An Improved UV-Optical Diagnostic for Rejuvenating Galaxies in the Local Universe and Implications for Galaxy Evolution

Rejuvenating galaxies are important probes of galaxy evolution, yet identifying them observationally is challenging as constraining recent star formation histories requires both photometric and spectroscopic data. We present a method for identifying rejuvenating galaxies in the local Universe using ultraviolet (UV) imaging and optical spectroscopy, building on a recent selection that identifies a system as rejuvenating if it is quenched in the near-UV (NUV; tracing $\sim\!100\,\mathrm{Myr}$ timescales) but star-forming in H$α$ (tracing $\sim\!10\,\mathrm{Myr}$ timescales). Shortly after a star formation episode, however, the NUV is dominated by the same massive stars that power H$α$, so these indicators do not always trace distinct timescales. To address this, we derive a relation that predicts the NUV emission associated with the ionizing O-star population traced by H$α$, enabling us to isolate the NUV contribution from longer-lived stars (primarily B/A stars with $M\lesssim\!20\,M_\odot$). Subtracting the predicted O-star NUV from the dust-corrected NUV yields a more reliable rejuvenation diagnostic. Using this method, we identify $\sim\!10^{4}$ rejuvenating galaxies in a sample of Sloan Digital Sky Survey (SDSS) galaxies ($\sim\!4.5\%$). These galaxies have intermediate stellar masses and are found primarily in lower-density environments, becoming increasingly rare toward the centers of groups and clusters. Rejuvenating galaxies also exhibit systematically lower gas-phase metallicities, consistent with fueling by the accretion of metal-poor gas.

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