The CO-H2 conversion factor in disc galaxies and mergers

Relating the observed CO emission from giant molecular clouds (GMCs) to the underlying H₂ column density is a long-standing problem in astrophysics. While the Galactic CO-H₂ conversion factor (X_CO) appears to be reasonably constant, observations indicate that X_CO may be depressed in high surface density starburst environments. Using a multiscale approach, we investigate the dependence of X_CO on the galactic environment in numerical simulations of disc galaxies and galaxy mergers. X_CO is proportional to the GMC surface density divided by the integrated CO intensity, W_CO, and W_CO is related to the kinetic temperature and velocity dispersion in the cloud. In disc galaxies (except within the central ∼ kpc), the galactic environment is largely unimportant in setting the physical properties of GMCs provided they are gravitationally bound. The temperatures are roughly constant at ∼10 K due to the balance of CO cooling and cosmic ray heating, giving a nearly constant CO-H₂ conversion factor in discs. In mergers, the velocity dispersion of the gas rises dramatically during coalescence. The gas temperature also rises as it couples well to the warm (∼50 K) dust at high densities (n > 10⁴ cm^-3). The rise in velocity dispersion and temperature combine to offset the rise in surface density in mergers, causing X_CO to drop by a factor of ∼2-10 compared to the disc simulation. This model predicts that high-resolution Atacama Large Millimeter/submillimeter Array observations of nearby ultraluminous infrared galaxies should show velocity dispersions of 10¹-10² km s^-1, and brightness temperatures comparable to the dust temperatures.