  Negative feedback from accreting supermassive black holes is regarded as a
key ingredient in suppressing star formation and quenching massive galaxies.
However, several models and observations suggest that black hole feedback may
have a positive effect, triggering star formation by compressing interstellar
medium gas to higher densities. We investigate the dual role of black hole
feedback using cosmological hydrodynamic simulations from the Feedback In
Realistic Environments (FIRE) project, including a novel implementation of
hyper-refined accretion-disc winds. Focusing on a massive, star-forming galaxy
at $z \sim 2$ ($M_{\rm halo} \sim 10^{12.5} \, {\rm M}_{\odot}$), we show that
strong quasar winds with kinetic power $\sim$10$^{46}$ erg/s acting for
$>$20$\,$Myr drive the formation of a central gas cavity and can dramatically
reduce the star formation rate surface density across the galaxy disc. The
suppression of star formation is primarily driven by reducing the amount of gas
that can become star-forming, compared to directly evacuating the pre-existing
star-forming gas reservoir (preventive feedback dominates over ejective
feedback). Despite the global negative impact of quasar winds, we identify
several plausible signatures of local positive feedback, including: (1) spatial
anti-correlation of wind-dominated regions and star-forming clumps, (2) higher
local star formation efficiency in compressed gas near the edge of the cavity,
and (3) increased local contribution of outflowing material to star formation.
Stars forming under the presence of quasar winds tend to do so at larger radial
distances. Our results suggest that positive and negative AGN feedback can
coexist in galaxies, but local positive triggering of star formation plays a
minor role in global galaxy growth.
