ESA Science & Technology - Colliding galaxies: triggering stellar activity

Far from being 'island universes' as was once thought, galaxies are now known to form and evolve by interacting with their environment and with neighbouring galaxies. Collisions and mergers are believed to be key mechanisms involved in this process. The gravitational interactions between galaxies produce giant tidal waves in the discs of the galaxies, giving rise to spiral arms and provoking outbursts of star formation which can be traced in X-rays. The Antennae Galaxy is one of the closest and most studied galaxy mergers in the Universe and consists of two gravitationally interacting galaxies that collided with one another a couple of hundred million years ago. This galaxy system has a high rate of supernova explosions which heats the gas to millions of degrees so that it glows in X-rays.

The left hand panel shows a false-colour XMM-Newton EPIC image of the interacting Antennae galaxies overlaid with a greyscale optical Digitised Sky Survey image. The right hand panel shows an enlargement around the interacting region in the X-ray only. The EPIC image is false-coloured as follows: Soft (0.2-1.0 keV) photons are shown in red, medium (1.0-2.0 keV) photons are shown in green and hard (2.0-12 keV) photons are shown in blue. A complex pattern of extended emission can be seen in the lowest energy band whilst higher energy photons are emitted in parts of the inner regions. Areas with a lot of high-energy emission can be identified as bluish in the image.