Recovering the Physical Properties of Molecular Gas in Galaxies from CO
  SLED Modeling
  Modeling of the spectral line energy distribution (SLED) of the CO molecule
can reveal the physical conditions (temperature, density) of molecular gas in
Galactic clouds and other galaxies. Recently, the Herschel Space Observatory
and ALMA have offered, for the first time, a comprehensive view of the
rotational J = 4-3 through J = 13-12 lines, which arise from a complex, diverse
range of physical conditions that must be simplified to one, two, or three
components when modeled. Here we investigate the recoverability of physical
conditions from SLEDs produced by galaxy evolution simulations containing a
large dynamical range in physical properties. These simulated SLEDs were
generally fit well by one component of gas whose properties largely resemble or
slightly underestimate the luminosity-weighted properties of the simulations
when clumping due to non-thermal velocity dispersion is taken into account. If
only modeling the first three rotational lines, the median values of the
marginalized parameter distributions better represent the luminosity-weighted
properties of the simulations, but the uncertainties in the fitted parameters
are nearly an order of magnitude, compared to approximately 0.2 dex in the
"best-case" scenario of a fully sampled SLED through J = 10-9. This study
demonstrates that while common CO SLED modeling techniques cannot reveal the
underlying complexities of the molecular gas, they can distinguish bulk
luminosity-weighted properties that vary with star formation surface densities
and galaxy evolution, if a sufficient number of lines are detected and modeled.
