 similar to 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.