Why is the Milky Way X-factor Constant?
  The CO-H2 conversion factor (Xco; otherwise known as the X-factor) is
observed to be remarkably constant in the Milky Way and in the Local Group
(aside from the SMC). 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. 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 the Milky Way'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.
