External pressure-triggering of star formation in a disc galaxy: a
  template for positive feedback
  Feedback from active galactic nuclei (AGN) has often been invoked both in
simulations and in interpreting observations for regulating star formation and
quenching cooling flows in massive galaxies. AGN activity can, however, also
over-pressurise the dense star-forming regions of galaxies and thus enhance
star formation, leading to a positive feedback effect. To understand this
pressurisation better, we investigate the effect of an ambient external
pressure on gas fragmentation and triggering of starburst activity by means of
hydrodynamical simulations. We find that moderate levels of over-pressurisation
of the galaxy boost the global star formation rate of the galaxy by an order of
magnitude, turn stable discs unstable, and lead to significant fragmentation of
the gas content of the galaxy, similar to what is observed in high redshift
galaxies.
