Galaxy quenching and black hole feedback

5 problems

• What fraction of black hole energy couples to galactic gas?

  Open

  Simulations assume that a few percent of the energy released by a growing black hole heats or pushes the surrounding gas. Whether real outflows carry this much energy has not been measured reliably.

• How do black hole jets keep cluster cores from cooling?

  Partially resolved

  Gas at the centers of galaxy clusters radiates away its heat in less than a billion years, yet it forms stars at only a few percent of the expected rate. Jets from the central black hole carry enough energy, but how that energy spreads evenly through the gas is not known.

• How did massive galaxies stop forming stars within 700 Myr?

  Contested

  JWST found massive galaxies that had already finished forming stars when the universe was under a billion years old. Making that many stars and then stopping so quickly is hard to reproduce in models.

• What sets the black hole mass versus bulge velocity dispersion relation?

  Open

  The mass of a galaxy's central black hole tracks the random speeds of stars in its central bulge with surprisingly little scatter, although the black hole is far too small to affect those stars by gravity. Either the black hole's energy output regulates the galaxy, or repeated mergers average the two masses together.

• What stops star formation in massive galaxies?

  Open

  Above a certain mass most galaxies stop forming stars and stay that way. Black hole outbursts, heating of infalling gas by shocks, and other effects are all candidates, and the relative roles are unknown.