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

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. In this paper, we search for breakBRD analogs in the IllustrisTNG simulation and describe their history and future. We find that a small fraction (similar to 4% at z = 0; similar to 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 analog 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 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 similar to 2 Gyr. The observed breakBRD galaxies may therefore be a unique sample of outside-in quenching galaxies.