




Ring galaxies are an especially striking example of how collisions between galaxies can dramatically change their structure, while also triggering the formation of new stars. They arise from a particular type of collision, in which one galaxy (the "intruder") plunges directly through the disk of another one (the "target"). In the case of AM 0644-741, the galaxy that pierced through the ring galaxy is out of the image but visible in larger-field images. The soft spiral galaxy that is visible to the left of the ring galaxy in the image is a coincidental background galaxy that in the same group as the other two, but is not interacting directly with the ring.

The resulting gravitational shock imparted due to the collision drastically changes the orbits of stars and gas in the target galaxy's disk, causing them to rush outward, somewhat like ripples in a pond after a large rock has been thrown in. As the ring plows outward into its surroundings, gas clouds collide and are compressed. The clouds can then contract under their own gravity, collapse, and form an abundance of new stars.

The rampant star formation explains why the ring is so blue: It is continuously forming massive, young, hot stars, which are blue in color. Another sign of robust star formation is the pink regions along the ring. These are rarefied clouds of glowing hydrogen gas, fluorescing because of the strong ultraviolet light from the newly formed massive stars.


The Hubble Heritage Team used the Hubble Advanced Camera for Surveys to take this image in January 2004. The team used a combination of four separate filters that isolate blue, green, red, and near-infrared light to create the color image.

Image Credit: NASA, ESA, and The Hubble Heritage Team (AURA/STScI) Acknowledgment: J. Higdon (Cornell U.) and I. Jordan (STScI)