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

Abstract: We present new CO ($J=5-4$ and $7-6$) and CI 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 (SED). 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 [CI] that all agree within the uncertainties. Given its large star formation rate (SFR) $\sim 500~M_\odot~{\rm yr}^{-1}$ and stellar mass $>10^{11.5}~M_\odot$, the short depletion time scale of $<0.3$ Gyr might indicate that D49 is experiencing its last growth spurt and will soon transit to quiescence.