The Cartwheel Galaxy, ESO 350-40
The Cartwheel Galaxy is a distant galaxy located 400 million light years away in the southern constellation Sculptor. It features a unique ring structure, most likely as a result of a head-on collision 200 million years ago with a smaller intruder galaxy that passed though the very core of the larger galaxy. The density wave set in motion by the passage of the intruder is visible as intense starburst activity along the edge of a massive ring that is slowly making its way outwards from the centre, like a ripple on a pond.
The galaxy is part of a small group of four, with the other members being the smaller blue and yellow galaxies right next to the Cartwheel and another companion a little further north (down). The latter is believed to be the intruder galaxy. In fact, high resolution radio observations have identified a trail of neutral hydrogen gas between the Cartwheel and the intruder, strongly suggesting that this is indeed the culprit now fleeing the scene.
Images from the Hubble Space Telescope show the bright knots in the ring to be giant clusters of super luminous young blue stars. These bright stars will live and die in a few million years, well before the density wave has moved on, and new stars will continue to be born of the same recycled matter in this chaotic setting of intense starburst activity and cataclysmic supernovae brought about by a cosmic encounter 200 million years ago.
Eventually the wave will dissipate and fade out into the outer regions of the galaxy where the gas and dust is too thin for new stars to be born. Looking like the spokes of a giant wheel, faint spiral arms can already be seen beginning to form again after the collision. The galaxy will probably take on the form of a normal spiral again in the future.
Looming in the background of the image are large numbers and groupings of faint distant galaxies, visibly clumping together and forming the large scale superstructure of the Universe. Many of these lie in the neighbourhood of the Sculptor Wall, a gigantic structure of galaxy clusters that stretches outwards for more than a billion light years.
Date: 22nd September and 9th/10th/11th October 2012
Exposure: LRGB: 630:70:65:65m, total 13hrs 50mins @ -30C
Telescope: 10" Serrurier Truss Newtonian f/5
Camera: QSI 683wsg with Lodestar guider
Filters: Astrodon LRGB E-Series Gen 2
Taken from my observatory in Auckland, New Zealand