We’ve found small black holes and we’ve found really, really big black holes. But what about the “inbetweener” black holes?

The very existence of this class of black hole is disputed, but a Japanese group of astronomers have found the potential locations of three intermediate black hole (IMBH) candidates inside previously unknown star clusters near the center of the Milky Way.

But what are IMBHs and why are they so important?

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Conventional stellar-mass black holes are the ones we are taught at school when discussing the life cycles of massive stars. When a star — over ten-times the mass of our sun — runs out of fuel, its death throes culminate in a supernova. This powerful explosion will create the extreme gravitational conditions ripe for a stellar black hole to form.

At the other end of the black hole spectrum are the supermassive ones. As the superlative suggests, these black holes are monsters. We know that the majority of galaxies we can observe — including our own — play host to supermassive black holes in their cores. These black holes are very different from their stellar tiddler counterparts; supermassive black holes grow from tens of thousands to billions of times the mass of our sun.

But some big questions have vexed astrophysicists as to where supermassive black holes come from. How did they become so massive? What’s the link between stellar-mass black holes and supermassive black holes? And is there an “intermediate” black hole phase?

Logic would dictate that IMBHs should be out there, but there were few candidates until the discovery of Hyper-Luminous X-ray Source 1 (HLX-1) was confirmed earlier this year by the Commonwealth Scientific and Industrial Research Organisation (CSIRO) telescope in Australia. The apparent dearth of the objects, however, is causing some puzzlement.

When Black Holes Unite

Black hole formation theories suggest that supermassive black holes were formed through the agglomeration of many intermediate-sized black holes. Therefore, it stands to reason that there should at least be some IMBHs near the centers of galaxies.

One environment that may be fertile for the growth of IMBHs is that of densely packed star clusters surrounding the galactic center — these clusters could be dense enough to regularly kick-off supernovae, creating a supply of stellar black holes that also accumulate and grow into intermediate black holes.

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So, with this theory in mind, using the 10-meter Atacama Submillimeter Telescope Experiment (ASTE) in the Atacama Desert, Chile, and the 45-meter Nobeyama Radio Observatory (NRO) in Japan, a research group headed by Keio University’s Tomoharu Oka hunted for the emissions from molecular gases associated with supernovae in star clusters.

“Huge star clusters at the center of the Milky Way Galaxy have an important role related to formation and growth of the Milky Way Galaxy’s nucleus,” said Oka.

Find the Gas; Find the Cluster

One would think that finding giant star clusters is easy, but as we look through the Milky Way’s disk toward the galactic center (30,000 light-years away), it is hard to see the star clusters through the gas, dust and stars in front. It’s a cosmic equivalent of “you can’t see the wood for the trees!” — we can’t see the star clusters for the stars (and dust)!.

“The huge amount of gas and dust lying between the solar system and the center of the Milky Way Galaxy prevent not only visible light, but also infrared light, from reaching the Earth,” said Oka. “Moreover, innumerable stars in the bulge and disc of the Milky Way Galaxy lie in the line of sight. Therefore, no matter how large the star cluster is, it is very difficult to directly see the star cluster at the center of the Milky Way Galaxy.”

So to detect the clusters, Oka and his team surveyed the center of our galaxy for the emissions from molecular clouds — particularly the millimeter wavelength emission from carbon monoxide. This wavelength can penetrate the obscuring galactic disk, providing the researchers with a window into the core of our galaxy. The distribution of warm gas of more than 50 Kelvin (-370 degrees Fahrenheit/-223 degrees Celcius) with a density of more than 10,000 hydrogen molecules per cubic centimeter could then be mapped.

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The group managed to find three previously unknown warm clumps of gas, all of which exhibit signs of rapid expansion. A fourth clump was found in the location of Sagittarius A (Sgr A), a very well-known radio source and lair of Sagittarius A* (Sgr A*) — the Milky Way’s very own supermassive black hole with a mass of 4 million suns.

“It can be inferred that the gas clump ‘Sgr A’ has a disk-shaped structure with radius of 25 light-years and revolves around the supermassive black hole (Sgr A*) at a very fast speed,” added Oka.

According to the National Astronomical Observatory of Japan press release, the expansion detected inside the other three previously unknown clumps of molecular gas can be attributed to recent supernova activity. The researchers believe these clumps therefore correspond to clusters of stars, where one of the clusters is comparable to the largest known star cluster in the Milky Way, with a mass of around 100,000 solar masses.

Seeds of the Supermassive

Where there’s regular supernovae popping-off inside a cluster, stellar black holes are forming. In one of the most active clusters, there is evidence to suggest that, on average, one supernova every 300 years is detonating. The other two clusters also show signs of recent supernova activity.

According to theory, inside these dense violent supernova pressure-cookers of star clusters, stellar black holes are being born, merging and then bulking-up to form IMBHs. Oka’s team predicts that there should be an IMBH inside each of these three clusters, weighing-in at several hundred solar masses. In the grand galactic scale, these IMBHs would eventually sink into the center of the galaxy and get swallowed by Sgr A*, potentially explaining how the supermassive black holes in the cores of galaxies get so massive.

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Although the possible IMBHs in the centers of the clusters have yet to be observed, the environment is certainly ripe for their creation. What’s more, Oka is confident that indirect evidence for these intermediate black holes may be forthcoming: “We would like to observe IMBHs in the star cluster. Actually, our observation data has already indicated traces of IMBHs.”

Inside one of the clusters, two clouds of gas have been observed apparently orbiting a central mass very quickly — should followup observations confirm this to be the case, it could revel that our galaxy not only contains a supermassive behemoth at its nucleus, it may have the ‘seeds’ of star clusters containing IMBHs slowly falling into the supermassive black hole at the core.

“In order to confirm the existence of IMBHs, we are planning to conduct further observations,” concludes Oka. “The new discovery is an important step toward unraveling the formation and growth mechanism of the supermassive black hole at the Milky Way Galaxy’s nucleus, which is a top-priority issue in galactic physics.”

I hate to be predictable, but isn’t it time for Muse to sing us out?

Press release: Keio University

Image: Artist’s impression of one of the star clusters containing an IMBH. Credit: Keio University