The insubordinate cluster of Milky Way
The observations made with the Gemini Multi-Object Spectrograph (imaging mode) confirmed that the globular cluster is one of the less visible clusters ever found in our galaxy.
A group of astronomers led by Dongwon Kim, Australian National University (ANU), found a small and distant cluster of stars that seems strangely out of place. Kim 2 has been localized to the extreme boundaries of our galaxy, the Milky Way, right where the experts have never observed a cluster so small. The discovery was made as part of the Milky Way Satellite Stromlo Survey carried out by ANU.
“The light of this cluster is faint, very faint,” said Kim, “and is 10 times more distant than the average of the other star clusters in the halo surrounding the Milky Way,” said Dougal Mackey.
Think of globular clusters as in the cities of spherical stars forming a large and extended halo around the core of our galaxy: the brightest cluster can be easily observed with amateur telescopes. However, to find Kim 2 was necessary to use several of the largest and most powerful telescopes in the world. The small cluster Kim 2 was localized using the Dark Energy Camera (Decam) mounted on the Blanco Telescope (4 meters) of the Cerro Tololo Inter-American Observatory (CTIO) in Chile. “This discovery sheds new light on the formation and evolution of the Milky Way “said Daniel Evans, program director of the National Science Foundation for the Gemini Observatory. “It is nice to see so many telescopes that combine to produce this result, as well as the Gemini Observatory with its incredible power.”
The ultra-deep observations made with the Gemini Multi-Object Spectrograph (imaging mode) have confirmed that the new globular cluster is one of the less visible clusters ever found in the Milky Way. Seven other globular clusters are relatively low light, but this is definitely the farthest ever observed on the edge of our galaxy. Kim 2 has 10 to 20 times less stars than any other external storage in the halo. Also, the density of its stars is less than half compared to the other clusters in the same brightness range.
The existence of the cluster has been confirmed for the first time studying an area of 500 square degrees observed with the Decam. “These objects are optically elusive to be seen with the naked eye,” emphasized Helmut Jerjen. In the image above, the stars in the cluster appear as small splashes of paint ‘and you look through them without notice them at all. They hide in the sea of stars in the Milky Way. The sophisticated computer programs are our tools to find them,” he added.
The images show evidence of a significant loss of mass during the history of Kim 2. Computer simulations predict that, as a result of their evolution over many billions of years, including the slow loss of some stars because of gravitational force of the Milky Way, star clusters like should be organized in such a way that their most massive stars are concentrated towards the center. “This ‘mass segregation’ was difficult to observe, especially in clusters of low mass, but the data collected by Gemini revealed that Kim 2 seems to have lost much of its initial mass,” added Gary Da Costa. What we understand from this? A considerable number of globular clusters of low luminosity already existed when the galaxy was young, but most of them could be evaporated due to complex internal dynamic processes.
So why Kim 2 survived without disappearing into thin air? One possible scenario is that Kim 2 is not actually a real member of the family of the Milky Way globular, but a cluster of stars originally located in a dwarf satellite galaxy that then later was absorbed by the Milky Way. In support of this hypothesis, experts said that the stars Kim 2 seem richer in heavier elements than other globular clusters in the halo outside and are relatively younger than the oldest globular clusters in the Milky Way. Kim 2, then, would have escaped by the destructive tidal influence forces of the Milky Way, surviving until the present time.
The discovery of Kim 2 suggests that there are still a large number of astronomical objects to be discovered in the Southern Hemisphere and is precisely the aim of the researchers working at Stromlo Milky Way Satellite Survey.