On the origin of the $\Sigma_1$-$M_\star$ quenching boundary

We have considered a phenomenologically motivated model in which galaxies are quenched when the energy output of the central black hole exceeds a hundred times the gravitational binding energy of the baryons in the host halo. The model reproduces the mass functions of star-forming and quiescent galaxies at 0<z<2.5 and the quenching boundary on a $\Sigma_1$-stellar mass diagram. The quenching boundary arises because of the colour-morphology relation. The stellar surface density $\Sigma_1$ in the central kiloparsec is a morphological indicator. Galaxies becomes redder as $\Sigma_1$ increases until they cross the quenching boundary and enter the passive population. Mergers drive the growth of supermassive black holes and the morphological evolution that accompany the migration to the red sequence. That is the origin of the population of high-mass passive galaxies. At lower masses, passive galaxies are mainly satellites that ceased to form stars because of environmental effects.