Indian telescope sheds light on the elusive ‘middleweight’ black holes

17 April 2025: Tracing an intermedia black hole (IMBH) located in a faint galaxy about 4.3 million light-years away using India’s largest optical telescope, astronomers have found that gas clouds orbit the black hole at a distance of 125 light-minutes (around 2.25 billion kilometre) with a velocity dispersion of 545 km per second.

The discovery refines our understanding about how black holes, specially those that weight between 100 and 100,000 Suns, grow and interact with their surroundings.

For decades, astronomers have searched for a missing link in the cosmic black hole family: the elusive Intermediate-Mass Black Holes (IMBHs). Bridging the gap between smaller stellar black holes (having a few dozen times the Sun’s mass) and massive supermassive black holes (having millions to billions of solar masses), IMBHs have remained elusive.

IMBHs, are thought to be the seeds that grow into supermassive black holes. However, their faint nature and location in small galaxies make them extremely difficult to observe. Unlike their larger counterparts, they don’t generate bright emissions unless they’re actively pulling in matter, making advanced observational techniques essential.

Using the 3.6m Devasthal Optical Telescope (DOT) a team led by scientists from Aryabhatta Research Institute of Observational Sciences (ARIES), an autonomous institute of the Department of Science and Technology (DST), have successfully detected and measured the properties of an IMBH in a faint galaxy called NGC 4395.

The team of astrophysicists, led by Shivangi Pandey studied NGC 4395—a low-luminosity active galaxy hosting one of the faintest actively feeding black holes ever observed.

They used the largest optical telescope in India, the 3.6m DOT, and its indigenously developed spectrograph and camera ADFOSC, along with the smaller 1.3m Devasthal Fast Optical Telescope (DFOT) located at the Devasthal Observatory of ARIES.

Since the size of the region surrounding the black hole is very difficult to resolve even with a sophisticated telescope, the team monitored the object continuously for two nights using both the telescopes and applied a special technique called spectrophotometric reverberation mapping.

This technique measures the delay between light emitted by the black hole’s accretion disk and the surrounding gas clouds (broad-line region). This delay, or time lag, revealed the region’s size and helped calculate the black hole’s mass.

Apart from racing the gas clouds, they also found that theIMBH weighs about 22,000 times the Sun’s mass, making it one of the most precisely measured intermediate-mass black holes. The black hole consumes matter at just 6% of its maximum theoretical rate.

The study published in the Astrophysical Journal validates the size-luminosity relationship for black holes in low-luminosity active galaxies, provides a more accurate black hole mass estimate than previous studies and offers a more precise benchmark for future research.

Dr Suvendu Rakshit, a scientist at ARIES involved in this study, notes: “The hunt for more IMBHs is far from over. Larger telescopes and advanced instruments will be key to uncovering these cosmic middleweights.”

As technology advances, future observations with larger telescopes and higher-resolution instruments will deepen our understanding of IMBHs and their role in shaping the universe.