The impact that accreting black holes can have on their surrounding galaxies is profound yet still poorly understood. A full description of why, how, and when black holes alter the evolutionary pathways of their host galaxies remains one of the major outstanding questions in astrophysics.
Semi-analytic models of the formation and evolution of galaxies from cosmological initial conditions cannot produce observed galaxy population properties without the additional injection of energy. This energy is required to prevent gas cooling and therefore the runaway formation of stars (Benson et al. 2003, Bower et al. 2006, Croton et al. 2006, Schawinski et al. 2006). As highly efficient sources of energy, accreting black holes at the centers of galaxies are now routinely invoked as the source of this energy and thus as a fundamental component in galaxy formation theory (Silk & Rees 1998).
This process, called `feedback,' thus casts the black hole into the role of a thermostat for the gas in galaxies. By heating and expelling gas that would otherwise cool and condense into stars, black hole feedback is capable of fundamentally changing and controlling the evolutionary trajectory of their host galaxies and in turn the further growth of the black hole as it starves itself of fuel. This close relationship between galaxies and their central black holes can thus be described as `co-evolution,' potentially beginning with the birth of both in the early Universe.