Approximately 100 million years ago, a smaller galaxy plunged through the heart of the Cartwheel galaxy, creating ripples of brief star formation. In this image, the first ripple appears as an ultraviolet-bright blue outer ring so powerful that it may be one of the most powerful UV-emitting galaxies in the nearby universe.
This false-color composite image shows the Cartwheel galaxy as seen by the Galaxy Evolution Explorer (GALEX), the Hubble Space Telescope (green); the Spitzer Space Telescope (red); and the Chandra X-ray Observatory (purple).
Although astronomers have not identified exactly which galaxy collided with the Cartwheel, two of three candidate galaxies can be seen in this image to the bottom left of the ring, one as a neon blob and the other as a green spiral.
Previously, scientists believed the ring marked the outermost edge of the galaxy, but the latest GALEX observations detect a faint disk, not visible in this image, that extends to twice the diameter of the ring.Image Credit: NASA/JPL-Caltech