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

Abstract

The observed breakBRD ("break bulges in red disks") galaxies are a nearby sample of faceon 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 (∼4% at z = 0; ∼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 * /M < 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 ∼2 Gyr. The observed breakBRD galaxies may therefore be a unique sample of outside-in quenching galaxies.