UVX

UVX stands for ultraviolet excess, an astronomical term describing an observed surplus of ultraviolet radiation from certain galaxies and active galactic nuclei beyond what is predicted by their visible-light spectra. The most notable example is the ultraviolet upturn observed in many elliptical and lenticular galaxies, where the spectral energy distribution rises toward shorter ultraviolet wavelengths despite an older, seemingly metal-rich stellar population.

Observations of UVX come from ultraviolet imaging and spectroscopy obtained with space-based instruments such as the International Ultraviolet Explorer, the Hubble Space Telescope, and the GALEX mission. UVX signals in galaxies are typically studied in the far-ultraviolet and near-ultraviolet bands and are analyzed in the context of the galaxies’ ages, metallicities, and star-formation histories. The strength of the UVX can vary with galaxy properties and environment, providing a diagnostic of underlying stellar populations.

Origins and interpretation remain active areas of research. The leading explanation attributes UVX to hot, evolved low-mass stars, including extreme horizontal-branch stars and their descendants, with mass loss and binary interactions potentially playing a role. Other contributors, such as post-asymptotic-giant-branch stars, may also contribute to the observed UV flux. The precise mix of sources likely varies among galaxies.

Significance of UVX lies in its utility for testing and refining models of stellar population synthesis, constraining the ages and metallicities of early-type galaxies, and improving understanding of late stages of stellar evolution. The acronym UVX may appear in other contexts outside astronomy, but this article focuses on ultraviolet excess in astrophysical objects.