Astronomers discover extremely elusive 'intermediate-mass' black hole

© X-ray: NASA/CXC/SAO/M.Mezcua et al & NASA/CXC/INAF/A.Wolter et al; Optical: NASA/STScI and DSS; Inset: Radio: EVN/VLBI

This is a composite image of the galaxy NGC 2276, with X-rays from Chandra (pink) and optical data (red, green, and blue). The inset zooms into just NGC 2276-3c, an intermediate-mass black hole and reveals its emission in radio waves, including a jet produced by the black hole that appears to be snuffing out star formation.

Astronomers have detected a black hole embedded in the spiral arm of a galaxy 100 million light-years from Earth — but this isn't any old black hole, it belongs to an extremely elusive class that may be the 'missing link' in black hole evolution.

Using observational data from NASA's Chandra X-ray Observatory and the European Very Long Baseline Interferometry (VLBI) Network, which detects radio waves from energetic sources in the cosmos, the researchers, led by Mar Mezcua of the Harvard-Smithsonian Center for Astrophysics, were able to also deduce that this particular 'intermediate-mass black hole' (IMBH) is creating a 'dead zone' inside its host galaxy, NGC 2276.

"In paleontology, the discovery of certain fossils can help scientists fill in the evolutionary gaps between different dinosaurs," said Mezcua. "We do the same thing in astronomy, but we often have to 'dig' up our discoveries in galaxies that are millions of light years away."

Black holes are known to come in two main classes: stellar-mass black holes, which are spawned by supernovae and are around 5-30 times the mass of the sun, and supermassive black holes, which occupy the cores of most galaxies and have solar masses of millions to . But to understand how black holes grow, there must be some black holes that have masses between the stellar and the supermassive. After all, logic dictates that if all black holes start small and grow over time, there must be some intermediate mass black holes out there with girths that range between a few hundred to a few hundred thousand solar masses.

"Astronomers have been looking very hard for these medium-sized black holes," said co-author Tim Roberts, of the University of Durham. UK. "There have been hints that they exist, but the IMBHs have been acting like a long-lost relative that isn't interested in being found."

So when astronomers detect hints of an intermediate black hole, they pounce, and it seems that the energetic object in the spiral arm of NGC 2276 is one of these elusive mid-mass monsters.

"We found that NGC 2276-3c has traits similar to both stellar-mass black holes and supermassive black holes" said co-author Andrei Lobanov of the Max Planck Institute for Radio Astronomy in Bonn, Germany. "In other words, this object helps tie the whole black hole family together."

But this bright object, known as NGC-2276-3c, has a dark side.

© NASA/CXC/SAO/M.Mezcua et al & NASA/CXC/INAF/A.Wolter et al

The Chandra X-ray Observatory observation of the galaxy NGC 2276.

Blasting up to 2,000 light-years from the black hole, a powerful radio jet penetrates through the galaxy's interstellar medium. Within this jet, up to 1,000 light-years from the black hole, there is a dramatic paucity of young stars; the jet appears to have cleared a cavity near the black hole, removing star-forming gases, snuffing out star birth.

The researchers are now trying to understand how the IMBH got there in the first place. It seems highly probable that NGC-2276-3c formed in the core of a smaller dwarf galaxy that, hundreds of millions or billions of years ago, merged with NGC 2276. In other regions of the galaxy, there appears to be a surge of star birth, which supports the idea that another galaxy may have been cannibalized.

It's studies like this that not only expose an apparently rare class of black hole, they also pose an interesting question. Is our conventional thinking about the growth of black holes correct? All known galaxies seem to have supermassive black holes in their cores, but how did they get so big? Recent research revealed a black hole of gargantuan proportions in the early universe — how did it get so big so fast? Do supermassive black holes undergo a sudden 'growth spurt' between 'stellar' and 'supermassive'? This would certainly explain why we're not spotting many intermediate-mass black holes — perhaps they simply don't stay that size for very long.

In the case of NGC-2276-3c, however, we know one thing, don't expect to be living in the neighborhood of that black hole, it's a galactic dead zone.