The massive black hole-velocity dispersion relation and the halo baryon
  fraction: a case for positive AGN feedback
  Force balance considerations put a limit on the rate of AGN radiation
momentum output, $L/c$, capable of driving galactic superwinds and reproducing
the observed $\mbh -\sigma $ relation between black hole mass and spheroid
velocity dispersion. We show that black holes cannot supply enough momentum in
radiation to drive the gas out by pressure alone. Energy-driven winds give a
$\mbh -\sigma $ scaling favoured by a recent analysis but also fall short
energetically once cooling is incorporated. We propose that outflow-triggering
of star formation by enhancing the intercloud medium turbulent pressure and
squeezing clouds can supply the necessary boost, and suggest possible tests of
this hypothesis. Our hypothesis simultaneously can account for the observed
halo baryon fraction.
