Mon. Not. R. Astron. Soc., 471, 2917-2931 (2017/November-1)
Warm and cold molecular gas conditions modelled in 87 galaxies observed by the Herschel SPIRE Fourier transform spectrometer.
KAMENETZKY J., RANGWALA N. and GLENN J.
Abstract (from CDS):
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 ∼105.0±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 LFIR. 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 αCO 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 αCO, though they do not have a discernible trend with LFIR; 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 (τdep) 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.
© 2017 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society
ISM: molecules - galaxies: ISM - submillimetre: galaxies - submillimetre: galaxies
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