The breakBRD Breakdown: Using IllustrisTNG to Track the Quenching of an Observationally-Motivated Sample of Centrally Star-Forming Galaxies

TL;DR

This study uses the IllustrisTNG simulation to identify and analyze galaxies with centrally concentrated star formation, revealing their properties, formation triggers, and transient nature, and comparing them with observed breakBRD galaxies.

Contribution

First simulation-based analysis of breakBRD galaxies, linking their properties and formation history to observational data and quenching processes.

Findings

BreakBRD analogues are rare (~4% at z=0) and similar to observed galaxies.

BreakBRD galaxies are in the process of quenching and have short-lived states.

Approximately 10% of quiescent galaxies experienced similar star formation concentration in the past.

Abstract

The observed breakBRD ("break bulges in red disks") galaxies are a nearby sample of face-on disk galaxies with particularly centrally concentrated star formation: they have red disks but recent star formation in their centers as measured by the D$_n$4000 spectral index (Tuttle & Tonnesen 2020). In this paper, we search for breakBRD analogues in the IllustrisTNG simulation and describe their history and future. We find that a small fraction (${\sim}4\%$ at $z=0$; ${\sim}1\%$ at $z=0.5$) of galaxies fulfill the breakBRD criteria, in agreement with observations. In comparison with the mass-weighted parent IllustrisTNG sample, these galaxies tend to consist of a higher fraction of satellite and splashback galaxies. However, the central, non-splashback breakBRD galaxies show similar environments, black hole masses, and merger rates, indicating that there is not a single formation trigger for inner star formation and outer quenching. We determine that breakBRD analogue galaxies as a whole are in the process of quenching. The breakBRD state - with its highly centrally concentrated star formation - is uncommon in the history of either currently quiescent or star-forming galaxies; however, approximately 10% of $10^{10} < M_\ast/M_{\odot} < 10^{11}$ quiescent galaxies at $z=0$ have experienced SFR concentrations comparable to those of the breakBRDs in their past. Additionally, the breakBRD state is short-lived, lasting a few hundred Myr up to ${\sim}2$ Gyr. The observed breakBRD galaxies may therefore be a unique sample of outside-in quenching galaxies.