A6 Active Galactic Nuclei and Tidal Disruption Events

Topic

The central figure in this model is the active galactic nucleus: the supermassive black hole occupying a galaxy's center. The rate at which this black hole accretes matter can vary dramatically over time; when this accretion rate surges, the nucleus's luminosity vastly exceeds the combined starlight of its host galaxy—an extreme state known as the quasar phase.

When a star passes too close to this central black hole, the tidal forces exerted by the hole can overcome the star's own self-gravity, causing it to be completely torn apart. The resulting stellar debris is then accreted onto the black hole, producing a temporary flare of brightness—a tidal disruption event—that fades once the available material has been fully consumed.

Fundamentally, both the quasar phase and tidal disruption events share the same origin: episodes of intense accretion onto the black hole that generate not only a flare of brightness but also radiation and jets capable of heating or expelling the gas surrounding the nucleus.

It is precisely this radiation and these jets that slow down star formation—at least temporarily—and inhibit future accretion by the nucleus, thereby closing the cycle: intense accretion produces radiation and jets; this radiation and these jets heat and expel the surrounding gas; and this gas expulsion, in turn, reduces the accretion the nucleus could sustain later on. Thus, from the supermassive black hole that opens this account to the self-imposed braking mechanism that concludes it—taking in the quasar phase and tidal disruption events along the way—the active galactic nucleus ensures that its own appetite, when pushed to the extreme, ultimately limits the very food supply upon which that appetite depends.

ID:3919