  We study the impact of outflows driven by active galactic nuclei (AGN) on
galaxy formation. Outflows move into the surrounding intergalactic medium (IGM)
and heat it sufficiently to prevent it from condensing onto galaxies. In the
dense, high-redshift IGM, such feedback requires highly energetic outflows,
driven by a large AGN. However, in the more tenuous low-redshift IGM,
equivalently strong feedback can be achieved by less energetic winds (and thus
smaller galaxies). Using a simple analytic model, we show that this leads to
the anti-hierarchical quenching of star-formation in large galaxies, consistent
with current observations. At redshifts prior to the formation of large AGN,
galaxy formation is hierarchical and follows the growth of dark-matter halos.
The transition between the two regimes lies at the z ~ 2 peak of AGN activity.
