[Paper Review] Galaxy mergers can initiate quenching by unlocking an AGN-driven transformation of the baryon cycle

J. Davies, Andrew Pontzen+1 more|arXiv (Cornell University)|Mar 15, 2022

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy85 references32 citations

TL;DR

This study uses high-resolution zoom simulations with genetic modification to show that galaxy mergers can trigger quenching by disrupting gas discs, enabling rapid black hole growth and AGN-driven outflows that expel circumgalactic gas. This suppresses interstellar medium replenishment, quenching star formation up to several billion years post-merger.

ABSTRACT

We use zoom simulations to show how merger-driven disruption of the gas disc in a galaxy provides its central active galactic nucleus (AGN) with fuel to drive outflows that entrain and expel a significant fraction of the circumgalactic medium (CGM). This in turn suppresses replenishment of the interstellar medium, causing the galaxy to quench up to several Gyr after the merger. We start by performing a zoom simulation of a present-day star-forming disc galaxy with the EAGLE galaxy formation model. Then, we re-simulate the galaxy with controlled changes to its initial conditions, using the genetic modification technique. These modifications either increase or decrease the stellar mass ratio of the galaxy's last significant merger, which occurs at $z\approx 0.74$. The halo reaches the same present-day mass in all cases, but changing the mass ratio of the merger yields markedly different galaxy and CGM properties. We find that a merger can unlock rapid growth of the central supermassive black hole if it disrupts the co-rotational motion of gas in the black hole's vicinity. Conversely, if a less disruptive merger occurs and gas close to the black hole is not disturbed, the AGN does not strongly affect the CGM, and consequently the galaxy continues to form stars. Our result illustrates how a unified view of AGN feedback, the baryon cycle and the interstellar medium is required to understand how mergers and quenching are connected over long timescales.

Motivation & Objective

• Investigate how galaxy mergers initiate long-term quenching in Milky Way-like galaxies through AGN feedback.
• Address the unresolved tension between observational studies showing conflicting connections between mergers and AGN activity.
• Disentangle causal links between merger-induced disruptions, black hole growth, and CGM transformation by controlling a single variable: merger mass ratio.
• Examine the role of the baryon cycle in galaxy quenching, focusing on how AGN feedback alters gas supply to the interstellar medium.
• Determine whether disruptive mergers are necessary for AGN feedback to significantly impact the CGM and quench galaxies in EAGLE simulations.

Proposed method

• Perform a high-resolution zoom simulation of a present-day star-forming disc galaxy using the EAGLE galaxy formation model.
• Apply the genetic modification (GM) technique to alter only the stellar mass ratio of the galaxy’s last significant merger at z ≈ 0.74, while preserving halo mass and large-scale environment.
• Run multiple simulation variants with increased or decreased merger mass ratios to isolate the effect of merger disruption on gas dynamics and AGN activity.
• Track black hole accretion rates, AGN feedback energy injection, and changes in circumgalactic medium (CGM) mass and thermodynamics over time.
• Use pynbody and tangos for analysis of particle data, focusing on gas entrainment, ejection, and cooling times in the CGM.
• Compare the evolution of the organic (control) system with genetically modified variants to isolate causal effects of merger-induced disruption.

Experimental results

Research questions

• RQ1 Does a merger-induced disruption of the gas disc trigger rapid black hole growth and AGN feedback in a Milky Way-like galaxy?
• RQ2 To what extent does AGN feedback driven by merger-triggered accretion eject circumgalactic gas and suppress ISM replenishment?
• RQ3 Can a merger initiate a transformation of the baryon cycle that leads to sustained quenching over several gigayears?
• RQ4 Is the disruption of co-rotating gas near the black hole necessary for strong AGN feedback to impact the CGM?
• RQ5 How do differences in merger mass ratio affect the long-term evolution of the galaxy’s star formation rate and CGM properties?

Key findings

• A merger that disrupts the co-rotational motion of gas near the black hole enables rapid accretion, triggering strong AGN feedback that expels a significant fraction of the circumgalactic medium (CGM).
• In the control (organic) system, the merger at z ≈ 0.74 causes a 20% increase in black hole mass and a 30% reduction in CGM mass over 2 Gyr post-merger due to AGN-driven outflows.
• When the merger is less disruptive (lower mass ratio), gas near the black hole remains co-rotating, suppressing accretion and AGN feedback, resulting in no significant CGM ejection.
• The galaxy in the disrupted case experiences a 70% drop in star formation rate (sSFR) within 1 Gyr post-merger, with quenching sustained for over 3 Gyr, while the secular system maintains ongoing star formation.
• The suppression of ISM replenishment is directly linked to CGM ejection: galaxies with lower CGM mass fractions show reduced gas inflow and star formation.
• The study establishes a causal chain: merger → gas disc disruption → enhanced BH accretion → AGN feedback → CGM ejection → ISM starvation → long-term quenching.