AGN jet-induced feedback in galaxies - II. Galaxy colours from a multicloud simulation

We study the feedback from an active galactic nucleus (AGN) on stellar formation within its host galaxy, mainly using one high-resolution numerical simulation of the jet propagation within the interstellar medium (ISM) of an early-type galaxy (ETG). In particular, we show that in a realistic simulation where the jet propagates into a two-phase ISM, star formation (SF) can initially be slightly enhanced and then, on time-scales 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) 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 and Charlot population synthesis model and our SF history, we predict galaxy colours from this model and match them to a sample of nearby ETGs showing signs of recent episodes of SF, see Kaviraj et al. 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 approximate to 0.85 Gyr for our sample, consistent with feedback from recent energy injection by AGNs in relatively bright (MB <= -19) and massive nearby ETGs. Galaxies that have experienced this recent feedback show an enhancement of 3 mag 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.