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Fig. 7.

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Left: depletion time as a function of redshift for our sample, colour coded as a function of their offset from the MS. The comparison samples are as indicated in Fig. 6, with their mean error shown by the vertical line at z = 0. The solid line shows the expected trend (Tacconi et al. 2018) for a main–sequence galaxy of stellar mass 2 × 109M, the median of our sample. With depletion timescales in the range 45−900 Myr, most of our sample falls below the locus for normal, star–forming galaxies, and will quickly deplete the gas and transition into quiescence in the absence of gas replenishment. Right: gas fraction (Mgas/(Mgas + M*)) as a function of redshift. The high–redshift unobscured QSOs are not plotted as they lack estimates of their stellar masses. For the QSO comparison samples, the gas mass estimates have been adjusted for αCO = 4. On average, the detected QSOs are very gas–rich, while the non–detected QSOs, which are likely transitioning into quiescence, have almost depleted the available molecular gas. The largest gas fractions are found in the QSOs with the largest offset from the main–sequence, suggesting that the large gas mass is sustaining the high SFRs.

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