AGN Jet-induced Feedback in Galaxies. II. Galaxy colours from a
  multicloud simulation
  We study the feedback from an AGN on stellar formation within its host
galaxy, mainly using one high resolution numerical simulation of the jet
propagation within the interstellar medium of an early-type galaxy. In
particular, we show that in a realistic simulation where the jet propagates
into a two-phase ISM, star formation can initially be slightly enhanced and
then, on timescales of few million years, rapidly quenched, as a consequence
both of the high temperatures attained and of the reduction of cloud mass
(mainly due to Kelvin-Helmholtz instabilities). We then introduce a model of
(prevalently) {\em negative} AGN feedback, where an exponentially declining
star formation is quenched, on a very short time scale, at a time t_AGN, due to
AGN feedback. Using the Bruzual & Charlot (2003) population synthesis model and
our star formation history, we predict galaxy colours from this model and match
them to a sample of nearby early-type galaxies showing signs of recent episodes
of star formation (Kaviraj et al. 2007). We find that the quantity t_gal -
t_AGN, where t_gal is the galaxy age, is an excellent indicator of the presence
of feedback processes, and peaks significantly around t_gal - t_AGN \approx
0.85 Gyr for our sample, consistent with feedback from recent energy injection
by AGNs in relatively bright (M_{B} \lsim -19) and massive nearby early-type
galaxies. Galaxies that have experienced this recent feedback show an
enhancement of 3 magnitudes in NUV(GALEX)-g, with respect to the unperturbed,
no-feedback evolution. Hence they can be easily identified in large combined
near UV-optical surveys.
