The habitability trade-off: Chemical decoupling and quenching in massive galaxies

TL;DR

This study uses the IllustrisTNG simulation to identify a significant subpopulation of massive galaxies with stellar-gas chemical decoupling, revealing a complex trade-off between reduced star formation and increased habitability potential.

Contribution

It uncovers the prevalence of chemically decoupled galaxies and explores their implications for galaxy habitability, highlighting a transient phase with unique properties.

Findings

Approximately 31.5% of massive galaxies show stellar-gas decoupling.

Chemically decoupled galaxies have suppressed star formation and gas fractions.

Despite dilution, these galaxies exhibit higher habitability proxy values.

Abstract

Massive galaxies experience complex evolutionary processes, including mergers and gas accretion, which can disrupt the chemical equilibrium between their stellar and gaseous components. Using the IllustrisTNG (TNG100) simulation at $z=0$, we investigated the prevalence and physical properties of such chemically decoupled systems within the massive star-forming galaxy population. We identify a substantial subpopulation ($\sim 31.5\%$ of the sample) that exhibits systematic stellar-gas decoupling, characterised by a metal-rich stellar component coexisting with a diluted gas reservoir. These non-equilibrium galaxies are closely linked to recent merger activity and partial quenching, and display systematically suppressed star-formation rates and reduced gas fractions, consistent with a transitional evolutionary phase. We then examined the implications of this phase for galaxy-scale habitability prescriptions by applying a terrestrial planet abundance proxy that combines stellar mass, gas-phase metallicity, and the rate of sterilising events. Despite their diluted gas reservoirs, non-equilibrium galaxies dominate the high end of the inferred present-day habitability proxy distribution, exceeding equilibrium systems by more than an order of magnitude. We interpret this as a habitability trade-off: the same gas dilution and quenching processes that reduce the efficiency of future terrestrial planet formation simultaneously create a transient phase of suppressed radiation hazards for existing planets. The Andromeda galaxy (M31) shows qualitative similarities to this chemically decoupled population, suggesting that galaxies exiting their peak star-forming phase represent a distinct and highly relevant demographic for galaxy-scale habitability. Galactic habitability is therefore intrinsically time-dependent.