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.

💡 Research Summary

This paper investigates how an active‑galactic‑nucleus (AGN) jet influences star formation and, consequently, the observable colours of its host early‑type galaxy. The authors first perform a high‑resolution (∼10 pc) hydrodynamic simulation in which a relativistic jet propagates through a two‑phase interstellar medium (ISM) composed of cold, dense clouds embedded in a warm, diffuse background. The jet, launched at ≈0.1 c, drives strong shocks into the clouds; Kelvin‑Helmholtz instabilities rapidly grow on the cloud surfaces, stripping material and heating the gas to ≳10⁶ K. During the first ≈0.5 Myr the compression of clouds can boost the local star‑formation rate (SFR) by roughly 10–20 %, but the subsequent temperature rise and mass loss dominate, causing the SFR to collapse within a few Myr.

Guided by these results, the authors construct a simple “negative feedback” model for the galaxy’s global star‑formation history (SFH). The baseline SFH follows an exponential decline (SFR ∝ e^{‑t/τ}) with τ≈1 Gyr. At a specific epoch t_AGN, representing the moment the jet reaches the bulk of the ISM, the SFR is assumed to be quenched almost instantaneously (on a timescale ≲10⁵ yr). The time interval between the galaxy’s formation (t_gal) and the quenching event (t_gal − t_AGN) thus becomes a key parameter.

To translate the SFH into observable colours, the authors employ the Bruzual & Charlot (2003) stellar population synthesis code. They generate synthetic broadband magnitudes in the near‑UV (NUV, GALEX) and optical (SDSS u,g,r,i,z) bands for both the unperturbed exponential model and the quenched model. The most striking prediction is that when t_gal − t_AGN ≈ 0.85 Gyr, the NUV − g colour is on average three magnitudes redder than in the no‑feedback case, while optical colours change only modestly. This large NUV shift arises because the NUV flux is highly sensitive to stars younger than ≈1 Gyr; a sudden cessation of star formation therefore removes the young‑stellar contribution almost immediately, dimming the NUV band dramatically.

The theoretical predictions are tested against a sample of nearby early‑type galaxies identified by Kaviraj et al. (2007) as having experienced recent star‑formation episodes. These galaxies are relatively bright (M_B ≲ ‑19) and massive, and they possess GALEX NUV measurements. By comparing the observed NUV − g distribution with the model grid, the authors find that the majority of the sample clusters around t_gal − t_AGN ≈ 0.8–0.9 Gyr. In other words, the data suggest that these galaxies were quenched by AGN activity roughly 0.85 Gyr ago, consistent with the simulation‑driven expectation. Moreover, the quenched galaxies exhibit an NUV − g enhancement of about three magnitudes relative to a smoothly evolving counterpart, making them readily identifiable in large NUV‑optical surveys.

The paper emphasizes that the feedback is predominantly negative. While the jet can temporarily compress clouds and modestly raise the SFR, the net effect is rapid heating and cloud destruction, which suppresses further star formation. The injected jet energy (≈10⁵⁹ erg) is sufficient to raise the ISM temperature and prevent cooling for timescales comparable to the galaxy’s dynamical time, thereby driving the galaxy toward a “red‑and‑dead” state.

In conclusion, the study provides a coherent, multi‑scale picture: (1) high‑resolution simulations reveal the microphysics of jet‑cloud interactions; (2) a simple analytic quenching model captures the macroscopic impact on the galaxy’s SFH; (3) population‑synthesis calculations translate this into observable colour signatures; and (4) comparison with real galaxies confirms that the NUV‑optical colour, especially the NUV − g index, is an effective diagnostic of recent AGN‑driven quenching. The authors suggest that future work should explore a broader range of jet powers, host‑galaxy masses, and environmental conditions, and that upcoming facilities (e.g., JWST, ALMA) could directly image the remnants of jet‑impacted clouds, further testing the negative‑feedback paradigm.