Binary Stars as the Source of the Far-UV Excess in Elliptical Galaxies

TL;DR

This paper proposes that binary star interactions produce hot helium-burning stars responsible for the far-UV excess in elliptical galaxies, challenging previous age-related explanations and suggesting a binary evolution origin.

Contribution

It demonstrates that binary interactions are the likely source of hot stars causing the UV excess, revising the understanding of elliptical galaxy evolution.

Findings

Binary interactions produce hot helium-burning stars.

UV excess is not strongly dependent on metallicity.

UV excess is not an indicator of galaxy age.

Abstract

The discovery of an excess of light in the far-ultraviolet (UV) spectrum in elliptical galaxies was a major surprise in 1969. While it is now clear that this UV excess is caused by an old population of hot helium-burning stars without large hydrogen-rich envelopes rather than young stars, their origin has remained a mystery. Here we show that these stars most likely lost their envelopes because of binary interactions, similar to the hot subdwarf population in our own Galaxy. This has major implications for understanding the evolution of the UV excess and of elliptical galaxies in general. In particular, it implies that the UV excess is not a sign of age, as had been postulated previously, and predicts that it should not be strongly dependent on the metallicity of the population.