Wet Compaction to a Blue Nugget: a Critical Phase in Galaxy Evolution

TL;DR

High-resolution simulations reveal that galaxies undergo a critical 'wet compaction' phase at a stellar mass of about 10^{10} solar masses, transforming their structure, dynamics, and star formation activity, leading to the formation of compact 'blue' and 'red' nuggets.

Contribution

This study introduces the concept of wet compaction as a key phase in galaxy evolution, linking it to observable properties and black hole growth, based on detailed cosmological simulations.

Findings

Galaxies experience a gaseous shrinkage to form 'blue nuggets'.

Post-compaction, galaxies develop extended gaseous discs and rings.

Black hole growth is enhanced during the compaction phase.

Abstract

We utilize high-resolution cosmological simulations to reveal that high-redshift galaxies tend to undergo a robust `wet compaction' event when near a `golden' stellar mass of $\sim 10^{10} M_{\odot}$. This is a gaseous shrinkage to a compact star-forming phase, a `blue nugget' (BN), followed by central quenching of star formation to a compact passive stellar bulge, a `red nugget' (RN), and a buildup of an extended gaseous disc and ring. Such nuggets are observed at cosmic noon and seed today's early-type galaxies. The compaction is triggered by a drastic loss of angular momentum due to, e.g., wet mergers, counter-rotating cold streams, or violent disc instability. The BN phase marks drastic transitions in the galaxy structural, compositional and kinematic properties. The transitions are from star-forming to quenched inside-out, from diffuse to compact with an extended disc-ring and a stellar envelope, from dark matter to baryon central dominance, from prolate to oblate stellar shape, from pressure to rotation support, from low to high metallicity, and from supernova to AGN feedback. The central black hole growth, first suppressed by supernova feedback when below the golden mass, is boosted by the compaction, and the black hole keeps growing once the halo is massive enough to lock in the supernova ejecta.