Modelling CO formation in the turbulent interstellar medium

Abstract

We present results from high-resolution three-dimensional simulations of turbulent interstellar gas that self-consistently follow its coupled thermal, chemical and dynamical evolution, with a particular focus on the formation and destruction of H2 and CO. We quantify the formation timescales for H2 and CO in physical conditions corresponding to those found in nearby giant molecular clouds, and show that both species form rapidly, with chemical timescales that are comparable to the dynamical timescale of the gas. We also investigate the spatial distributions of H2 and CO, and how they relate to the underlying gas distribution. We show that H2 is a good tracer of the gas distribution, but that the relationship between CO abundance and gas density is more complex. The CO abundance is not well-correlated with either the gas number density n or the visual extinction A_V: both have a large influence on the CO abundance, but the inhomogeneous nature of the density field produced by the turbulence means that n and A_V are only poorly correlated. There is a large scatter in A_V, and hence CO abundance, for gas with any particular density, and similarly a large scatter in density and CO abundance for gas with any particular visual extinction. This will have important consequences for the interpretation of the CO emission observed from real molecular clouds. Finally, we also examine the temperature structure of the simulated gas. We show that the molecular gas is not isothermal. Most of it has a temperature in the range of 10--20 K, but there is also a significant fraction of warmer gas, located in low-extinction regions where photoelectric heating remains effective.Comment: 37 pages, 15 figures; minor revisions, matches version accepted by MNRA

Dusty star-forming galaxies at high redshift

“…Glover et al (2010) and Glover & Mac Low (2011) utilized magnetohydrodynamic models of GMC evolution combined with chemical reaction networks to show that H 2 can survive in low metallicity environments, while CO can be more easily destroyed. These models were expanded upon by Shetty et al (2011a,b), who coupled these models with large velocity gradient radiative transfer simulations to explicitly predict the CO emission.…”

“…In the regime of large gas surface density, which is more pertinent to the galaxies at hand, Feldmann et al (2012a) coupled the GMC models of Glover et al (2010) to a cosmological zoom simulation of an individual galaxy at z ∼ 2. These authors found that at high surface densities, one might expect the X-factor to drop, similar to the empirical findings of Ostriker & Shetty (2011).…”

The turbulent life of dust grains in the supernova-driven, multiphase interstellar medium

“…) to take into account dust shielding and molecular self-shielding. In the warm and cold gas, the primary cooling processes are Lyman-α cooling, H2 ro-vibrational line cooling, finestructure emission from C + and O, and rotational emission from CO (Glover et al 2010;. In hot gas, electronic excitation of helium and of partially ionised metals must also be taken into account, which is done using the Gnat & Ferland (2012) cooling rates assuming collisional ionisation equilibrium.…”

Using CO line ratios to trace the physical properties of molecular clouds

“…This is a reasonable assumption if the CO-emitting gas is isothermal, but we know from numerical simulations of molecular clouds that this is only approximately true and that temperature variations of a factor of two or more within CO-rich gas are not uncommon (see e.g. Glover et al 2010). Further complicating matters is the fact that the variations in density and temperature are not independent: the density structure depends sensitively on the temperature of the gas, while the temperature depends both on the density, and also on other factors such as the local extinction, the metallicity of the gas and the strength of the ISRF.…”