2018MNRAS.473..271V

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_* ≃ 10¹⁰ M_☉, SFR ≃ 100 M_☉ yr^- 1^) galaxy, 'Althaea', at z ≃ 6. We find that the CO emission traces the inner molecular disc (r ≃ 0.5 kpc) of Althaea 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 ≃ 220 M_☉ pc^- 2^) gas surface density in Althaea, 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_~1.5   M_☉ (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 Althaea is ≃13 h (≃38 h).