Hickson Compact Group 31 (also known as NGC 1741 Group) is a group of interacting galaxies consisting of eight galaxies: the Wolf-Rayet galaxy NGC 1741 (actually two colliding dwarf galaxies) and its irregular dwarf companions. The entire group spans about 150 thousand light-years, and is located about 166 million light-years away in the constellation Eridanus. The distorted galaxies are quickly producing massive, hot, young stars that are pumping out ultraviolet radiation, heating up surrounding gas clouds, and causing them to glow.
Compact groups are small systems of galaxies, with each member apparently in close proximity to the other members. They are presumed to be physically associated and often show morphological peculiarities indicative of gravitational interaction. Hickson Compact Group 31 is one of 100 compact galaxy groups catalogued by the Canadian astronomer Paul Hickson. Compact groups offer a window into what commonly happened in the Universe’s formative years when large galaxies were created from smaller building blocks.
The slowly merging galaxies of HCG 31 waited billions of years to come together, forming thousands of new star clusters. The oldest stars in a few of its ancient globular clusters are about 10 billion years old, while the brightest clusters, hefty groups each holding at least 100,000 stars, are less than 10 million years old. The entire system is rich in hydrogen gas, the stuff of which stars are made.
The bright, distorted object at middle, left, is NGC 1741, consisting of NGC 1741A and 1741B. The numerous bluish star clusters have formed in the streamers of debris pulled from the galaxies and at the site of their head-on collision. The cigar-shaped object above the galaxy duo is another member of the group. A bridge of star clusters connects the trio. A longer rope of bright star clusters points to a fourth member of the group, at lower right. The bright object in the center is a foreground star.
The interacting galaxies of Hickson Compact Group 31 will continue to destroy each other, millions of stars will form and explode, and thousands of nebulae will form and dissipate before the dust settles and the galaxies will form one large elliptical galaxy about one billion years from now.
Such encounters between dwarf galaxies are normally seen billions of light-years away and therefore occurred billions of years ago. But the galaxies of Hickson Compact Group 31 are relatively nearby, what makes them easier to study. The relatively young and bright star clusters within HCG 31, allows astronomers to calculate the clusters’ age, trace the star-formation history, and determine that the galaxies are undergoing the final stages of galaxy assembly.
This composite image was composed from observations made by the Hubble Space Telescope’s Advanced Camera for Surveys, NASA’s Spitzer Space Telescope, and the Galaxy Evolution Explorer (GALEX).
Image Credit: NASA, ESA, S. Gallagher (The University of Western Ontario), and J. English (University of Manitoba)