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

We present new CO ($J=5-4$ and $7-6$) [CI] ($^3P_2\,-\, ^3P_1$ $^3P_1\,-\, ^3P_0$) emission line observations of the star-forming galaxy D49 at massive end Main Sequence $z=3$. incorporate previous ($J=3-2$) optical-to-millimetre continuum to fit its spectral energy distribution (SED). Our results hint high-$J$ luminosities exceeding expected location on empirical correlations with infrared luminosity. fully consistent literature trends is found. do not retrieve any signatures a bright active galactic nucleus that could boost $J=5-4,\,7-6$ lines in either or X-ray bands, but warm photon-dominated regions, shocks turbulence principle so. suggest mechanical heating be favourable mechanism able enhance gas fixed luminosity other main-sequence galaxies high redshift, further investigation necessary confirm this explanation. derive molecular masses from dust, CO, all agree within uncertainties. Given large star formation rate (SFR) $\sim 500~M_\odot~{\rm yr}^{-1}$ stellar mass $>10^{11.5}~M_\odot$, short depletion time scale $<0.3$ Gyr might indicate experiencing last growth spurt will soon transit quiescence.