Galaxy collisions are spectacular in their own right, but even among them there are specimens that are just unbelievable. This is the case of NGC 6240, which is a merger of two active supermassive black holes, heaps of newborn stars, and vast circumgalactic gas structures.
An international team, led by researchers at Hiroshima University in Japan using the Subaru Telescope in Hawaii, was able to show exactly what these gas structures look like. The complex configuration is due to galaxy-wide winds in NGC 6240. The quality and detail of the image is unprecedented, and it indicates an intense and widespread star-formation.
The galactic winds are generated by the stars near the center of NGC 6240. Young stars emit strong ultraviolet light that propels gas and dust forward, and supernova explosions add to the pressure blowing gas away from the galaxy and into intergalactic space. X-ray observations indicate that the galaxy has two supermassive black holes at the center, which increase the energy output from the core and the strength of the wind.
These winds are hot and carry lots of heavier elements, and can significantly affect the galaxy as a whole. Superwinds like the one in NGC 6240 could quickly (in cosmic terms) disperse a galaxy’s gas reservoir and choke off new star formation.
The giant ionized gas nebula created by the superwinds of NGC 6240. Hiroshima University / NAOJ
NGC 6240 is one of the better-studied mergers in the universe. It is located 350 million light-years away, and it produces between 25 to 80 time more stars than the Milky Way every year. The star-formation is so intense that NGC 6240 is part of the class of galaxies known as starbursts. The starburst is connected to its peculiar shape; NGC 6240 is a merger of two spiral galaxies, and the intense forces between the galaxies are compressing gas clouds, forcing new stars to form.
The outflow structure stretches for about 300,000 light-years and it contains loops, filaments, and blobs. This complex arrangement is an indication of how the star-formation and consequent galactic wind have not been constant. The team thinks that NGC 6240 has experienced at least three violent bursts of new stars, with the oldest having started 80 million years ago. As the merger is believed to have commenced at least 1 billion years ago, these episodes belong to the late stages of the merger.
NGC 6240 will continue to merge, and it will eventually become an elliptical galaxy with very little star formation, passively aging until it collides with another galaxy.