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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. Monthly Notices of the Royal Astronomical Society, 499 (1). pp. 837-850. ISSN 0035-8711. doi:10.1093/mnras/staa2839.

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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. At the resolution of these simulations, 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:Article
Related URLs:
URLURL TypeDescription Paper
Keating, Laura C.0000-0001-5211-1958
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
Alternate Title:Reproducing the CO-to-H2 conversion factor in cosmological simulations of Milky-Way-mass galaxies
Additional Information:© 2020 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model ( Accepted 2020 September 3. Received 2020 August 19; in original form 2020 January 22. Published: 19 September 2020. We thank Desika Narayanan and Xiangcheng Ma for their comments on this draft, and Yuxuan Yuan for his assistance in post processing the particle split snapshot. LCK acknowledges the support of a CITA fellowship and a Beatrice and Vincent Tremaine Fellowship. AJR was supported by a COFUND/Durham Junior Research Fellowship under EU grant 609412, and by the Science and Technology Facilities Council [ST/P000541/1]. 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, and 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 (Loken et al. 2010; Ponce et al. 2019). Data Availability: The data underlying this paper will be shared on reasonable request to the corresponding author.
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
European Research Council (ERC)609412
Science and Technology Facilities Council (STFC)ST/P000541/1
Subject Keywords:methods: numerical – ISM: molecules – galaxies: evolution – galaxies: ISM
Issue or Number:1
Record Number:CaltechAUTHORS:20200310-084830640
Persistent URL:
Official Citation:Laura C Keating, Alexander J Richings, Norman Murray, Claude-André Faucher-Giguère, Philip F Hopkins, Andrew Wetzel, Dušan Kereš, Samantha Benincasa, Robert Feldmann, Sarah Loebman, Matthew E Orr, Reproducing the CO-to-H2 conversion factor in cosmological simulations of Milky-Way-mass galaxies, Monthly Notices of the Royal Astronomical Society, Volume 499, Issue 1, November 2020, Pages 837–850,
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:16 Nov 2021 18:06

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