SImulator of GAlaxy Millimetre/submillimetre Emission (SIGAME): CO
  emission from massive z=2 main-sequence galaxies
  We present SIGAME (SImulator of GAlaxy Millimetre/submillimetre Emission), a
new numerical code designed to simulate the 12CO rotational line emission
spectrum of galaxies. Using sub-grid physics recipes to post-process the
outputs of smoothed particle hydrodynamics (SPH) simulations, a molecular gas
phase is condensed out of the hot and partly ionized SPH gas. The gas is
subjected to far-UV radiation fields and cosmic ray ionization rates which are
set to scale with the local star formation rate volume density. Level
populations and radiative transport of the CO lines are solved with the 3-D
radiative transfer code LIME. We have applied SIGAME to cosmological SPH
simulations of three disc galaxies at z=2 with stellar masses in the range
~(0.5-2)x10^11 Msun and star formation rates ~40-140 Msun/yr. Global CO
luminosities and line ratios are in agreement with observations of disc
galaxies at z~2 up to and including J=3-2 but falling short of the few existing
J=5-4 observations. The central 5 kpc regions of our galaxies have CO 3-2/1-0
and 7-6/1-0 brightness temperature ratios of ~0.55-0.65 and ~0.02-0.08,
respectively, while further out in the disc the ratios drop to more quiescent
values of ~0.5 and <0.01. Global CO-to-H2 conversion (alpha_CO) factors are
~=1.5 Msun*pc^2/(K km s/1), i.e. ~2-3 times below typically adopted values for
disc galaxies, and alpha_CO increases with radius, in agreement with
observations of nearby galaxies. Adopting a top-heavy Giant Molecular Cloud
(GMC) mass spectrum does not significantly change the results. Steepening the
GMC density profile leads to higher global line ratios for J_up>=3 and CO-to-H2
conversion factors [~=3.6 Msun*pc^2/(K km/s)].
