Lying about 500 million light-years away in the constellation of Sculptor, the cartwheel shape of this galaxy is the result of a violent galactic collision. A smaller galaxy has passed right through a large disk galaxy and produced shock waves that swept up gas and dust – much like the ripples produced when a stone is dropped into a lake. The expanding wave of increased density creates a ring of compressed gas that forms stars at a furious rate.
This object is one of the most dramatic examples of the small class of ring galaxies, making up about 0.1% of disk galaxies in the local Universe. The estimated collision happened about 200 million years ago.
Most images of the Cartwheel display three galaxies close together. The smaller ones are named G1 and G2. A fourth physically associated companion known as G3 (out of the frame) is widely believed to be the “bullet” galaxy that plunged through the disk of the cartwheel, creating its current shape, not G1 or G2. Credit: ESA/Hubble & NASA
Cartwheel Galaxy Group. In this picture G3 is also seen at the far right. It is a composite of a visual image superposed with high resolution radio observations of neutral hydrogen (traced by the green contours). The neutral hydrogen trail suggestively leads to the culprit galaxy G3 at the far right, presently about 250,000 light years distant from the Cartwheel.
Credit: J.Higdon (NRAO), C.Struck, P.Appleton (ISU), K.Borne (Hughes STX), R.Lucas (STScI), NASA
How to make a ring galaxy
This animation shows the formation of a ring galaxy, when a smaller galaxy collide with a much larger disc galaxy, passing right through it’s center. The collision shock waves sweep up gas and dust – much like the ripples produced when a stone is dropped into a lake. The expanding wave of increased density creates a ring of compressed gas that forms stars at a furious rate. Credit: James Josephides, Swinburne Astronomy Productions
Cartwheel Galaxy Credit: NASA, Hubble Space Telescope
Here is another ring galaxy. Hoag’s Object is a perfect example of a ring galaxy, about 600 million light years away. The blue ring, which is dominated by clusters of young, massive stars, contrasts sharply with the yellow nucleus of mostly older stars. Curiously, an object that bears an uncanny resemblance to Hoag’s Object can be seen in the gap at the one o’clock position. The object is probably a background ring galaxy. Credit: NASA and The Hubble Heritage Team (STScI/AURA); Acknowledgment: Ray A. Lucas (STScI/AURA)