Star eating black hole spotted by Hubble and Chandra

NASA’s Hubble Space Telescope and the Chandra X-ray Observatory have collaborated to identify a potential example of an elusive category of black holes. Known as NGC 6099 HLX-1, this luminous X-ray source appears to be located within a compact star cluster on the outskirts of a massive elliptical galaxy.

Image credit: NASA, ESA, CXC, Yi-Chi Chang (National Tsing Hua University); Image Processing: Joseph DePasquale (STScI)

Star eating black hole spotted by Hubble and Chandra

Since the early 1990s, observations from Hubble have confirmed that most galaxies host supermassive black holes, millions to billions of times the mass of the Sun, at their centers. Additionally, stellar-mass black holes, which form when massive stars die, are known to be common. However, intermediate-mass black holes (IMBHs), ranging from several hundred to a few hundred thousand solar masses, remain rare and difficult to detect.

IMBHs typically do not consume enough surrounding matter to generate the telltale radiation observed from more massive counterparts. They must be observed during rare events in which they disrupt and consume a nearby star, a phenomenon known as a tidal disruption event. During such an occurrence, the black hole emits an intense burst of X-ray radiation.

The candidate IMBH identified in this study is situated about 40,000 light-years from the center of galaxy NGC 6099, which lies approximately 450 million light-years away in the constellation Hercules. The discovery is detailed in a recent publication in the Astrophysical Journal.

The object, HLX-1, was first detected by Chandra in 2009 as a powerful X-ray source. Follow-up observations with ESA’s XMM-Newton space observatory revealed its evolution over time. According to Yi-Chi Chang of National Tsing Hua University in Taiwan, who led the study, such extremely luminous X-ray sources outside galactic nuclei can provide critical clues about the existence of IMBHs, serving as a vital missing link in black hole evolution.

The X-ray emission from HLX-1 registers at a temperature of 3 million degrees Kelvin, consistent with radiation produced by a tidal disruption event. Hubble imaging identified a tightly packed star cluster surrounding the black hole, where stars are spaced only a few light-months apart, roughly 500 billion miles.

HLX-1 reached its peak brightness in 2012 before gradually fading by 2023. However, because the optical and X-ray data do not fully overlap, interpretations remain uncertain. The decline in brightness could reflect the formation and gradual dissipation of a gas disk from a disrupted star, or ongoing, uneven accretion activity.

Study co-author Roberto Soria of Italy’s National Institute for Astrophysics (INAF) noted the variability of HLX-1’s luminosity. It was relatively bright in 2009, surged to 100 times that brightness in 2012, and then steadily dimmed again. Future monitoring will help determine whether HLX-1 experiences recurrent flaring or is steadily fading.

While a dormant supermassive black hole likely resides at the center of NGC 6099, this suspected IMBH is located in the galaxy’s outer regions. This spatial arrangement may offer clues to black hole formation and galactic evolution.

A Hubble Space Telescope image of a pair of galaxies: NGC 6099 (lower left) and NGC 6098 (upper right).

Image credit: NASA, ESA, CXC, Yi-Chi Chang (National Tsing Hua University); Image Processing: Joseph DePasquale (STScI)

Building bigger black holes

Researchers aim to understand whether IMBHs could serve as precursors to supermassive black holes. One hypothesis suggests IMBHs are "seeds" that merge as galaxies grow by absorbing smaller galaxies, thereby enlarging their central black holes. Observations by Hubble have shown a proportional relationship between galaxy mass and central black hole mass, supporting this theory.

An alternative hypothesis proposes that gas clouds at the centers of dark-matter halos in the early universe collapsed directly into supermassive black holes without forming stars first. Observations from the James Webb Space Telescope of distant, disproportionately large black holes appear to support this scenario.

However, astronomers caution that there may be an observational bias, with current telescopes more likely to detect only the most massive black holes at great distances. A broader population of IMBHs may exist but remain undetected due to their faintness or inactivity.

Soria emphasized that identifying more IMBHs undergoing tidal disruption events could reveal how frequently such black holes occur and how they contribute to the growth of galaxies through mergers and star disruption.

Future surveys and observations

Current telescopes like Chandra and XMM-Newton cover only small areas of the sky, limiting the discovery of new tidal disruption events. However, the upcoming Vera C. Rubin Observatory in Chile, designed to conduct an all-sky optical survey, may identify more of these events from hundreds of millions of light-years away. Hubble and Webb could then provide detailed follow-up imagery to identify star clusters and better understand the environments surrounding these elusive black holes.

The Hubble Space Telescope, now operating for over three decades, continues to deliver transformative insights into the structure and evolution of the universe. Managed by NASA’s Goddard Space Flight Center in Maryland, Hubble is a joint project between NASA and the European Space Agency. Lockheed Martin Space supports mission operations, and the Space Telescope Science Institute in Baltimore oversees scientific operations.

This video is an illustration of an intermediate-mass black hole capturing and gravitationally shredding a star

Animation credit: NASA, ESA, Ralf Crawford (STScI)