Using Machine Learning to Estimate Near-ultraviolet Magnitudes and Probe Quenching Mechanisms of z = 0 Nuggets in the RESOLVE and ECO Surveys

We present a z = 0 census of nuggets—compact galaxies that form via gas-rich violent disk instability—within the luminosity- and volume-limited RESOLVED Spectroscopy Of a Local VolumE (RESOLVE) and Environmental COntext (ECO) surveys. We use random forest (RF) models to predict near-ultraviolet (NUV) magnitudes for ECO galaxies that lack high-quality NUV magnitudes, thereby doubling the number of ECO galaxies with reliable extinction-corrected star formation rates (SFRs) and red/green/blue classifications based on specific SFRs (sSFRs). The resulting RF-enhanced RESOLVE+ECO nugget sample allows us to analyze rare subpopulations—green nuggets and nuggets with active galactic nuclei (AGN)—likely associated with quenching. Green nuggets are more similar to red nuggets than to blue nuggets in halo mass (Mhalo) distribution, with both red and green nuggets being found mainly at Mhalo ≥ 1011.4M⊙, where permanent halo quenching is predicted. At these masses, the AGN frequency for green nuggets is higher (48.2% −5.3%5.3% ) than for either blue (39.2% −2.8%2.9% ) or red (29.3% −2.8%3.0% ) nuggets. Between Mhalo = 1011.4–1012M⊙, at the onset of permanent quenching, the AGN frequency for green nuggets is nearly double the frequency for blue or red nuggets, implying AGN are associated with this transition. At Mhalo < 1011.4M⊙, where temporary cyclic quenching is expected, the AGN frequency for blue nuggets (7.5% −1.2%1.4% ) is lower than for either green (31.3% −7.5%8.7% ) or red (18.8% −7.8%11.5% ) nuggets. At all masses, nuggets with AGN have reduced sSFRs and likely also atomic gas content compared to nuggets without AGN, but the quenching is more extreme below Mhalo = 1011.4M⊙.