Chemical evolution of giant molecular clouds in simulations of galaxies

Abstract

We present an analysis of Giant Molecular Clouds (GMCs) within hydrodynamic simulations of isolated, low-mass (M* ~ 10^9 M_sol) disc galaxies. We study the evolution of molecular abundances and the implications for CO emission and the X_CO conversion factor in individual clouds. We define clouds either as regions above a density threshold n_H,min = 10 cm^-3, or using an observationally motivated CO intensity threshold of 0.25 K km s^-1. Our simulations include a non-equilibrium chemical model with 157 species, including 20 molecules. We also investigate the effects of resolution and pressure floors (i.e. Jeans limiters). We find cloud lifetimes up to ~40 Myr, with a median of 13 Myr, in agreement with observations. At one tenth solar metallicity, young clouds (<10-15 Myr) are underabundant in H2 and CO compared to chemical equilibrium, by factors of ~3 and 1-2 orders of magnitude, respectively. At solar metallicity, GMCs reach chemical equilibrium faster (within ~1 Myr). We also compute CO emission from individual clouds. The mean CO intensity, I_CO, is strongly suppressed at low dust extinction, A_v, and possibly saturates towards high A_v, in agreement with observations. The I_CO - A_v relation shifts towards higher A_v for higher metallicities and, to a lesser extent, for stronger UV radiation. At one tenth solar metallicity, CO emission is weaker in young clouds (<10-15 Myr), consistent with the underabundance of CO. Consequently, X_CO decreases by an order of magnitude from 0 to 15 Myr, albeit with a large scatter.

28 pages, 29 figures (including appendices), Accepted for publication in MNRAS. Minor changes relative to v1

References in corpus (31)

• The EAGLE project: Simulating the evolution and assembly of galaxies and their environments
• Theory of Star Formation
• The effect of photo-ionization on the cooling rates of enriched, astrophysical plasmas
• The Boston University-Five College Radio Astronomy Observatory Galactic Ring Survey
• The Resolved Properties of Extragalactic Giant Molecular Clouds
• The Atomic to Molecular Transition in Galaxies. II: HI and H_2 Column Densities
• The 2013 Release of Cloudy
• Modelling discontinuities and Kelvin-Helmholtz instabilities in SPH
• Inviscid SPH
• Simulating the formation of molecular clouds. I. Slow formation by gravitational collapse from static initial conditions
• Molecular Hydrogen and Global Star Formation Relations in Galaxies
• CO Isotopologues in the Perseus Molecular Cloud Complex: the X-Factor and Regional Variations
• Simulating the formation of molecular clouds. II. Rapid formation from turbulent initial conditions
• The Dynamics of Radiation Pressure-Dominated HII Regions
• Photoprocesses in protoplanetary disks
• 2MASS wide field extinction maps - I. The Pipe nebula
• Environmental Dependence of the Kennicutt-Schmidt Relation in Galaxies
• The Atomic to Molecular Transition in Galaxies. I: An Analytic Approximation for Photodissociation Fronts in Finite Clouds
• HI-to-H2 Transitions and H I Column Densities in Galaxy Star-Forming Regions
• Bent by baryons: the low mass galaxy-halo relation
• CO-dark gas and molecular filaments in Milky Way type galaxies
• Molecular line intensities as measures of cloud masses - I. Sensitivity of CO emissions to physical parameter variations
• The frequency and nature of `cloud-cloud collisions' in galaxies
• Galaxies that Shine: radiation-hydrodynamical simulations of disk galaxies
• Giant Molecular Clouds in M31: I - Molecular Cloud Properties
• Non-equilibrium chemistry and cooling in the diffuse interstellar medium - II. Shielded gas
• What can simulated molecular clouds tell us about real molecular clouds?
• The Relationship between CO Emission and Visual Extinction Traced by Dust Emission in the Magellanic Clouds
• Atomic Chemistry In Turbulent Astrophysical Media I: Effect of Atomic Cooling
• Molecular cloud regulated star formation in galaxies
• Effects of Galactic Disc Inclination and Resolution on Observed GMC Properties and Larson's Scaling Relations

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• Live Fast, Die Young: GMC lifetimes in the FIRE cosmological simulations of Milky Way-mass galaxies
• Evolution of giant molecular clouds across cosmic time
• 3D chemical structure of diffuse turbulent ISM. I. Statistics of the HI-to-H${}_2$ transition
• Synthetic observations of star formation and the interstellar medium
• Populating H${}_2$ and CO in galaxy simulation with dust evolution
• Simulating the atomic and molecular content of molecular clouds using probability distributions of physical parameters
• Recent progress in simulating galaxy formation from the largest to the smallest scales
• Hybrid-chimes: A model for radiative cooling and the abundances of ions and molecules in simulations of galaxy formation
• Resolution criteria to avoid artificial clumping in Lagrangian hydrodynamic simulations with a multi-phase interstellar medium
• A comparison of ${\text{H}}_2$ formation models at high redshift
• Modelling H${}_2$ and its effects on star formation using a joint implementation of GADGET-3 and KROME
• The life cycle of giant molecular clouds in simulated Milky Way-mass galaxies