Active Galactic Nuclei (AGN) are believed to be intricately involved with the evolution of their host galaxies through a process called AGN feedback.
The mechanisms by which this occurs are not well-understood.
It has been known for some time that the population of high-luminosity AGN, associated with large supermassive black holes, peaked during the early universe, and are extinct in the modern universe.
AGN activity in the universe peaked around the same time as galactic star formation, and while this correlation is not completely understood, astronomers consider it a strong possibility that AGN play an important role in the regulation of star formation rates.
AGN feedback may be responsible for quenching cold stellar winds, reducing star formation rates.
Chaotic cold accretion is shown to be a likely mechanism by which AGN feedback occurs, allowing the AGN to ``cycle'' through a process of heating and cooling that is ultimately able to find equilibrium with the host galaxy, until available cold gasses are consumed.
Modeling AGN feedback presents significant challenges, but has improved upon observational predictions derived from the \LCDM\ cosmology-based Millenium Simulation.
Only a few subclasses of AGN have been studied regarding their feedback mechanisms, and with only the most rudimentary first-principle approaches, but the necessity of including AGN feedback to correctly predict colors and luminosities of galaxy mergers and quasars seems increasingly likely.
Refining modeling methods will be useful once observational resolution is increased by future telescope missions.
Quasars are the most luminous objects in the cosmos ($10^{12}-10^{15}$\Lo.
In 1964, Zel’dovich and Novikov described for the first time the luminosity of a quasar emerging from the accretion of matter by a supermassive black hole (SMBH).
\cite{koristafeedback}
We now know that SMBHs on the order of $10^6$ - $10^9$\Mo\ are common at the center of galaxies, and when their accretion process results in strong luminosities across the electromagnetic spectrum, we call these Active Galactic Nuclei (AGN).
Gas is readily available in the central region, shed by stars during their evolution, and from tidal disruptions caused by the SMBH.
The Eddington luminosity $L_E$ describes the luminosity necessary to induce radiation-driven outward matter flows, and for pure ionized hydrogen,
Quasars are AGN radiating near or above their Eddington luminosities, making this an important measure of their brightness, and at $>10^{45}\textrm{ergs s}^{-1}$ they can outshine their entire host galaxy!
For a $10^9$\Mo black hole to maintain its Eddington luminosity, it must accrete $40$~\Mo per year.
Central black holes in present-day galaxies apparently accrete at medium-to-low rates compared to rates needed to produce $L_E$, and observational studies show that quasars were much more common in the past, with their activity having peaked about 10 Gyr ago.
The Millenium Run and its following iterations are N-body simulations that predict the evolution of the matter in the universe. It is considered a (successful) test of the \LCDM cosmology.
The Millenium Simulation is used as a basis for simulations of other
Schematic of the ``cosmic cycle'' that plays out between galaxy formation and evolution regulated by black hole growth in galaxy mergers.
Galaxies in the modern universe typically contain dead quasars.
Mergers and other sources of inward gas flows are believed to trigger black hole growth and strong luminosities, which lead to AGN feedback and the possible continuation of the cycle at each iteration.
