Ultra-massive black hole feedback in compact galaxies
  Recent observations confirm the existence of ultra-massive black holes (UMBH)
in the nuclei of compact galaxies, with physical properties similar to NGC
1277. The nature of these objects poses a new puzzle to the `black hole-host
galaxy co-evolution' scenario. We discuss the potential link between UMBH and
galaxy compactness, possibly connected via extreme active galactic nucleus
(AGN) feedback at early times ($z > 2$). In our picture, AGN feedback is driven
by radiation pressure on dust. We suggest that early UMBH feedback blows away
all the gas beyond a $\sim$kpc or so, while triggering star formation at inner
radii, eventually leaving a compact galaxy remnant. Such extreme UMBH feedback
can also affect the surrounding environment on larger scales, e.g. the
outflowing stars may form a diffuse stellar halo around the compact galaxy, or
even escape into the intergalactic or intracluster medium. On the other hand,
less massive black holes will drive less powerful feedback, such that the stars
formed within the AGN feedback-driven outflow remain bound to the host galaxy,
and contribute to its size growth over cosmic time.
