Why is the Milky Way X-factor constant?

Abstract

The CO-H2 conversion factor (XCO; otherwise known as the X-factor) is observed to be remarkably constant in the MilkyWay and in the Local Group (aside from the Small Magellanic Cloud). To date, our understanding of why XCO should be so constant remains poor. Using a combination of extremely high-resolution (~1 pc) galaxy evolution simulations and molecular line radiative transfer calculations, we suggest that XCO displays a narrow range of values in the Galaxy due to the fact that molecular clouds share very similar physical properties. In our models, this is itself a consequence of stellar feedback competing against gravitational collapse. Giant molecular clouds (GMCs) whose lifetimes are regulated by radiative feedback show a narrow range of surface densities, temperatures and velocity dispersions with values comparable to those seen in the Milky Way. As a result, the X-factors from these clouds show reasonable correspondence with observed data from the Local Group, and a relatively narrow range. On the other hand, feedback-free clouds collapse to surface densities that are larger than those seen in the Galaxy, and hence result in X-factors that are systematically too large compared to theMilkyWay's. We conclude that radiative feedback within GMCs can generate cloud properties similar to those observed in the Galaxy, and hence a roughly constant Milky Way X-factor in normal, quiescent clouds. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.