A black hole and a star cluster are the survivors of a galaxy merger

The galaxy ESO 243-49. The circle pinpoints the black hole XLX-1 (Image NASA, ESA, and S. Farrell (University of Sydney, Australia and University of Leicester, UK))

The galaxy ESO 243-49. The circle pinpoints the black hole XLX-1 (Image NASA, ESA, and S. Farrell (University of Sydney, Australia and University of Leicester, UK))

The Hubble Space Telescope has allowed the discovery of a cluster of young blue stars around a black hole that was called HLX-1 (Hyper-Luminous X-ray source 1), which has a mass estimated at 20,000 times the Sun. This black hole is at the edge of a galaxy denominated ESO 243-49, distant about 290 million light years from Earth.

The black hole HLX-1 was discovered in 2009 by a team led by astronomer Sean Farrell using the ESA X-ray space telescope XMM-Newton and was the first black hole of intermediate mass discovered. Scientists believe that black holes of this class can provide a basis for supermassive black holes at the center of galaxies like the Milky Way. To get an idea of the ​​proportions, the supermassive black hole at the center of our galaxy has a mass estimated at four million times the Sun.

It was clear from the beginning that HLX-1 deserved special observation, in fact scientists started studying it using the Hubble Space Telescope as well as the Swift Space Telescope. In this way, it was possible to observe the emission of ultraviolet, visible and infrared light from HLX-1.

The images from the Hubble Space Telescope showed an interesting excess of red light that can’t be explained by emission from an accretion disc, the normal structure formed by material falling into a gravitational source such as a black hole. According to scientists, that light is the proof of the existence of a cluster of stars surrounding the black hole because the brightness and light color are similar to those of star clusters in nearby galaxies.



The existence of a cluster of stars around the black hole HLX-1 provides a clue about the origin of the black hole mass and why it’s in the galaxy ESO 243-49 limits. According to Dr. Sean Farrell that black hole was originated at the center of a dwarf galaxy that was later swallowed by a larger one, giving birth to the current one.

The black hole HLX-1 and part of the material around it survived the merger but now its future is uncertain. Its trajectory is presently unknown and further observations are required to determine it. It’s possible that HLX-1 will get closer to the center of the galaxy to merge with the supermassive black hole that already exists or it could remain in a stable orbit at the edge of the galaxy ESO 243-49 and in this case its X-ray brightness will gradually fade with the consumption of the material in its surroundings.

This case is particularly interesting because it shows the consequences of a merger of two galaxies, a phenomenon which in future will also involve the Milky Way, which in a few billion years will merge with Andromeda to form a giant galaxy.

About Massimo Luciani


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