At 36 billion solar masses, is the heaviest black hole too massive?

Within practically every modern galaxy, a central supermassive black hole can be found. Our own Milky Way houses Sagittarius A*, which weighs in at just over 4 million solar masses. Although that might sound like an impressively massive black hole, it’s actually on the small side for a galaxy as massive as ours. Andromeda is only a little bit more massive than the Milky Way, but its supermassive black hole is more than 100 million solar masses. Nearby, in the Virgo Cluster, the giant elliptical galaxy Messier 87 has an even more massive black hole: at 6.5 billion solar masses. For the most massive galaxies in the nearby Universe, supermassive black holes of ~1 billion solar masses or more are actually quite common.

But even in these giant elliptical galaxies, the most massive known supermassive black holes make up only about ~0.1% of the stellar mass (i.e., the mass that’s present in the form of stars) of the galaxies themselves. This ratio, of 1000-to-1 (for stellar mass to mass of the central black hole), is very common in the nearby Universe. In practice, pinning down the masses of these central black holes is difficult, and can normally only be done for the closest galaxies of all: those within ~1 billion light-years of us.

But thanks to the power of gravitational lensing, we’ve just robustly measured the mass of the black hole at the center of the cosmic horseshoe, despite it being ~5 billion light-years away. It weighs in at 36 billion solar masses: the heaviest black hole ever measured so exquisitely well. Given the other properties of this galaxy, the black hole appears to be “overmassive,” raising more questions than it answers and providing hints to the biggest chicken-and-egg question in all the Universe.

At the centers of galaxies, orbiting stars experience close interactions with the central supermassive black holes at a galaxy’s core. Stars that pass too close, particularly in multi-star systems, are at risk of receiving hypervelocity kicks and being ejected from the galaxy entirely. While all galaxies have a dark matter halo, many low-mass galaxies, long thought to not have supermassive black holes, may in fact possess them.

The classic question, often posed to children, is “what came first: the chicken or the egg?” Chickens lay eggs, and every chicken is born from a chicken egg, so it’s natural to ask which one came first: the chicken or the egg. In light of modern evolutionary biology, we now know that the egg came first: predating chickens by many hundreds of millions of years, in fact.

For cosmology, we understand that modern galaxies all have large numbers of stars throughout them, but are generally anchored by a central supermassive black hole. The Universe’s chicken-and-egg question, then, is “what came first: the stars in a galaxy or the supermassive black hole?”

You can imagine it could go either way. You could have a cloud of gas collapse, form stars for the first time, those stars live-and-die, where at least some of those stars end their lives in ways that lead to black holes: through core-collapse supernovae or through direct collapse mechanisms, for instance. Then, those black holes would grow, possibly merge, and devour other matter, becoming the supermassive ones we see at late times. Alternatively, we could have formed the seeds of these black holes prior to and independent of any stars, such as through the direct collapse of matter or, more exotically, the presence of “primordial” black holes created through exotic mechanisms in the early Universe, where stars would then form around them as they grew through accretion and mergers over time.

If the Universe was born with primordial black holes, a completely non-standard scenario, and if those black holes served as the seeds of the supermassive black holes that permeate our Universe, there will be signatures that current and future observatories, like JWST and LISA, will be sensitive to. Measuring the growth rate of black holes over time is one key test; looking for fully evaporating black holes (for which there is no evidence) is a second.

The key observational way to distinguish between these different scenarios is to look at galaxies in exquisite detail all across the Universe: nearby, at intermediate distances, and at the greatest cosmic distances we can make those observations at. For more distant galaxies, we generally have to rely on indirect estimates of the black hole’s mass, as direct measurements of:

• the motions of individual stars near the galactic center,
• the velocity dispersions of groups of stars close to the galactic center (inferred from emission line broadening),
• or of the event horizon’s size directly,

become challenging-to-impossible the farther away a galaxy is. Remember that the more distant a galaxy is, the smaller it appears in the sky in terms of angular size, and so beyond a certain distance, our finite-sized (and finite resolution) telescopes lose the ability to make such measurements.

For the galaxies we find nearby, it’s only the most extreme examples, either the closest galaxies or the ones with the largest supermassive black holes, that we can make measurements of their individual interior stellar motions or can image the event horizon directly. For all others, we have to use other methods (such as X-ray emissions, where they exist) or aggregate estimates (of the dynamics of the stars in the galactic center) to infer the central black hole’s mass.

This scatter plot shows the mass of the central, supermassive black hole within a galaxy (y-axis) versus the central stellar velocity dispersion (x-axis) of the same galaxy. Data points that fall well above the black line are candidates for having overmassive black holes for their measured velocity dispersions.

Importantly, for nearby galaxies — the ones we can measure the best — we find that there’s a very steady correlation between the inferred mass of the supermassive black hole at the galaxy’s center (y-axis, above) and the central stellar velocity dispersion of the host galaxy (x-axis, above), as shown from the black line. You might look at the above graph and notice that there’s a large scatter, and indeed there is: some galaxies depart from this relationship by quite a large amount. In fact, perhaps the most interesting galaxies aren’t the blue ones in the upper-left quadrant or the green ones in the lower-right, but rather the one point located in pink: the one representing the brightest cluster galaxy of galaxy cluster Abell 1201.

You might then ask what it is about that one galaxy (shown below) that makes it so different from all the others in this sample. Whereas nearly all of the other galaxies are found nearby, within a billion light-years or so, the galaxy found at the center of Abell 1201 is more than two billion light-years away. It seems like it would be very difficult to measure both the central velocity dispersion of its stars (which is indeed a challenge, but can be done with integral field spectroscopy), but incredibly difficult to estimate the mass of the central supermassive black hole. However, because the galaxy itself is so massive that it acts as a gravitational lens, we can use the magnification enhancement of the light sources from behind it to probe the internal properties, and infer that there is a very heavy black hole in there: of around 20-30 billion solar masses, or about a factor of ~30 heavier than would be expected for the galaxy’s central velocity dispersion.

This is an image of galaxy cluster Abell 1201, and in particular of the brightest galaxy in the cluster, found at the cluster’s center. This galaxy, located 2.3 billion light-years away, gravitationally lenses at least one of the galaxies behind it, enabling more precise measurements of its central black hole’s mass.

This is interesting and highly suggestive of the notion that black holes, and not stars, came first. If you had stars come first, then what you’d expect to find is the following.

• When you looked at galaxies today, you’d find a correlation between how much mass is in the form of stars within the galaxy and how heavy the supermassive black hole is. (We have done this, and the ratio of stellar mass to supermassive black hole mass is about 1000-to-1.)
• Then, when you looked at galaxies at earlier times, you’d expect that the correlation would remain the same (with the same ratio) for some time, before “tilting” at early times to favor more stellar mass and lower supermassive black hole mass (i.e., with ratios exceeding 1000-to-1).
• And then, for the very earliest galaxies of all, you’d expect that you might find collections of stars with no supermassive black hole in there at all, or at the very least, with tiny black holes that just appeared to be the result of the end-state of stellar evolution.

But if, instead, you see that you’re finding overmassive black holes when you look to galaxies found at earlier times and in earlier stages of evolution — where the mass of the black hole is heavier than ~0.1% of the stellar mass of the host galaxy — then you’d conclude that the black hole came first.

In other words, the key to answering this chicken-and-egg question for the stars within galaxies and the supermassive black holes also found within them is to sample those galaxies across cosmic time.

This galaxy, discovered in 2007 by the Sloan Digital Sky Survey and formally known as LRG 3-757, creates a beautiful, nearly perfect ring through the phenomenon of gravitational lensing. This cosmic horseshoe is a background galaxy being gravitationally lensed by a massive foreground galaxy.

Normally, our estimates of black hole masses become poorer and poorer the farther away we look, because we lose resolution at great distances. But thanks to gravitational lensing, a properly aligned system could let you probe different regimes: the total mass thanks to the big lensed, distorted, circular arc of the background object, but the mass and properties near the center can also be measured under the right circumstances: when a radial arc, near the center of the lensing source, is present.

That brings us to the object known as “the cosmic horseshoe,” which was discovered by the Sloan Digital Sky Survey in 2007 and was then spectacularly imaged by Hubble back in 2011. For nearly 15 years, we’ve marveled at this exquisite example of a gravitational lens, which aligns the foreground lensing source with a background, blue, star-forming galaxy so perfectly that it almost makes a complete Einstein Ring.

However, it isn’t the ring itself that’s of great interest, although we can indeed use it to help better understand the foreground source’s mass distribution and to model the stellar mass of the galaxy. Instead, you can see (highlighted below) that, in addition to the main horseshoe-shape, there’s also a pair of features — a “stretched image” and a “radial arc” — that correspond to the same background object, with the radial arc appearing very close to the center of the host galaxy, and by extension, close to the supermassive black hole.

Although the “horseshoe” feature of the lensed background galaxy in the “cosmic horseshoe” system is clearly the most visually striking feature, the smaller features, in the two boxes, are gravitationally lensed copies of the same background galaxy, where the radial arc near the galactic center provides astronomers with an exquisite window into the dynamics of the stars near the central black hole.

That feature, and its analysis with both Hubble data and with new MUSE integral field spectroscopy data (which is important for science, but not so useful for making pretty pictures), is what enabled a team of scientists to detect and make an estimate for the mass of the supermassive black hole at the galaxy’s center. Despite this galaxy being located an impressive ~5 billion light-years away, meaning its light was emitted from a time when the Universe was only about two-thirds of its present age (or about the time the Solar System was forming), the black hole is among the most massive ever found: at 36 billion solar masses.

The fact that this black hole is so massive, and that the errors on it are so relatively small (at about the ~30% level), makes it arguably the most massive known black hole with such small uncertainties on its mass. When compared with galaxies found more locally, the team of scientists found that, just like the brightest cluster galaxy in Abell 1201, its black hole is much more massive than its central stellar velocity dispersion would indicate, and that additionally, the black hole appears to be overmassive compared to the total stellar mass of the galaxy. From this galaxy alone, when added to the data from nearby galaxies, it appears to favor the “black hole came first” option as respects our cosmic chicken-and-egg question.

Whereas “light” black hole seeds, such as black holes that arise from the first generations of stars that form, could theoretically explain many of the supermassive black holes that exist at both early and late times, there is now strong evidence, especially early on, that at least some population of heavy seeds, of around ~10,000-100,000 solar masses, is needed to explain the early, overmassive black holes spotted by JWST.

It’s true that this represents one of the best, most accurate, and most distant supermassive black hole measurements of all. At greater distances and earlier times, without this sort of enhancement from gravitational lensing, we can only obtain much poorer estimates of a black hole’s mass.

However, we can still obtain such estimates, as well as stellar mass estimates for their host galaxies, here in the JWST era: particularly if we combine high-quality X-ray data (such as with NASA’s Chandra X-ray observatory) with that JWST data. In fact, we can use these observatories in tandem to look at the earliest objects of all: quasars and ultra-distant galaxies, including little red dot galaxies.

What we find, remarkably, for the earliest galaxies of all, from the first ~1 billion years of cosmic history — going all the way back to just ~420 million years after the Big Bang for the most extreme galaxies with black hole mass estimates — is that nearly all of the ones with black holes display overmassive black holes. Instead of stellar mass to black hole mass ratios of 1000-to-1, as is typical here in our modern Universe, we find that galaxies have ratios that are more like 100-to-1, 10-to-1, or in a couple of spectacular cases (made more difficult by how faint galaxies with low stellar masses are at such distances), even 1-to-1. In other words, early on, “overmassive” black holes are actually typical.

When the data from a variety of “little red dot” galaxies are broken up into their stellar mass component versus the component arising from an active supermassive black hole, the mass ratios of the galaxy’s total stellar mass compared with the supermassive black hole’s mass can be determined. Many, and perhaps even most, of these black holes are found to be significantly overmassive: at much more than 0.1% of the mass of the stellar component.

We have long looked at the supermassive black holes we’ve been finding in the Universe, particularly at early times and in the most extreme, high-mass situations, and wondered, “how did these black holes get so big, so fast?” Many people who studied stellar evolution kept telling us that even if you assume that the first generation of stars was dominated by high-mass, short-lived stars that could directly collapse once formed relatively quickly, the scenario didn’t add up. That starting with a “seed” black hole of a few hundred solar masses or even a thousand solar masses, even just ~100 million years after the Big Bang, didn’t give a massive enough seed early enough to explain the black holes that we see.

Now, we don’t just have coarse estimates of black holes and their galaxies at early times to combine with the late-time, modern galaxies (and supermassive black holes) that we find nearby, we also have our first intermediate, in-between example of a massive, star-rich galaxy with an exquisitely measured heavy supermassive black hole inside. Although there are many different possible pathways we can imagine for galaxies to evolve and grow up, the full suite of data:

• early galaxies with JWST,
• modern galaxies with a variety of tools,
• and now the cosmic horseshoe, thanks to the effects of gravitational lensing,

points to the “black holes came first” scenario for the big chicken-and-egg question. We are now closer than ever to a unified picture of what’s happening as far as the coevolution of stars and black holes within the most massive galaxies over the course of cosmic history.

Whereas the blue and red-dotted lines at the lower-right of this diagram indicate the populations of modern day galaxies with black holes and stars in them, the JWST data from examining early galaxies, shown in multicolored data points elsewhere on the graph, indicate a severe departure from the modern-day relationship. This has severe implications for the seeds and origins of supermassive black holes.

Sure, it’s very cool that we’ve found such a profoundly heavy supermassive black hole: at 36 billion solar masses, and with such small uncertainties, we’re now legitimately approaching the theoretical limit for how heavy a supermassive black hole can be. Depending on the black hole’s spin and evolutionary merger history, that limit is somewhere between 50-270 billion solar masses, and “36 billion” is tantalizingly close to it. It’s through the remarkable alignment of this galaxy with our line-of-sight and the background object that literally “lights the way” for us, showing what’s going on in the galaxy’s core, that we’re able to obtain this critical information about a remarkable object in the Universe at such great distances.

But even more important is what it means for cosmic evolution, and the chicken-and-egg question about stars and black holes within a galaxy. When combined with the other data that we have, the evidence from the cosmic horseshoe favors the notion that black holes and galaxies coevolve, but that:

• black holes most likely came first (particularly in the high-mass black hole cases),
• originated from massive seeds (of tens of thousands of solar masses),
• and in the right environments, can continue to grow quickly even as their stellar masses lag behind.

The cosmic horseshoe isn’t just one unique data point either, however, as the earlier (and often overlooked) galaxy at the center of Abell 1201 reminds us. Future surveys, and possibly even current ones with Vera Rubin Observatory, for example, will certainly uncover more Einstein rings (or near-rings), more centrally located radial arcs, and will enable the detection and superior measurements of supermassive black holes all over the Universe. We may even spot them at still-greater distances at high precision, and could potentially extend our reach to significantly lower-mass galaxies. With this latest find, we have another clue to the greatest chicken-and-egg puzzle in all the Universe. With a little luck, we’ll have an even better picture of how our Universe actually grew up in just a few years.