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 tAGN, 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 tgal–tAGN, where tgal is the galaxy age, is an excellent indicator of the presence of feedback processes, and peaks significantly around tgal–tAGN≈ 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.