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Published October 2017 | Published + Supplemental Material + Submitted
Journal Article Open

Deriving a multivariate α_(CO) conversion function using the [CII]/CO(1-0) ratio and its application to molecular gas scaling relations


We present Herschel PACS observations of the [C II] 158 μm emission line in a sample of 24 intermediate mass (9 < log M_*/M_⊙ < 10) and low metallicity (0.4 < Z/Z_⊙ < 1.0) galaxies from the xCOLD GASS survey. In combination with IRAM CO (1−0) measurements, we establish scaling relations between integrated and molecular region L_([CII])/L_(CO (1–0)) ratios as a function of integrated galaxy properties. A Bayesian analysis reveals that only two parameters, metallicity and offset from the main sequence, Δ(MS), are needed to quantify variations in the luminosity ratio; metallicity describes the total dust content available to shield CO from UV radiation, while Δ(MS) describes the strength of this radiation field. We connect the L_([CII])/L_(CO (1–0)) ratio to the CO-to-H_2 conversion factor and find a multivariate conversion function, which can be used up to z ∼ 2.5. This function depends primarily on metallicity, with a second-order dependence on Δ(MS). We apply this to the full xCOLD GASS and PHIBSS1 surveys and investigate molecular gas scaling relations. We find a flattening of the relation between gas mass fraction and stellar mass at log M_* < 10.0. While the molecular gas depletion time varies with sSFR, it is mostly independent of mass, indicating that the low L_(CO)/SFR ratios long observed in low-mass galaxies are entirely due to photodissociation of CO and not to an enhanced star formation efficiency.

Additional Information

© 2017 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. Received: 21 August 2016. Revision Received: 17 June 2017. Accepted: 20 June 2017. Published: 22 June 2017. GA would like to thank the UK Science and Technologies Facilities Council (STFC) for their support via a postgraduate studentship. AS acknowledges the support of the Royal Society through the award of a University Research Fellowship. BC is the recipient of an Australian Research Council Future Fellowship (FT120100660). BC and LC acknowledge support from the Australian Research Councils Discovery Projects funding scheme (DP150101734). This work is based on observations carried out with the IRAM 30m telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain). We thank the staff of the telescope for their help in conducting the COLD GASS observations. PACS, aboard Herschel, has been developed by MPE (Germany); UVIE (Austria); KU Leuven, CSL, IMEC (Belgium); CEA, LAM (France); MPIA (Germany); INAF- IFSI/OAA/OAP/OAT, LENS, SISSA (Italy) and IAC (Spain). This development has been supported by BMVIT (Austria), ESA-PRODEX (Belgium), CEA/CNES (France), DLR (Germany), ASI/INAF (Italy) and CICYT/MCYT (Spain).

Attached Files

Submitted - 1702.03888.pdf

Published - Accurso_2017p4750.pdf

Supplemental Material - stx1556_Supp.zip


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August 19, 2023
August 19, 2023