HLX-1

A missing link

Black holes fall into familiar size classes: stellar-mass objects of a few to a few dozen Suns, and the supermassive monsters of millions to billions of Suns at galactic centres. The middle ground between them, called the intermediate-mass range, has been stubbornly hard to confirm. HLX-1 is the strongest candidate yet found for this missing link.

A brilliant X-ray beacon

HLX-1 stands for “Hyper-Luminous X-ray source 1”. It was discovered in 2009 as an unusually bright X-ray source sitting just outside the disc of the galaxy ESO 243-49, rather than at its centre. Its luminosity peaks far above what ordinary stellar black holes produce, suggesting something heavier is doing the pulling.

The source was first catalogued as 2XMM J011028.1-460421, and follow-up observations with the Hubble Space Telescope showed its glow coming from a compact region separate from the host galaxy's core.

How its mass is measured

Astronomers weigh HLX-1 the same way they weigh other black holes: by studying the hot disc of gas spiralling inward and measuring how fast it orbits. Those measurements point to a mass around 20,000 Suns, more than a thousand times heavier than Cygnus X-1 but only a small fraction of Sagittarius A*. The exact value remains uncertain, which is why HLX-1 is a candidate rather than a confirmed case.

Why it matters

Finding intermediate-mass black holes would help explain how supermassive black holes grew so quickly in the early universe. They may be the seeds that later merged and fed their way up to billions of solar masses, so objects like HLX-1 are a key test of how the largest black holes came to be.