English: This unique image from ESO’s VLT Survey Telescope (VST) reveals two galaxies at the very beginning of the merging process. The interactions between the duo have created a rare effect known as a light echo, where light reverberates around the material within each galaxy. This is analogous to the acoustic echo where the reflected sound arrives at the listener with a delay after the direct sound. This is the first case of a light echo observed between two galaxies.

The larger galaxy, seen here in yellow, is ShaSS 073 — an active galaxy with an extremely luminous core. Its less massive companion, in blue, is named ShaSS 622, and together the pair make up the intriguing ShaSS 622-073 system. The bright core of ShaSS 073 is exciting a region of gas within the disc of its blue companion: it bombards the material there with radiation, causing it to glow brightly as it absorbs and then re-emits this light. This glowing region extends across 1.8 billions of square light-years.

However, while studying this merger, astronomers found the luminosity of the large central galaxy to be 20 times lower than needed to excite the gas in this way. This indicates that the centre of ShaSS 073 has dramatically faded over the last 30 000 years or so — but the highly-ionised region between the two galaxies still retains the memory of its former glory.