Predictions for CO emission and the CO-to-H2 conversion factor in galaxy simulations with non-equilibrium chemistry

Oliver A. Thompson; Alexander J. Richings; Brad K. Gibson; Claude André Faucher-Giguère; Robert Feldmann; Christopher C. Hayward

doi:10.1093/mnras/stae1486

Predictions for CO emission and the CO-to-H₂ conversion factor in galaxy simulations with non-equilibrium chemistry

Oliver A. Thompson^*, Alexander J. Richings, Brad K. Gibson, Claude André Faucher-Giguère, Robert Feldmann, Christopher C. Hayward

^*Corresponding author for this work

Physics and Astronomy

Research output: Contribution to journal › Article › peer-review

Abstract

Our ability to trace the star-forming molecular gas is important to our understanding of the Universe. We can trace this gas using CO emission, converting the observed CO intensity into the H₂ gas mass of the region using the CO-to-H₂ conversion factor (X_CO). In this paper, we use simulations to study the conversion factor and the molecular gas within galaxies. We analysed a suite of simulations of isolated disc galaxies, ranging from dwarfs to Milky Way-mass galaxies, that were run using the FIRE-2 subgrid models coupled to the CHIMES non-equilibrium chemistry solver. We use the non-equilibrium abundances from the simulations, and we also compare to results using abundances assuming equilibrium, which we calculate from the simulation in post-processing. Our non-equilibrium simulations are able to reproduce the relation between CO and H₂ column densities, and the relation between X_CO and metallicity, seen within observations of the Milky Way. We also compare to the xCOLD GASS survey, and find agreement with their data to our predicted CO luminosities at fixed star formation rate. We also find the multivariate function used by xCOLD GASS overpredicts the H₂ mass for our simulations, motivating us to suggest an alternative multivariate function of our fitting, though we caution that this fitting is uncertain due to the limited range of galaxy conditions covered by our simulations. We also find that the non-equilibrium chemistry has little effect on the conversion factor (<5 per cent) for our high-mass galaxies, though still affects the H₂ mass and L_CO by ≈25 per cent.

Original language	English (US)
Pages (from-to)	1948-1965
Number of pages	18
Journal	Monthly Notices of the Royal Astronomical Society
Volume	532
Issue number	2
DOIs	https://doi.org/10.1093/mnras/stae1486
State	Published - Aug 1 2024

Funding

We thank the reviewer for their report, and we thank Joop Schaye for useful comments and suggestions. CAFG was supported by NSF through grants AST-2108230, AST-2307327, and CAREER award AST-1652522; by NASA through grants 17-ATP17-0067 and 21-ATP21-0036; by STScI through grants HST-GO-16730.016-A and JWST-AR-03252.001-A; and by CXO through grant TM2-23005X. The simulations used in this work were run on the DiRAC@Durham facility managed by the Institute for Computational Cosmology on behalf of the STFC DiRAC HPC Facility (www.dirac.ac.uk). The equipment was funded by BEIS capital funding via STFC capital grants ST/K00042X/1, ST/P002293/1, ST/R002371/1, and ST/S002502/1, Durham University and STFC operations grant ST/R000832/1. DiRAC is part of the National eInfrastructure. The radiative transfer post-processing calculations and other analysis of the simulation outputs used Viper, the University of Hull High Performance Computing Facility.

Keywords

ISM: atoms
ISM: molecules
astrochemistry
galaxies: ISM
galaxies: evolution

ASJC Scopus subject areas

Astronomy and Astrophysics
Space and Planetary Science

Access to Document

10.1093/mnras/stae1486

Cite this

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title = "Predictions for CO emission and the CO-to-H2 conversion factor in galaxy simulations with non-equilibrium chemistry",

abstract = "Our ability to trace the star-forming molecular gas is important to our understanding of the Universe. We can trace this gas using CO emission, converting the observed CO intensity into the H2 gas mass of the region using the CO-to-H2 conversion factor (XCO). In this paper, we use simulations to study the conversion factor and the molecular gas within galaxies. We analysed a suite of simulations of isolated disc galaxies, ranging from dwarfs to Milky Way-mass galaxies, that were run using the FIRE-2 subgrid models coupled to the CHIMES non-equilibrium chemistry solver. We use the non-equilibrium abundances from the simulations, and we also compare to results using abundances assuming equilibrium, which we calculate from the simulation in post-processing. Our non-equilibrium simulations are able to reproduce the relation between CO and H2 column densities, and the relation between XCO and metallicity, seen within observations of the Milky Way. We also compare to the xCOLD GASS survey, and find agreement with their data to our predicted CO luminosities at fixed star formation rate. We also find the multivariate function used by xCOLD GASS overpredicts the H2 mass for our simulations, motivating us to suggest an alternative multivariate function of our fitting, though we caution that this fitting is uncertain due to the limited range of galaxy conditions covered by our simulations. We also find that the non-equilibrium chemistry has little effect on the conversion factor (<5 per cent) for our high-mass galaxies, though still affects the H2 mass and LCO by ≈25 per cent.",

keywords = "ISM: atoms, ISM: molecules, astrochemistry, galaxies: ISM, galaxies: evolution",

author = "Thompson, {Oliver A.} and Richings, {Alexander J.} and Gibson, {Brad K.} and Faucher-Gigu{\`e}re, {Claude Andr{\'e}} and Robert Feldmann and Hayward, {Christopher C.}",

note = "Publisher Copyright: {\textcopyright} The Author(s) 2024. Published by Oxford University Press on behalf of Royal Astronomical Society.",

year = "2024",

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T1 - Predictions for CO emission and the CO-to-H2 conversion factor in galaxy simulations with non-equilibrium chemistry

AU - Thompson, Oliver A.

AU - Richings, Alexander J.

AU - Gibson, Brad K.

AU - Faucher-Giguère, Claude André

AU - Feldmann, Robert

AU - Hayward, Christopher C.

N1 - Publisher Copyright: © The Author(s) 2024. Published by Oxford University Press on behalf of Royal Astronomical Society.

PY - 2024/8/1

Y1 - 2024/8/1

N2 - Our ability to trace the star-forming molecular gas is important to our understanding of the Universe. We can trace this gas using CO emission, converting the observed CO intensity into the H2 gas mass of the region using the CO-to-H2 conversion factor (XCO). In this paper, we use simulations to study the conversion factor and the molecular gas within galaxies. We analysed a suite of simulations of isolated disc galaxies, ranging from dwarfs to Milky Way-mass galaxies, that were run using the FIRE-2 subgrid models coupled to the CHIMES non-equilibrium chemistry solver. We use the non-equilibrium abundances from the simulations, and we also compare to results using abundances assuming equilibrium, which we calculate from the simulation in post-processing. Our non-equilibrium simulations are able to reproduce the relation between CO and H2 column densities, and the relation between XCO and metallicity, seen within observations of the Milky Way. We also compare to the xCOLD GASS survey, and find agreement with their data to our predicted CO luminosities at fixed star formation rate. We also find the multivariate function used by xCOLD GASS overpredicts the H2 mass for our simulations, motivating us to suggest an alternative multivariate function of our fitting, though we caution that this fitting is uncertain due to the limited range of galaxy conditions covered by our simulations. We also find that the non-equilibrium chemistry has little effect on the conversion factor (<5 per cent) for our high-mass galaxies, though still affects the H2 mass and LCO by ≈25 per cent.

AB - Our ability to trace the star-forming molecular gas is important to our understanding of the Universe. We can trace this gas using CO emission, converting the observed CO intensity into the H2 gas mass of the region using the CO-to-H2 conversion factor (XCO). In this paper, we use simulations to study the conversion factor and the molecular gas within galaxies. We analysed a suite of simulations of isolated disc galaxies, ranging from dwarfs to Milky Way-mass galaxies, that were run using the FIRE-2 subgrid models coupled to the CHIMES non-equilibrium chemistry solver. We use the non-equilibrium abundances from the simulations, and we also compare to results using abundances assuming equilibrium, which we calculate from the simulation in post-processing. Our non-equilibrium simulations are able to reproduce the relation between CO and H2 column densities, and the relation between XCO and metallicity, seen within observations of the Milky Way. We also compare to the xCOLD GASS survey, and find agreement with their data to our predicted CO luminosities at fixed star formation rate. We also find the multivariate function used by xCOLD GASS overpredicts the H2 mass for our simulations, motivating us to suggest an alternative multivariate function of our fitting, though we caution that this fitting is uncertain due to the limited range of galaxy conditions covered by our simulations. We also find that the non-equilibrium chemistry has little effect on the conversion factor (<5 per cent) for our high-mass galaxies, though still affects the H2 mass and LCO by ≈25 per cent.

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KW - galaxies: evolution

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