Warm and cold molecular gas conditions modelled in 87 galaxies observed by the Herschel SPIRE Fourier transform spectrometer

Abstract

We have conducted two-component, non-local thermodynamic equilibrium modelling of the CO lines from J = 1-0 through J = 13-12 in 87 galaxies observed by the Herschel SPIRE Fourier Transform Spectrometer (FTS). We find the average pressure of the cold molecular gas, traced especially by CO J = 1-0, is ∼10^5.0±0.5 K cm^-3. The mid- to high-J lines of CO trace higher pressure gas at 10^{6.5 ± 0.6} K cm^-3; this pressure is slightly correlated with L_FIR. Two components are often necessary to accurately fit the Spectral Line Energy Distributions; a one-component fit often underestimates the flux of carbon monoxide (CO) J = 1-0 and the mass. If low-J lines are not included, mass is underestimated by an order of magnitude. Even when modelling the low-J lines alone or using an α_CO conversion factor, the mass should be considered to be uncertain to a factor of at least 0.4 dex, and the vast majority of the CO luminosity will be missed (median, 65 per cent). We find a very large spread in our derived values of α_CO, though they do not have a discernible trend with L_FIR; the best fit is a constant 0.7 M_⊙ (K km s^{- 1} pc²)^-1, with a standard deviation of 0.36 dex, and a range of 0.3-1.6 M_⊙ (K km s^{- 1} pc²)^-1. We find average molecular gas depletion times (τ_dep) of 10⁸ yr that decrease with increasing star formation rate. Finally, we note that the J = 11-10/J = 1-0 line flux ratio is diagnostic of the warm component pressure, and discuss the implications of this comprehensive study of SPIRE FTS extragalactic spectra for future study post-Herschel.