Astronomers have recently discovered a potentially existing intermediate-mass black hole that is currently consuming a star. This level of black hole is extremely rare and contributes to further understanding these mysterious celestial bodies.
The United States National Aeronautics and Space Administration (NASA) announced in a press release that astronomers observed this bright X-ray emitting black hole using NASA’s Hubble Space Telescope and the Chandra X-ray Observatory.
Named NGC 6099 HLX-1 (HLX-1), this black hole is located on the outskirts of the giant elliptical galaxy NGC 6099, approximately 40,000 light-years away from the center of the galaxy. The galaxy is situated in the Hercules constellation, about 450 million light-years from Earth.
Since the launch of the Hubble Space Telescope in 1990, astronomers have discovered that the centers of galaxies around the universe may contain supermassive black holes, with individual masses reaching millions to billions of times that of the Sun. Additionally, galaxies may also house up to millions of small black holes, with individual masses less than 100 times that of the Sun, formed when massive stars reach the end of their lifecycles.
The elusive intermediate-mass black holes, like HLX-1, have masses ranging from hundreds to tens of thousands of times that of the Sun. This intermediate size of black holes is challenging to observe as they consume less gas and stars compared to supermassive black holes which emit strong radiation.
How was HLX-1 discovered? Astronomers first detected an unusual X-ray source in images captured by the Chandra X-ray Observatory in 2009. Subsequently, they tracked its evolution using the European Space Agency’s (ESA) XMM-Newton satellite.
Yi-Chi Chang, a doctoral student at Taiwan’s National Tsing Hua University Institute of Astronomy and the lead author of the study, noted that such a bright X-ray source outside the nucleus of a galaxy is rare and serves as a crucial detection result in identifying elusive intermediate-mass black holes. These black holes represent an essential missing link in the evolution of stellar-mass and supermassive black holes.
The X-ray emissions from HLX-1 reach temperatures as high as 3 million degrees, aligning with a tidal disruption event, where a star gets close to a black hole and is torn apart by its strong gravitational forces. Evidence of a small cluster of stars surrounding the black hole was found by the Hubble Space Telescope. Due to the close proximity of these stars, only a few light-months apart, the star cluster can provide sufficient energy to the black hole.
This black hole candidate, possibly classified as an intermediate-mass black hole, reached its peak brightness in 2012 before steadily declining until 2023. Optical and X-ray observations during this period did not overlap, making interpretations more complex. The black hole may have torn apart a captured star, leading to the formation of a plasma (also known as a disk) structure, or possibly a disk that flickers whenever gas falls into the black hole.
Researchers suggest that studying intermediate-mass black holes helps in understanding how larger supermassive black holes initially formed. There are currently two different theories.
The first theory proposes that intermediate-mass black holes serve as seeds for the formation of larger black holes, as giant galaxies are formed through the merging of smaller galaxies. During these merging processes, black holes at the center of galaxies also grow.
The second theory suggests that early gas clouds in the universe do not initially form stars but directly collapse into supermassive black holes. The James Webb Space Telescope by NASA has found that the masses of extremely distant black holes are abnormally large compared to their host galaxies, supporting this viewpoint.
Researchers state that if they are fortunate, they will discover more black holes emitting bright X-rays due to tidal disruption events. Through statistical studies, they can determine the frequency of intermediate-mass black holes consuming stars and how larger galaxies grow by merging smaller ones.
The findings of this study were published in The Astrophysical Journal.