CO line emission from galaxies in the Epoch of Reionization.

We study the CO line luminosity (L_CO), the shape of the CO spectral line energy distribution (SLED), and the value of the CO-to-H₂ conversion factor in galaxies in the Epoch of Reionization (EoR). For this aim, we construct a model that simultaneously takes into account the radiative transfer and the clumpy structure of giant molecular clouds (GMCs) where the CO lines are excited. We then use it to post-process state-of-the-art zoomed, high resolution (30 pc), cosmological simulation of a main-sequence (M* ≈ 10¹⁰M_☉, SFR ≈ 100M_☉ yr^-1) galaxy, 'Althæa', at z ≈ 6.We find that the CO emission traces the inner molecular disc (r ≈ 0.5 kpc) of Althæa with the peak of the CO surface brightness co-located with that of the [C II] 158 μm emission. Its L_CO(1-0) = 10^4.85 L_☉ is comparable to that observed in local galaxies with similar stellar mass. The high (Σ_gas ≈ 220M_☉ pc^-2) gas surface density in Althæa, its large Mach number (M≈ 30) and the warm kinetic temperature (T_k ≈ 45 K) of GMCs yield a CO SLED peaked at the CO(7-6) transition, i.e. at relatively high-J and a CO-to-H₂ conversion factor α_CO ≈ 1.5M_☉(K km s^-1 pc²)^-1 lower than that of the Milky Way. The Atacama Large Millimeter/submillimeter Array observing time required to detect (resolve) at 5σ the CO(7-6) line from galaxies similar to Althæa is ≈13 h (≈38 h). [ABSTRACT FROM AUTHOR]