A comma-shaped molecular cloud near the center of the Milky Way appears to be orbiting one of astronomy’s most sought-after objects.
At the center of the “tadpole’s” orbit, a team of astronomers saw precisely… nothing. And a nothing that attracts something just screams ‘black hole’.
Modeling suggests that this would not be just an ordinary black hole, but one belonging to the rarely seen class of middleweights; the “missing link” intermediate-mass black holes.
If so, it would be the fifth intermediate black hole candidate found near the galactic center.
This growing number of previously elusive objects could help astronomers figure out how supermassive black holes at the centers of galaxies form and then grow to such colossal size.
“In this paper, we report the discovery of a peculiar isolated compact cloud,” writes a team of astronomers led by Miyuki Kaneko of Keio University in Japan.
“Tadpole’s spatial compactness and lack of bright counterparts at other wavelengths indicate that the object may be an intermediate-mass black hole.”
Black holes in the Universe tend to be found in two distinct mass regimes. There are stellar-mass black holes, up to about 100 times the mass of the Sun. These are black holes that form from the collapse of the core of a massive star at the end of its life, or mergers between these black holes.
Then there are supermassive black holes. These are the giant chonkers that lie at the center of galaxies, with masses of millions to billions of times that of the Sun.
It’s unclear how these objects form, and it’s a cosmic conundrum that astronomers would love to solve.
One place where answers can be found is among black holes with intermediate masses. Finding these intermediate-mass black holes (IMBH) would be evidence that black holes uniformly span a whole range of masses, and that intermediates are a growth stage between titch and behemoth.
But only a few of these medium-weight objects have been identified, and most of the time only tentatively.
One of the problems is that solitary black holes don’t emit any light on their own. They can only be detected by the effect their immense gravity has on their environment, making matter spin in an incandescent fury, or tugging at the fabric of space-time in different ways.
That unsubtle tug can affect the orbital dance of distant objects, like the stars that astronomers have studied to check for the presence of Sagittarius A*, the black hole at the center of the Milky Way.
The galactic center is a pretty crowded place, actually. It’s dense with molecular clouds, the kind that give rise to stars. It is known as the Central Molecular Zone, and its molecular gas density is several orders of magnitude greater than that of the Milky Way’s disk.
Because the region is so dense, it can be difficult to see what’s inside, but a powerful radio telescope can reveal the activity there.
That’s how researchers found the cloud they nicknamed Tadpole. They were using the James Clerk Maxwell Telescope to look for gas that was perturbed by gravity.
That was what Tadpole was: a molecular cloud very close to the galactic center, 27,000 light-years away, moving differently than other nearby material.
Its stretched shape, the team found, was likely the result of being pulled by the strong tidal pull – a gravitational interaction.
And their modeling showed that the mass responsible for this interaction is about 100,000 times the mass of the Sun. This strongly suggests an intermediate black hole.
Where it could have come from and how it formed are questions that still need to be answered.
First, the team needs to confirm their suspicions. They intend to use the powerful Atacama Large Millimeter/submillimeter Array in Chile to carry out follow-up observations of Tadpole to determine if they can find signs of a black hole, or something else, at the orbital center.
If it’s an intermediate-mass black hole, that could have profound implications for our understanding of the supermassive variety.
The research was published in The Astrophysical Journal.