Beyond the Merger-Quasar-Quench Paradigm I: Mergers are neither necessary nor sufficient to quench central galaxies in IllustrisTNG

TL;DR

This study uses the IllustrisTNG simulation to show that galaxy mergers are neither necessary nor sufficient for quenching star formation in central galaxies, emphasizing the dominance of secular processes.

Contribution

It provides a comprehensive analysis demonstrating that mergers do not predominantly cause quenching, highlighting the importance of secular processes in galaxy evolution within the simulation.

Findings

Only 3% of major mergers lead to quenching within 1 Gyr.

Most quenching events are not preceded by recent mergers.

Secular processes dominate black hole growth and galaxy quenching.

Abstract

The cessation of star formation in galaxies, known as 'quenching', is a complex, multi-scale process which has been theorized to be linked to galaxy mergers. In this paper, we investigate the potential role of mergers in quenching galaxies in the IllustrisTNG cosmological hydrodynamical simulation. We track the evolution of over 11,000 central galaxies in the simulation with stellar mass $M_\star \ge 10^9 M_\odot$ at $z = 0$ throughout the entirety of cosmic history. We compare their star formation and merger histories to test whether mergers are necessary or sufficient for inducing quenching in the simulation. Only a very small fraction of mergers (about 3 per cent of major mergers and about 12 per cent of all mergers) lead to quenching within 1 Gyr, indicating that mergers are not sufficient by themselves to cause quenching. Furthermore, the vast majority of quenching events are not preceded by a merger within 1 Gyr. Once random coincidences are accounted for and a stellar mass-matched control sample is applied, no merger excess is observed. Hence, mergers are clearly not necessary for quenching to occur in the simulation. Finally, we perform a series of random forest classification and regression analyses to assess the integrated role of mergers in galaxy quenching and supermassive black hole growth in IllustrisTNG. We determine that secular processes dominate the growth of supermassive black holes and the quenching of central galaxies in this simulation, in stark contrast to prior theoretical expectations from idealized hydrodynamical simulations.