  Planets orbiting within the close-in habitable zones of M dwarf stars will be
exposed to elevated high-energy radiation driven by strong magneto-hydrodynamic
dynamos during stellar youth. Near-ultraviolet (NUV) irradiation can erode and
alter the chemistry of planetary atmospheres, and a quantitative description of
the evolution of NUV emission from M dwarfs is needed when modeling these
effects. We investigated the NUV luminosity evolution of early M-type dwarfs by
cross-correlating the Lepine & Gaidos (2011) catalog of bright M dwarfs with
the GALEX catalog of NUV (1771-2831A) sources. Of the 4805 sources with GALEX
counterparts, 797 have NUV emission significantly (> 2.5 sigma) in excess of an
empirical basal level. We inspected these candidate active stars using
visible-wavelength spectra, high-resolution adaptive optics imaging,
time-series photometry, and literature searches to identify cases where the
elevated NUV emission is due to unresolved background sources or stellar
companions; we estimated the overall occurrence of these "false positives" as
~16%. We constructed a NUV luminosity function that accounted for false
positives, detection biases of the source catalogs, and GALEX upper limits. We
found the NUV luminosity function to be inconsistent with predictions from a
constant star-formation rate and simplified age-activity relation defined by a
two-parameter power law.
