There is an interesting correlation between the central objects and their host galaxies in recent high resolution HST photometry of early type galaxies and Near-IR images of nearby quasar hosts .
It has been shown that a ) the hosts of these very luminous quasars are likely to be early type galaxies and that b ) the mass ratio of central black holes ( BHs ) and their host spheroidal components ( M _ { bh } / M _ { spheroid } ) is \sim 0.002 within a factor of three .
Using the hierarchical galaxy evolution scheme for the formation of early-type galaxies , we present here a general viscous accretion disk model to trace the star formation and central engine evolution before and after mergers .
In our model , starbursts and AGN coexist ; these two activities compete for the gas supply , interact with each other , probably feed back on each other and lock into a final status .
They thus constrain the ratio of central black hole mass and its host spheroidal mass to a universal value of order 10 ^ { -3 } .