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. Scientists built a computer model of this dance to understand how the leak arises.

In the model, the two progenitor galaxies approached, separated, and approached again. Each time they passed close, the ultraviolet escape fraction grew by roughly a factor of ten. The reason is simple: the gas compressed, and gaps appeared through which light could burst out. Starbursts also boosted the leak — young stars pushed the gas around and punched holes in it.

Curiously, the escape of another kind of light, the Lyman-alpha line, did not grow as much. This light is emitted by hydrogen and often gets trapped in dense gas. So looking at the Lyman-alpha line can make you underestimate how much ultraviolet actually escapes a galaxy. In the model, the brightest source was a dense knot of stars called 'clump C'—it turned out to be the main contributor to the escaping radiation.

The simulated galaxy ended up resembling the real Haro 11—both in shape and brightness. So such computer dances help us understand how, in the distant past, the first galaxies cleared the universe of neutral hydrogen fog.