dead (early-type) spheroid.

From the point of view of the central BH, during the early stages plenty of gas is available from the surroundings, so that considerable accretion rates can sustain mildly super-Eddington emission. However, during these early stages the BH bolometric luminosity is substantially smaller than that of the host galaxy, so that the dust-enshrouded AGN may be observable only as a faint X-ray nucleus. However, the nuclear power in self-regulated conditions increases exponentially and after some $10^8$ yr progressively attains or even exceeds that from star formation in the host; interstellar gas and dust are removed and star formation is appreciably reduced or quenched, so that the overall system behaves as an optical quasar. Residual gas present in the central regions of the galaxy can be accreted onto the BH at progressively lower, sub-Eddington accretion rates, as observed on the average in unobscured AGNs.

The coevolution scenario sketched above has been extensively tested (see Lapi et al. 2014, 2017; Mancuso et al. 2016a,b,2017 and references therein) on a wealth of data including : SFR/stellar mass functions of galaxies and AGN luminosity/BH mass functions; IR/sub-mm/radio counts and redshift distributions; main sequences of star-forming galaxies (SFR vs. stellar mass correlation) 