Echoes of far away light
A unique shot taken by the European Southern Observatory’s VLT Survey Telescope (VST) uncovers two galaxies right at the beginning of the merging process. The smash up between the duo has created a rare effect known as a light echo, where light bounces off the material within each galaxy. The effect is analogous to the acoustic echo where reflected sound arrives at the listener with a delay after the sound is emitted.
The larger galaxy of the two is seen here in yellow and is known as ShaSS 073. It’s an active galaxy with an extremely luminous, highly energetic core. Meanwhile, its less massive companion, named ShaSS 622, glows in blue and completes the intriguing ShaSS 622-073 system. The pairing’s radiation causes a bright glow as it absorbs and then re-emits light, extending across 1.8 billion square light years.
While studying the merger, astronomers discovered that the luminosity of the large central galaxy is 20-times lower than required to excite the gas in this way – an indication that the centre of ShaSS 073 has faded dramatically over the last 30,000 years or so.