  Massive galaxies today typically are not forming stars despite being
surrounded by hot gaseous halos with short central cooling times. This likely
owes to some form of "quenching feedback" such as merger-driven quasar activity
or radio jets emerging from central black holes. Here we implement heuristic
prescriptions for these phenomena on-the-fly within cosmological hydrodynamic
simulations. We constrain them by comparing to observed luminosity functions
and color-magnitude diagrams from SDSS. We find that quenching from mergers
alone does not produce a realistic red sequence, because 1 - 2 Gyr after a
merger the remnant accretes new fuel and star formation reignites. In contrast,
quenching by continuously adding thermal energy to hot gaseous halos
quantitatively matches the red galaxy luminosity function and produces a
reasonable red sequence. Small discrepancies remain - a shallow red sequence
slope suggests that our models underestimate metal production or retention in
massive red galaxies, while a deficit of massive blue galaxies may reflect the
fact that observed heating is intermittent rather than continuous. Overall,
injection of energy into hot halo gas appears to be a necessary and sufficient
condition to broadly produce red and dead massive galaxies as observed.
