Why does quasar activity eventually stop?

Quasar activity is an episodic phenomenon that eventually ceases as the central black hole exhausts its fuel supply.1

During its active phase, a quasar generates intense radiation, powerful mass outflows, and jets. These energetic processes heat the interstellar medium within the host galaxy and can even remove the gas entirely. This depletion of gas has two primary effects: it shuts down star formation and effectively chokes off the supply of material spiraling into the black hole.1

Once this fuel source is depleted, the quasar activity stops, leaving behind a dormant massive black hole at the center of the galaxy. This life cycle typically lasts around 10 million years, with the most massive black holes generally becoming dormant earlier than those with less mass.1