An image of Hubble Space Telescope shared this week by The NASA shows an unusual interaction of four dwarf galaxies. There are two small galaxies that are so close together that they appear to be one object, called NGC 1741, located at the top of the image. Then there is another cigar-shaped galaxy on the near right, and a fourth galaxy on the lower left that is connected to the other three by a stream of young stars.
Together, the four galaxies form an ensemble called Hickson Compact Group 31, or HCG 31. The group is 166 million light-years distant from Earth, which is relatively close to seeing interacting dwarf galaxies. The galaxies are currently so close together, less than 75,000 light-years from each other, that all four would fit inside the Milky Way.
This image is a revised version of an image originally released in 2010, which has been processed to highlight the star-forming regions in the cluster. As the gravitational forces of the galaxies’ mass interact, this stimulates the formation of stars, which glow blue when young.
Dwarf galaxy mergers are usually seen very far away, meaning they are very old, but this group is comparatively young. Astronomers were able to use the Hubble data to narrow down their positions to see when the galaxies started interacting, a few hundred million years ago, and predict when they would eventually merge.
“This is a clear example of a group of galaxies on their way to a merger because there’s so much gas that it’s going to mix everything together,” study lead author Sarah Gallagher said in an email. release when the image was posted.
“The galaxies are relatively small, comparable in size to the Large Magellanic Cloud, a satellite galaxy of our Milky Way. Their speeds, measured from previous studies, show that they move very slowly relative to each other, at just 134,000 miles per hour (60 kilometers per second). So it’s hard to imagine how this system wouldn’t end up as a single elliptical galaxy in another billion years.”