'Free-floating' black hole appears to be feeding on a star

Astronomers have spotted what may be a rare type of black hole hiding on the outskirts of a distant galaxy. This discovery could help fill in a key missing piece in our understanding of how black holes grow over time – and how galaxies build themselves around them.

The new object, known as NGC 6099 HLX-1, was picked up through a collaborative effort using NASA’s Hubble Space Telescope and the Chandra X-ray Observatory.

The bright X-ray signal coming from this object suggests it might belong to a hard-to-find class of black holes known as intermediate-mass black holes (IMBHs).

The elusive intermediate black hole

The suspected black hole is located within a compact star cluster about 40,000 light-years from the center of the galaxy NGC 6099. This galaxy lies approximately 450 million light-years away in the constellation Hercules.

Most galaxies have two kinds of black holes. The massive ones – millions to billions of times the mass of our Sun – are located in the center of galaxies. Then, there are the much smaller black holes – typically less than 100 times the Sun’s mass – formed from dying stars.

Between the smallest and largest black holes lies a largely uncharted middle ground: intermediate-mass black holes. Ranging from hundreds to hundreds of thousands of solar masses, they’ve been incredibly difficult to detect.

Unlike their larger cousins, IMBHs usually aren’t feeding. They don’t suck in enough material to glow brightly. Unless something triggers a flare – like a passing star getting too close – they remain invisible.

A black hole caught in the act

In this case, astronomers believe the black hole may have been caught red-handed during one of those feeding episodes, what scientists call a “tidal disruption event.” When a black hole tears apart a star, it creates a superheated disk of plasma that shines in X-rays.

Chandra first picked up the signal in 2009. ESA’s XMM-Newton observatory then tracked it over time. In 2012, the object reached peak brightness – about 100 times brighter than it was in 2009. Since then, the brightness has faded.

“If the IMBH is eating a star, how long does it take to swallow the star’s gas? In 2009, HLX-1 was fairly bright. Then in 2012, it was about 100 times brighter. And then it went down again,” noted study co-author Roberto Soria of the Italian National Institute for Astrophysics (INAF).

“So now we need to wait and see if it’s flaring multiple times, or there was a beginning, there was peak, and now it’s just going to go down all the way until it disappears.”

The X-ray emission from HLX-1 is at a temperature of about 3 million degrees, which is consistent with a violent event. Hubble added more detail by spotting a dense cluster of stars around the black hole.

In this packed environment, stars are only a few light-months apart – about 500 billion miles – providing plenty of fuel for a black hole to feast on.

Why intermediate-mass black holes matter

“X-ray sources with such extreme luminosity are rare outside galaxy nuclei and can serve as a key probe for identifying elusive IMBHs. They represent a crucial missing link in black hole evolution between stellar mass and supermassive black holes,” said lead author Yi-Chi Chang of National Tsing Hua University.

Scientists want to study intermediate-mass black holes more closely because they might be the building blocks for the enormous black holes at galactic centers.

When smaller galaxies merge into larger ones, their black holes may combine too. Over time, this could form the supermassive black holes we see today. Hubble data has shown that bigger galaxies tend to have bigger central black holes, suggesting they grew together.

Alternatively, some theories suggest that black holes in the early universe didn’t start small. Instead, clouds of gas in dark-matter halos may have skipped star formation entirely and collapsed straight into giant black holes.

NASA’s James Webb Space Telescope has recently found very distant black holes that seem too massive for the size of their galaxies. That might support the direct-collapse idea. Or it could be that we’re just missing the smaller ones because they’re too faint to detect at such great distances.

Telescopes that scan the entire sky

“So if we are lucky, we’re going to find more free-floating black holes suddenly becoming X-ray bright because of a tidal disruption event,” said Soria.

“If we can do a statistical study, this will tell us how many of these IMBHs there are, how often they disrupt a star, how bigger galaxies have grown by assembling smaller galaxies.”

The main problem is coverage. Chandra and XMM-Newton observe only a tiny portion of the sky. That makes it hard to catch these sudden flares in real time. But there’s hope on the horizon.

The upcoming Vera C. Rubin Observatory in Chile will scan the entire sky regularly in optical light. It could help catch more of these events.

Then, space telescopes like Hubble and Webb can follow up to confirm whether an IMBH was behind the show – and maybe help us figure out how these cosmic heavyweights really come to be.

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