Predictions for CO emission and the CO-to-H_2 conversion factor in galaxy simulations with non-equilibrium chemistry
arxiv(2024)
摘要
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 H_2 gas mass of the region
using the CO-to-H_2 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 H_2 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 H_2 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%) for
our high-mass galaxies, though still affects the H_2 mass and Lco by
≈25%.
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