ultravioletexcess

Ultravioletexcess is a term used in astronomy to describe an observed surplus of ultraviolet emission from an astronomical object relative to its emission at longer wavelengths. It is closely related to, and often used interchangeably with, ultraviolet excess (UVX). In practice, ultravioletexcess is quantified by the ratio of ultraviolet flux to optical flux or by color indices that appear bluer than expected for a given optical color.

In stars, ultravioletexcess helps identify hot, young, or metal-poor populations, including certain halo stars and blue horizontal-branch stars. In galaxies and active galactic nuclei, ultraviolet excess can indicate recent or ongoing star formation or the presence of an accretion disk around a supermassive black hole. The interpretation of ultravioletexcess depends on context and models of stellar atmospheres or galaxy spectra.

Measurement and interpretation typically rely on photometric data from ultraviolet and optical bands, such as ultraviolet space-based surveys or ground-based U-band measurements, compared with B- and V-band data. Corrections for interstellar extinction, reddening, and redshift are essential for accurate assessment. The term encompasses diverse physical causes: hot stellar components, young stellar populations, or non-stellar emission from accretion processes.

Limitations include sensitivity to dust, distance, and calibration uncertainties, which can mimic or obscure true ultraviolet excess. Ultravioletexcess remains a useful diagnostic in surveys seeking hot or young components, helping to classify objects and study star formation histories, stellar evolution in unusual populations, and the energetic processes in galactic nuclei. Related concepts include the UV upturn, blue horizontal-branch stars, and UV–optical color diagnostics.