  We exploit the recent, wide samples of far-infrared (FIR) selected galaxies
followed-up in X rays and of X-ray/optically selected active galactic nuclei
(AGNs) followed-up in the FIR band, along with the classic data on AGN and
stellar luminosity functions at high redshift z>1.5, to probe different stages
in the coevolution of supermassive black holes (BHs) and host galaxies. The
results of our analysis indicate the following scenario: (i) the star formation
in the host galaxy proceeds within a heavily dust-enshrouded medium at an
almost constant rate over a timescale ~0.5-1 Gyr, and then abruptly declines
due to quasar feedback; over the same timescale, (ii) part of the interstellar
medium loses angular momentum, reaches the circum-nuclear regions at a rate
proportional to the star formation and is temporarily stored into a massive
reservoir/proto-torus wherefrom it can be promptly accreted; (iii) the BH grows
by accretion in a self-regulated regime with radiative power that can slightly
exceed the Eddington limit L/L_Edd< 4, particularly at the highest redshifts;
(iv) for massive BHs the ensuing energy feedback at its maximum exceeds the
stellar one and removes the interstellar gas, thus stopping the star formation
and the fueling of the reservoir; (v) afterwards, if the latter has retained
enough gas, a phase of supply-limited accretion follows exponentially declining
with a timescale of about 2 e-folding times. We show that the ratio of the FIR
luminosity of the host galaxy to the bolometric luminosity of the AGN maps the
various stages of the above sequence. Finally, we discuss how the detailed
properties and the specific evolution of the reservoir can be investigated via
coordinated, high-resolution observations of starforming, strongly-lensed
galaxies in the (sub-)mm band with ALMA and in the X-ray band with Chandra and
the next generation X-ray instruments.
