Following the launch of NASA’s GALEX satellite some twenty years ago, we were surprised to discover that 30% of all galaxies have a very extended ultraviolet (UV) disk. These galaxies were dubbed “eXtended UV” (XUV). Other highly extended galaxies are “giant” galaxies with low surface brightness, such as Malin 1. These giant galaxies, initially observed at wavelengths in the visible range, also appeared to be XUV when observed in the ultraviolet.
A team at LAM has now asked the opposite question: are XUV galaxies systematically “giant” also in the visible, when we have data deep enough to study the question? To answer this question, the team gathered 9 galaxies for which GALEX data is available in the UV, but also data at visible wavelengths with the very deep “DES” survey.
They found that 3 XUV galaxies resemble giant galaxies with low surface brightness, while the others are normal or even have completely opposite structures. Despite this diversity, the team observed a certain continuity between the different categories when using a parameter measuring the diffuse (or non-diffuse) nature of the outer parts…
However, one galaxy stood out from the rest: its UV-extended disk harbors a stellar population older than the inner parts. Thanks to deep images, the team noticed the presence of shells in the outer parts of the galaxy, as well as a double core (see illustration), leading to the conclusion that this galaxy is the product of a rare phenomenon: the merger of two gas-rich dwarf galaxies.
This work was carried out as part of Eloïse Bernaud’s doctoral thesis, and is the subject of an article recently accepted in A&A (https://arxiv.org/abs/2506.11568).
