Two Black Holes Discovered in M22

New radio observations suggest that one of the brightest globular clusters in the sky, Messier 22, hosts two stellar mass black holes.

Astronomers have discovered two radio sources, likely to be black holes emitting ionized plasma jets, in the dense core of globular cluster M22.
Jean-Charles Cuillandre (CFHT), Hawaiian Starlight, CFHT
More than 150 swarming hives of stars known as globular clusters orbit the Milky Way galaxy, many of them beautiful telescope targets. Theorists have long argued whether black holes could hide inside these thriving stellar cities. New observations suggest they probably do — and perhaps in greater numbers than expected.

On the one hand, it would be natural for hundreds of stellar-mass black holes to form early in a globular cluster’s lifetime. The hottest, most massive stars in the cluster would evolve the most quickly, transitioning from blazing star to bloated giant to collapsed black hole in less than a billion years. These black holes would then flock to the cluster’s core — being more massive, they would move more slowly than smaller, lighter stars, and thus slide deeper into the cluster’s gravitational well.

But simulations have shown that gravitational interactions within the dense cluster core would eject all but a handful (and probably all but one) of the black holes shortly after they form. Even in less dense environments, such as the young open star clusters that bejewel the Milky Way’s disk, runaway stellar mergers might lead to one giant black hole in the cluster’s core.

Now Jay Strader (Michigan State University and Harvard-Smithsonian Center for Astrophysics) and his colleagues say they’ve discovered not one, but two black holes in Messier 22, one of the brightest globulars in the sky. Published in Nature today, their paper presents observations of two radio sources deep inside the cluster’s core that the astronomers found using the Karl G. Jansky Very Large Array (VLA) in New Mexico. After considering several possible explanations for the radio emission, the authors conclude that the most likely explanation is black holes shooting out jets as they gobble material from companion stars.

“Like all scientific results, confirmation is needed, but in my view, things look very good right now,” says Jon Miller (University of Michigan), who was not involved in the study. “Their interpretation of two black holes is very reasonable and consistent with the data.”

The Hubble Space Telescope zooms in on the core of globular cluster M22. Though the cluster is brilliant in visible light, long radio observations reveal only two sources close to the cluster core.
K. Sahu (STScI) et al., WFPC2, HST, NASA Inset: N. A. Sharp, REU program / NOAO / AURA / NSF
Black holes themselves emit no light, but they typically make themselves known when they feed on gas sucked from a stellar companion. This gas forms a disk-shaped smorgasbord of infalling material around the black hole, and some of it can shoot out as beams of plasma along the black hole’s poles as well. Before the gas in the disk careens past the point of no return, it heats up to over a million degrees Celsius, emitting X-rays in the process.

Yet oddly, the newly discovered black holes emit no detectable X-rays, making them unique among all other stellar-mass black hole discovered in the Milky Way. Given the nature of the radio emission from M22, Strader and his colleagues think the black holes are probably swallowing gas at very low rates. In that case, the disk won’t be as important a source of light as the jets, which emit both in radio and X-ray but put out far less X-ray emission than the disk. Astronomers don’t understand exactly how the jet’s radio and X-ray emission relate to each other, but they do know that, given a certain amount of radio emission, a certain amount of X-rays should appear, too.

But neither black hole shows up in archival observations from the Chandra X-ray Observatory. The authors suggest that could be because black holes forming in the dense globular cluster environment can weigh as much as 20 Suns, roughly twice that of a typical stellar-mass black hole found elsewhere in the galaxy. If the black holes are more massive, they’ll emit less in X-rays than in radio, thus escaping detection by Chandra.

Two black holes might not sound like a lot in the grand scheme of things, but here’s the kicker. We’re only seeing these black holes because they’re accreting gas, and that’s only because they’re lucky enough to have a stellar companion contributing gas to their diet. While these two black holes eat their meager feast, there might be many more fasting in the cluster around them.

To come up with an estimate, astronomers have to know how many black holes are likely to end up in close orbit with another star, and that’s a problem that’s not yet well understood. So the authors settle for a broad estimate: somewhere between 5 and 100 black holes might lurk in the swarming core of M22. That should give theorists something to think about.

16 thoughts on “Two Black Holes Discovered in M22

  1. Bruce

    Globular clusters look like tiny versions of giant elliptical galaxies. GEG’s are formed by the mergers of other galaxies which often had SMBHs at their cores. Couldn’t globular clusters also have intermediate mass BHs at their cores? Globular clusters also typically look perfectly round, but M22 here appears to be rather elliptical in shape. Could it be possible that M22 is the product of a merger of two globulars? P.S. Peter Wilson, evidence for processes in harmony with your ideas are present in this article, wouldn’t you say?

  2. Monica YoungMonica Young

    Hi Bruce, Though this article discusses stellar-mass black holes, it’s indeed quite possible that there are intermediate-mass black holes at the centers of other globular clusters. There hasn’t been conclusive proof, because it’s difficult to weigh black holes, but here’s a couple of our previous stories on that:, There was also a more recent result in Nature ( which might also have been an intermediate-mass black hole.

  3. Peter WilsonPeter Wilson

    Bruce: Yes. Duality is like one of those new words you learn, and suddenly it seems to be everywhere. Matter does not fall into a black hole or get shot out its poles in a jet…it does both. A big black hole does not swallow little black holes or kick them out of the cluster…it does both! Like the Higgs field, duality is everywhere you look. The most astounding fact in astronomy today is that duality is not part of the astronomical lexicon.

  4. John Haynes

    My understanding is that globular clusters are extremely old, containing some of the most ancient stars we’ve observed. If they are so old, then it seems highly likely that little gas and dust remains within them, that it has, by and large, been vacuumed up by the stars and possibly black holes in the cluster. Thus, there would be little matter for the black holes to feed on and produce x-ray radiation which is typical of matter falling into black holes.

    I would wager that globular clusters are probably pretty clean environments, with little gas and dust and quite probably little other than the stars themselves. Planets would likely not remain, if they formed at all, and even structures analogous to our kuiper belt and oort cloud are probably not present, stripped away and devoured or ejected long ago.

  5. Bruce

    Peter, your right about duality not being in the astronomical lexicon, I know because I looked and couldn’t find it. That’s why I couldn’t quite bring myself to use the word in my above post, but instead wrote “processes in harmony with your ideas.” The way you describe duality makes it sound like ye old ‘for every action there is an equal and opposite reaction’ which is one of the first things we all learn about physics. How would you define duality? If people knew more about the concept they might be more open to your theory.

  6. Mike W. Herberich

    Hey Peter, long time no hear! I hope you’re doing ok. While we’re at it: how is the Higgs field dual, as you mentioned two or three posts up? I understood the other examples you gave, you know: both things happening at the same time. What are the two things with reference to the Higgs field? Bruce, you’re referring to your favorite Newton’s (second?) law, are you? Actio equals re-actio.

  7. Bruce

    Very funny Mike. Was “hggs” really unintentional? Sigmond would have something to say I would think. And thanks for the tip of the hat to Newton; I was alluding to his third law. But I’ll offer an unsubconsious writen hgg to Monica in thanks for her taking the time to answer to my intial post in this thread. Notice how she doesn’t really answer my question about a globular merger in M22’s past though. I’ll read that as a “not likely” and leave it be then. But she did offer up some tasty carots however in the references about intermediate-mass black holes in globulars. The first reference includes a nice log-log chart showing a linear relationship between the mass of central black holes in globulars of various sizes, galactic central bulges and on up to those in giant eliptical galaxies. The S&T article implies that a BH of aproximately 0.5 % of the mass of the given spherical stellar system’s total mass can be expected.

  8. Peter Wilson

    From the paper: "Duality means a twofold nature, especially a contradictory one, as in the wave/particle duality of quantum mechanics. With respect to gravity, it simply means that complex systems cannot be described monolithically as either contracting or expanding. Both processes occur simultaneously and must be included in a complete description." Duality results from action/reaction applied to three or more bodies. For example, the system of Earth-Sun-Cassini spacecraft. In what NASA describes as a gravity-assist, the Earth lost an amount of orbital energy exactly equal to the energy gained by the spacecraft. The Earth’s orbit contracted, and Cassini’s expanded, enough to propel it to Saturn. But because Earth is so massive compared to Cassini, the contraction of its orbit is infinitesimal, so the dualistic nature of the situation can be ignored. Today, the focus is always on one side or the other, either expansion or contraction. Here, astronomers describe the collapse of a cloud of gas and dust to form new stars; there, they describe how the stellar winds from these new stars drives away the rest of the gas from which they formed. The focus is always on one process or the other–the collapse of the cloud to form new stars, or its dissipation because new stars have formed–and the dualistic nature of the situation is overlooked.

  9. Bruce

    Thanks Peter. That helped me and I hope it helps others understand the foundation of your theory. Correct me if I’m wrong, Peter, but Mike, I don’t think Peter was using the Higgs field as an example of duality, he was merely saying that duality is pervasive, as the Higgs field is thought (proven?) to be.

  10. Mike W. Herberich

    … (I inserted an extra few "i"s here to avoid being looked at as latently and Freudian gay ;-]). Okay, Bruce, that figures what you explain as for the all pervasive Higgs field. Going back to Peter’s original line it’s even clearer. Stupid question, actually. — So, what Peter is saying is that there is no net expansion (or contraction) in the universe, apart maybe from some "initial" and "outside"("godly"?) "push", whatever meaning could be attributed to these terms in quotes. Rather to each expansion at any scale there must be some equivalent contraction to cancel that out overall. Is that correct? If that were so or not so, we should still be able to measure the same red shifts and expansion rate, shouldn’t we? Its only that we are not busying ourselves (enough/ yet?) with the corresponding contractions, be it their nature nor where they occur?

  11. Bruce

    Very funny Mike. Ok, since you addressed the question to me I’ll attempt to answer, even though Peter is the teacher and I’m merely his supporter. (Actually, I kinda like playing the roll as his sidekick in his Don Quixotelike quest to vanquish Dark Energy, the evil, misterious force that threatens to rip the universe apart.) Again, please correct me if I’m wrong Peter, but Wilson is most cerntainly NOT “saying that there is no net expansion … in the universe.” He accepts both the expansion and its observed acceleration. What he’s suggesting is that the acceleration of the expansion is a dualistic consequence of the observed contraction of galactic clusters. As galactic clusters contract and merge the larger voids between the clusters expand (due to an exchange of gravitational potiental energy? Vague on this Peter. And watch out for that windmill.) Please confirm or clarify Peter.

  12. Peter WilsonPeter Wilson

    At the risk of adding to the confusion, I draw an analogy with the word “evolution.” It can be used in two senses: observation and explanation. In the former, we might say cars and languages evolve. In the latter, it is said evolution explains the development of complex life forms. Nobody can deny the observation that languages and automobiles change with time–evolve–but one can argue about its explanatory power. Likewise, the term duality has two senses. In the first sense, complex systems are inevitably observed to be dualistic. In the second sense, duality may or may not stand as an explanation. Two examples will be given below, one in each category, to clarify. The paper introduces duality as an observation, then suggests an evaluation of its explanatory power. The paper does not claim that duality explains the expansion of the universe. What the paper does claim is that duality is a real phenomenon with an expansive effect (it “acts like” dark energy), and the magnitude of its contribution needs to be evaluated.

  13. Peter WilsonPeter Wilson

    The Mars, Phobos and Deimos system is dualistic: Phobos’ orbit is contracting and it will eventually fall to the planet, while Deimos’ orbit is expanding and it will eventually “fall away” into interplanetary space. Nonetheless, the “cause” of both the expansion of Deimos’ orbit and the contraction of Phobos’ orbit is tidal drag with respect to Mars. This is an example of a system that is observed to be dualistic, but the dualism does not explain the system’s behavior. Specifically, the energy to expand Deimos’ orbit comes from Mars’ rotation, not Phobos’ inward fall. On the other hand, the jets from a black how are an example of a system that is observed to be dualistic, and the dualism DOES explain the system’s behavior. Specifically, matter falling into a black hole along its accretion disc does supply the energy to power the jets that shoot out. More matter falls in than gets shot out, but it is the inflowing material that powers the outflow. The contraction of one part causes expansion of the other. In these two examples, it is easy to determine, yes/no, the contraction of one part powers the expansion of the other, or not. In the case of the universe, it is more complicated. Again, the paper does not claim to have made this more difficult evaluation. What it does claim is that duality needs to be put into the equation, so that the evaluation can be made.

  14. Mike W. Herberich

    Ok, got that, down to the conclusion. And I think it is a very nice concise explanation and description to be understood. — I suspect that we need more and more precise observations of orbits of (groups of) bodies of all scales, especially the biggest ones, galaxies and clusters thereof, because it sort of mostly relies on (contracting/ expanding) orbits and tidal friction of these entities in space when we/ you (Peter) speak of duality. — Was it in these posts somewhere or in S&T magazine that I read that, as an example, the orbit of Andromeda is still (or again) pretty controversial and might remain so, simply for its enormous duration in time to be measured? — Or is it possible to ignore most of these practically immeasurable objects and gain analogous information from things you mentioned in your 2nd example, namely duality around black holes and the like? In other words: do you hope to see observations that could speak in favor of what you ask?

  15. Ted Hauter

    If there are Black Holes in M22 then this object is a galaxy. Perhaps cannibalized by a distant galaxy billions of years ago. This goes for any globular I suspect.

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