This image shows an object that astronomers think could be an intermediate-mass black hole (IMBH), a rare class of black holes. Called NGC 6099 HLX-1 and labeled in this image, this black hole seems to reside in a compact star cluster in a giant elliptical galaxy, as described in a new press release from Space Telescope Science Institute.

In this composite image, optical light data from NASA’s Hubble Space Telescope (colored red, green, and blue and appearing as white and yellow) have been combined with X-rays from NASA’s Chandra X-ray Observatory (purple). NGC 6099 is the elliptical galaxy to the lower left, and a companion elliptical galaxy (NGC 6098) is located in the upper right.

[Enlarge] NGC 6099 HLX-1 (Credit: X-ray: NASA/CXC/Inst. of Astronomy, Taiwan/Y-C Chang; Optical/UV: NASA/ESA/STScI/HST; Image Processing: NASA/STScI/J. DePasquale)

Scientists think there are three main categories of black holes. Stellar-mass black holes are formed from the collapse of a very massive star and weigh up to about 100 times the mass of the Sun. On the other end of the size spectrum, supermassive black holes, which are found in the centers of most galaxies, can be millions or even billions of solar masses.

IMBHs weigh between a few hundred to a few 100,000 times the mass of our Sun and have been much more difficult to find and study. Typically, they must be caught in the act of foraging in order to be found. When they occasionally tear apart and devour a hapless bypassing star — in what astronomers call a tidal disruption event — they pour out a gusher of radiation, including copious amounts of X-rays.

The newest probable IMBH, caught snacking in telescope data, is located on the galaxy NGC 6099’s outskirts at approximately 40,000 light-years from the galaxy’s center. The galaxy is located about 450 million light-years away from Earth in the constellation Hercules.

Astronomers first saw an unusual source of X-rays in an image taken by Chandra in 2009. They then followed its evolution with ESA’s XMM-Newton space observatory. X-ray sources as bright as HLX-1 are rare outside the centers of galaxies and can serve as a key probe for identifying elusive IMBHs.

The hot gas producing X-ray emission from HLX-1 has a temperature of 3 million degrees, which is consistent with a tidal disruption event. Hubble found evidence for a small cluster of stars around the black hole. This stellar cluster would give the black hole a lot to feast on, because the stars are so closely crammed together that they are just a few light-months apart (about 500 billion miles).

The suspected IMBH reached maximum brightness in 2012 and then continued declining to 2023. The optical and X-ray observations over the period do not overlap, so this complicates the interpretation. The black hole may have ripped apart a captured star, creating a plasma disk that displays variability, or it may have formed a disk that flickers as gas plummets toward the black hole.

By combining data from Chandra and Hubble along with new large surveys of the sky from facilities like the new Vera C. Rubin Observatory, astronomers hope to amass information on a much larger population of tidal disruption events and potentially intermediate-mass black holes. This could help astronomers pinpoint how IMBHs form, which remains an important open question as of today.

A paper describing these results appears in a recent issue of The Astrophysical Journal and is available here. The authors of the study are Yi-Chi Chang (National Tsing Hua University in Taiwan), Roberto Soria (INAF), Albert Kong (National Tsing Hua University), Alister Graham (Swinburne University of Technology in Australia), Kirill Grishin (University Paris Cite in France), and Igor Chilingarian (Center for Astrophysics | Harvard & Smithsonian).

NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program. The Smithsonian Astrophysical Observatory's Chandra X-ray Center controls science operations from Cambridge, Massachusetts, and flight operations from Burlington, Massachusetts.