- The external spacetime of all black holes is described by the Kerr metric, independent of their mass.
- This is the massive black hole at the center of the Milky Way; it drives our galaxies.
- These images are formed by photons originating in the deep gravitational fields of black holes and, therefore, carry imprints of the spacetime properties in the strong-field regime. The only stationary spacetime vacuum that is symmetrically flat and contains a horizon. Black holes are essential as shapers and controllers of how galaxies evolve. They are the engines of their universe.
- Gravity draws dust and gas inward to the galactic center. When a swirling accretion disk is formed around the supermassive black hole, this disk heats up and turns into a white-hot plasma. When the black hole starts eating the hot plasma in bite-size pieces, energy is spat out into the galaxy highly efficiently. This process is more than just a galaxy clean-up process. This feedback process affects star formation rates and gas flow patterns throughout the galaxy. We do not know how the triggering mechanism and shut-off switch work.
- A supernova explosion does stellar feedback on a much smaller basis. Only the giant engines of supermassive black holes can direct the evolution of medium and large galaxies. Our Milky Way spiral galaxy is a medium-sized one.
- Active galactic nuclei (AGN) at the center of a galaxy produce tremendous amounts of energy, often outshining the rest of the galaxy by orders of magnitude. An active galactic nucleus (AGN) grows into an active supermassive black hole (SMBH). It releases energy to stop or slow star formation within the galaxy, thereby controlling its growth.
- Galaxies and black holes convolved. Galaxies start small and dense. Gravity smashes the more minor things together. Mergers and acquisitions follow. Rings and whirlpools form up. Galaxies get bigger. Galaxies smash together, resulting in shapeshifting. Space smooths out. Young stars glow blue; old stars glow red on their way to dying. Black holes swallow gas and release energy until things start falling apart. Nothing lasts forever and strikes again. This feedback from black holes governs the galaxy from within. The energy ejected from black holes fills the space between galaxies with hot gas instead of cooling down and forming new stars. It is an automatic balancing act. Black holes are the engine that could. The matter is attracted to the accretion disc. Friction causes the energy to be pushed back out. This is how they dump their energy into the neighborhood. Timing is everything. The galaxy’s growth must be stunted at the right time to keep it balanced. Every component must work in unison to get the job done correctly.
- Strong magnetic fields cause matter to fly outward from the accretion disk. This matter exists in diffuse galactic winds or in powerful, narrow jets. Some of the energy is emitted as radiation. This keeps the center of the galaxy glowing brightly. The galaxy absorbs some energy, and the rest spews out in the beam. AGN feedback is critical in disk galaxies. The jets are composed of matter escaping from the black hole’s accretion disk.
- When AGN and stellar feedback intertwine with the hot gas surrounding galaxies, different flow patterns dominate in different galaxy types at other times. Black holes are activated when gas clumps are nearby. This is followed by eating and digesting the new energy. Then, it emits energy, such as jets and galactic winds. This cycle is intermittent. These high-powered galactic winds and jets push outward against this accretion flow. Big and small effects interact on a galaxy-wide basis. Scales can interact up and down. A supermassive black hole is a small spot in a vast galaxy. Yet a black hole’s information seamlessly moves up and down all the scales within the galaxy. This information loop keeps both ends in touch. The info goes round and round. The flow patterns of the diffuse gas in and around galaxies have a constantly updated road map. The gas flows within the galactic ecosystems are closely monitored. Black holes evaporate slowly. In the long run, their energy is returned to the universe, bit by bit.
