The CO-H2 Conversion Factor in Disc Galaxies and Mergers

TL;DR

This paper investigates how the CO-H2 conversion factor varies in different galactic environments, finding it remains constant in discs but drops significantly in mergers due to increased temperature and velocity dispersion.

Contribution

It provides a multi-scale simulation-based model explaining the environmental dependence of Xco in disc galaxies and mergers, highlighting the physical reasons for its variation.

Findings

Xco is nearly constant in disc galaxies outside the central kpc.

Xco decreases by a factor of 2-10 in galaxy mergers.

Gas velocity dispersion and temperature increase significantly during mergers.

Abstract

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 (Xco) appears to be reasonably constant, observations indicate that Xco may be depressed in high-surface density starburst environments. Using a multi-scale approach, we investigate the dependence of Xco on the galactic environment in numerical simulations of disc galaxies and galaxy mergers. Xco is proportional to the GMC surface density divided by the integrated CO intensity, Wco, and Wco 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 > 10^4 cm^-3). The rise in velocity dispersion and temperature combine to offset the rise in surface density in mergers, causing Xco to drop by a factor of ~2-10 compared to the disc simulation. This model predicts that high-resolution ALMA observations of nearby ULIRGs should show velocity dispersions of ~10-100 km/s, and brightness temperatures comparable to the dust temperatures.