How can galaxy mergers trigger quasar activity?
Shown are apparently normal, solitary galaxies (left), colliding galaxies (centre), and merging galaxies (right).
Galaxy mergers are a primary mechanism believed to trigger quasar activity by providing a sudden influx of fuel to the central supermassive black hole.
When two galaxies collide, tidal forces can disrupt the structure of the galaxies, causing gas and dust to lose angular momentum and fall toward the center of the more massive galaxy. This process effectively "feeds" the central black hole, which then accretes the material at a high rate. As this matter spirals inward, it forms an accretion disk where friction and turbulence heat the gas to millions of kelvins, resulting in the intense radiation that characterizes a quasar.1234
This episodic activity is a significant phase in the evolution of galaxies. Eventually, the energy, jets, and outflows generated by the active nucleus can heat or entirely remove the interstellar medium from the host galaxy, which simultaneously halts star formation and chokes off the fuel supply to the black hole, leading to the end of the quasar phase.1