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Reproducing the CO-to-H₂ conversion factor in cosmological simulations of Milky Way-mass galaxies

Keating, Laura C. and Richings, Alexander J. and Murray, Norman and Faucher-Giguère, Claude-André and Hopkins, Philip F. and Wetzel, Andrew and Kereš, Dušan and Benincasa, Samantha M. and Feldmann, Robert and Loebman, Sarah and Orr, Matthew E. (2020) Reproducing the CO-to-H₂ conversion factor in cosmological simulations of Milky Way-mass galaxies. . (Unpublished)

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We present models of CO(1-0) emission from Milky Way-mass galaxies at redshift zero in the FIRE-2 cosmological zoom-in simulations. We calculate the molecular abundances by post-processing the simulations with an equilibrium chemistry solver while accounting for the effects of local sources, and determine the emergent CO(1-0) emission using a line radiative transfer code. We find that the results depend strongly on the shielding length assumed, which in our models sets the attenuation of the incident UV radiation field. Commonly used choices for the shielding length, such as the Jeans length, result in CO abundances that are too high at a given H₂ abundance. We find that a model with a distribution of shielding lengths, which has a median shielding length of ∼3 pc in cold gas (T<300 K) for both CO and H₂, is able to reproduce both the observed CO(1-0) luminosity and inferred CO-to-H₂ conversion factor at a given star formation rate compared with observations. We suggest that this short shielding length can be thought of as a subgrid model which controls the amount of radiation that penetrates giant molecular clouds.

Item Type:Report or Paper (Discussion Paper)
Related URLs:
URLURL TypeDescription Paper
Faucher-Giguère, Claude-André0000-0002-4900-6628
Hopkins, Philip F.0000-0003-3729-1684
Wetzel, Andrew0000-0003-0603-8942
Kereš, Dušan0000-0002-1666-7067
Benincasa, Samantha M.0000-0003-4826-9079
Feldmann, Robert0000-0002-1109-1919
Loebman, Sarah0000-0003-3217-5967
Orr, Matthew E.0000-0003-1053-3081
Additional Information:© 2019 The Authors. We thank Desika Narayanan and Xiangcheng Ma for their comments on this draft. LCK acknowledges the support of a CITA fellowship. We acknowledge the support of the Natural Sciences and Engineering Research Council of Canada (NSERC). This research was undertaken, in part, because of funding from the Canada Research Chairs program. CAFG was supported by NSF through grants AST-1517491, AST-1715216, and CAREER award AST-1652522; by NASA through grant 17-ATP17-0067; and by a Cottrell Scholar Award from the Research Corporation for Science Advancement. Support for PFH was provided by an Alfred P. Sloan Research Fellowship, NSF Collaborative Research Grant 1715847 and CAREER grant 1455342, and NASA grants NNX15AT06G, JPL 1589742, 17-ATP17-0214. AW received support from NASA, through ATP grant 80NSSC18K1097 and HST grants GO-14734 and AR-15057 from STScI, the Heising-Simons Foundation, and a Hellman Fellowship. DK was supported by NSF grant AST-1715101 and the Cottrell Scholar Award from the Research Corporation for Science Advancement. RF acknowledges financial support from the Swiss National Science Foundation (grant no 157591). Support for SRL was provided by NASA through Hubble Fellowship grant HST-JF2-51395.001-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555. Computations were performed on the Niagara supercomputer at the SciNet HPC Consortium. SciNet is funded by: the Canada Foundation for Innovation; the Government of Ontario; Ontario Research Fund - Research Excellence; and the University of Toronto (Ponce et al. 2019; Loken et al. 2010).
Group:Astronomy Department, TAPIR
Funding AgencyGrant Number
Canadian Institute for Theoretical AstrophysicsUNSPECIFIED
Natural Sciences and Engineering Research Council of Canada (NSERC)UNSPECIFIED
Canada Research Chairs ProgramUNSPECIFIED
Cottrell Scholar of Research CorporationUNSPECIFIED
Alfred P. Sloan FoundationUNSPECIFIED
NASA Hubble FellowshipGO-14734
NASA Hubble FellowshipAR-15057
Space Telescope Science InstituteUNSPECIFIED
Heising-Simons FoundationUNSPECIFIED
Hellman FellowshipUNSPECIFIED
Swiss National Science Foundation (SNSF)157591
NASA Hubble FellowshipHST-JF2-51395.001-A
Canada Foundation for InnovationUNSPECIFIED
Government of OntarioUNSPECIFIED
Ontario Research Fund-Research ExcellenceUNSPECIFIED
University of TorontoUNSPECIFIED
Subject Keywords:ISM: molecules - galaxies: evolution - galaxies: ISM - methods: numerical
Record Number:CaltechAUTHORS:20200310-084830640
Persistent URL:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:101813
Deposited By: Tony Diaz
Deposited On:10 Mar 2020 16:02
Last Modified:10 Mar 2020 16:02

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