SImulator of GAlaxy Millimetre/submillimetre Emission (SÍGAME): CO emission from massive z = 2 main-sequence galaxies

We present SÍGAME (SImulator of GAlaxy Millimetre/submillimetre Emission), a new numerical code designed to simulate the ¹²CO rotational line 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 3D radiative transfer code LIME. We have applied SÍGAME to cosmological SPH simulations of three disc galaxies at z = 2 with stellar masses in the range ∼0.5-2 × 10¹¹ M_⊙ and star formation rates ∼40-140 M_⊙ yr^-1. 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 (α_CO) factors are {∼eq } 1.5 {{M_{⊙}} pc^{-2} (K km s^{-1})^{-1}}, I.e. ∼2-3 times below the typically adopted values for disc galaxies, and α_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 profiles leads to higher global line ratios for J_up ≥ 3 and CO-to-H₂ conversion factors [{∼eq } 3.6 {{M_{⊙}} pc^{-2} (K km s^{-1})^{-1}}].