Molecular gas content and high excitation of a massive main-sequence galaxy at z = 3

We present new CO (J = 5 − 4 and 7 − 6) and [C I] (³P₂−³P₁ and ³P₁−³P₀) emission line observations of the star-forming galaxy D49 at the massive end of the main sequence at z = 3. We incorporate previous CO (J = 3 − 2) and optical-to-millimetre continuum observations to fit its spectral energy distribution. Our results hint at high-J CO luminosities exceeding the expected location on the empirical correlations with the infrared luminosity. [CI] emission fully consistent with the literature trends is found. We do not retrieve any signatures of a bright active galactic nucleus that could boost the J = 5 − 4,  7 − 6 lines in either the infrared or X-ray bands, but warm photon-dominated regions, shocks, or turbulence could in principle do so. We suggest that mechanical heating could be a favourable mechanism able to enhance the gas emission at fixed infrared luminosity in D49 and other main-sequence star-forming galaxies at high redshift, but further investigation is necessary to confirm this explanation. We derive molecular gas masses from dust, CO, and [C I] that all agree within the uncertainties. Given its high star formation rate ∼500 M_⊙ yr⁻¹ and stellar mass > 10^11.5 M_⊙, the short depletion timescale of < 0.3 Gyr might indicate that D49 is experiencing its last growth spurt and will soon transit to quiescence.