  Without AGN feedback, simulated massive, star-forming galaxies become too
compact relative to observed galaxies at z<2. In this paper, we perform
high-resolution re-simulations of a massive (M_star~10^11 M_sol) galaxy at
z~2.3, drawn from the Feedback in Realistic Environments (FIRE) project. In the
simulation without AGN feedback, the galaxy experiences a rapid starburst and
shrinking of its half-mass radius. We experiment with driving mechanical AGN
winds, using a state-of-the-art hyper-Lagrangian refinement technique to
increase particle resolution. These winds reduce the gas surface density in the
inner regions of the galaxy, suppressing the compact starburst and maintaining
an approximately constant half-mass radius. Using radiative transfer, we study
the impact of AGN feedback on the magnitude and extent of the multi-wavelength
continuum emission. When AGN winds are included, the suppression of the
compact, dusty starburst results in lowered flux at FIR wavelengths (due to
decreased star formation) but increased flux at optical-to-near-IR wavelengths
(due to decreased dust attenuation, in spite of the lowered star formation
rate), relative to the case without AGN winds. The FIR half-light radius
decreases from ~1 kpc to ~0.1 kpc in <40 Myr when AGN winds are not included,
but increases to ~2 kpc when they are. Interestingly, the half-light radius at
optical-NIR wavelengths remains approximately constant over 35 Myr, for
simulations with and without AGN winds. In the case without winds, this occurs
despite the rapid compaction, and is due to heavy dust obscuration in the inner
regions of the galaxy. This work highlights the importance of forward-modelling
when comparing simulated and observed galaxy populations.
