Minor merger growth in action: JWST detects faint blue companions around massive quiescent galaxies at 0.5 < z < 3

TL;DR

This study uses JWST imaging to reveal that massive quiescent galaxies at 0.5 < z < 3 are surrounded by numerous faint, low-mass companions, indicating minor mergers significantly contribute to their evolution.

Contribution

It demonstrates that minor mergers extend down to very low mass ratios (<1:100), revealing a large population of faint companions undetectable by previous HST imaging.

Findings

Massive quiescent galaxies host ~5 low-mass companions within 35 kpc.

Companions with mass ratios <1:10 are bluer and have lower mass-to-light ratios.

Tiny companions contribute at least 30% of the mass growth via minor mergers.

Abstract

Minor mergers are thought to drive the structural evolution of massive quiescent galaxies; however, existing HST imaging is primarily sensitive to stellar mass ratios >1:10. Here, we report the discovery of a large population of low-mass companions within 35 kpc of known logM*/Msun > 10.5 quiescent galaxies at 0.5 < z < 3. While massive companions like those identified by HST are rare, JWST imaging from JADES reveals that the average massive quiescent galaxy hosts ~5 nearby companions with stellar mass ratios <1:10. Despite a median stellar mass ratio of just 1:900, these tiny companions are so numerous that they represent at least 30\% of the total mass being added to quiescent galaxies via minor mergers. While relatively massive companions have colors similar to their hosts, companions with mass ratios <1:10 typically have bluer colors and lower mass-to-light ratios than their host galaxies at similar radii. The accretion of these tiny companions is likely to drive evolution in the color gradients and stellar population properties of the host galaxies. Our results suggest that the well-established ``minor merger growth" model for quiescent galaxies extends down to very low mass ratios of <1:100, and demonstrates the power of JWST to constrain both the spatially-resolved properties of massive galaxies and the properties of low-mass companions beyond the local universe.