Jérémy Blaizot

Two galaxies approach each other like a pair of dancers who drift apart and then are drawn back together. At the moment of a particularly close encounter, their gas compresses, and bright ultraviolet light bursts from within. This is exactly what happens in the Haro 11 system — the nearest pair of merging galaxies that lets such light escape.

A galaxy is like a house with drawn curtains: bright ultraviolet light is born inside, but from outside you only see what manages to sneak through dust and gas. Astronomers need to know how much of this light escapes into intergalactic space, because it ionizes hydrogen in the early Universe and makes it transparent.