Everywhere we look in the Universe, in all directions and at all distances, we find the same thing: galaxies. These galaxies come in a wide variety of sizes, masses, and shapes, but nearly all of them — especially the largest, most easily-observed ones — have something in common: they’re full of stars, and they possess a supermassive black holes at their cores. For a long time, scientists have wondered not only where these supermassive black holes came from, but how they got so big even early on in cosmic history, and whether they formed before the stars in the galaxy itself, alongside them contemporaneously, or only afterwards: from the corpses of those early stars that lived, burned through their fuel, and died.

While we’ve certainly gained a wide variety of clues here in the JWST era, a definitive answer to the big chicken-and-egg question of what came first, the supermassive black hole at the centers of galaxies or the stars that swirl around them, has thus far remained elusive. But all of that appears to have changed with a brand new discovery in the background of galaxy cluster Abell 2744. Deeply imaged by JWST in a variety of filters and for long stretches of time, an incredible object was spotted in the background: a gravitationally lensed “little red dot,” stretched, magnified, and appearing three different times across the image.

With the light coming from just 700 million years after the Big Bang, the observations enabled us to measure not only the black hole’s mass, at a remarkable 50 million solar masses, but the rotational behavior and chemical enrichment of the material surrounding it. It provides the strongest evidence yet that supermassive black holes formed before, and independently of, the stars in the galaxy surrounding it: a fascinating conclusions with cosmic implications.

This three-panel image shows the extended radio emissions of Messier 87*, at top-left, the Hubble optical image of the jet, at top-right, and a radio image using very-long baseline technology of the region close to the black hole, clearly collimated by a magnetic field, with higher radio energies shown in red. The mass of the black hole at the center of this galaxy is around 6.5 billion solar masses, but approximately 2.4 trillion solar masses worth of stars inside it for comparison: a ratio of approximately 1-to-1000.

When we look at the nearby Universe, we see large, massive galaxies in great abundances, with the Milky Way and our nearest large neighbor, Andromeda, serving as two stunning examples of spiral galaxies. While the overall mass of a galaxy is dominated by a mix of dark matter, gas, and stars, there’s also a substantial supermassive black hole at the center of each one. However, our supermassive black hole, Sagittarius A*, only reaches up to around 4.3 million solar masses: just 0.001% of the total mass in the form of stars. Other galaxies are known to have black holes in the billions or even tens of billions of solar masses, making up about 1-part-in-1000 of the total stellar mass.

However, the nearby galaxies that we see represent the cosmos as it is today: after 13.8 billion years of evolution. The majority of stars that formed in these galaxies did so between 2 and 7 billion years after the Big Bang, which we know from measuring the star-formation rate across cosmic history. However, we had long seen distant galaxies and quasars from even earlier cosmic epochs, from the first billion years of cosmic history, where the central supermassive black hole had already grown up to be a billion solar masses or more. The mystery of “how did these black holes get so big so early on?” has plagued the field ever since.

If you begin with an initial, seed black hole when the Universe was only 100 million years old, there’s a limit to the rate at which it can grow: the Eddington limit. If seeds of several tens-of-thousands of solar masses arise early on and these SMBH seeds grow rapidly thereafter, there may be no conflict with what’s observed, after all.

In theory, there are three main scenarios for explaining how these supermassive black holes came to be.

1. In the most conservative scenario, there are no black holes that arise until stars form. Only after the first stars — many of which may have been much more massive than even the most massive stars today, perhaps up to 1000 solar masses or even more — are born, live, and die, do the first seed black holes form. Then, they sink to the centers of galaxies, where they merge, accrete more matter, and grow, eventually becoming the seeds for the supermassive black holes we observe today.
2. In a more recently-developed scenario that still invokes no new physics, it isn’t just stars that form out of the pristine material left over after the Big Bang, but rather you can get converging, cold streams of gas that collide, and lead to very large clumps of matter — in the range of tens of thousands of solar masses — that can directly collapse to form black holes without going through a stellar phase first. These direct collapse black holes, of around 40,000 solar masses apiece, then become the seeds for growing into supermassive black holes, forming as little as 100-150 million years after the Big Bang.
3. Or, in the most exotic scenario, it’s possible that the Universe was born with an array of black holes at the outset: the primordial black holes scenario. This would require some form of new physics, as primordial black holes are forbidden unless extremely large density fluctuations (on the order of 100% of the average cosmic density) are present, while the fluctuations in the cosmic microwave background instead point to even the largest-magnitude fluctuations being smaller than the 0.01% level.

The problem is, in order to create the supermassive black holes that we see in the early Universe, the first scenario doesn’t work; the black hole seeds are too small.

This snippet from a supercomputer simulation shows just over 1 million years of cosmic evolution between two converging cold streams of gas. In this short interval, just a little over 100 million years after the Big Bang, clumps of matter grow to possess individual stars containing tens of thousands of solar masses each in the densest regions, and could lead to direct collapse black holes of an estimated ~40,000 solar masses. This could provide the needed seeds for the Universe’s earliest, most massive black holes, as well as the earliest seeds for the formation of stars and the growth of galactic structures.

However, if you form black holes independently of stars, rather than from their corpses, then not only can you obtain larger black hole seeds at very early cosmic times, but you should expect that, early on, the ratios of “black hole mass” to “stellar mass” would have been much larger than the 1-part-in-1000 that we see today. The presence of overmassive black holes, which could exhibit a 1-to-100, a 1-to-10, or even a 1-to-1 ratio, would strongly disfavor the simplest, most conservative scenario for how these supermassive black holes came to be: the scenario where they formed only in the aftermath of stars living and dying.

We’ve indeed learned exactly this: that stars alone can’t explain the supermassive black holes we see, particularly here in the JWST era. We’ve found many galaxies with a 1-to-100 ratio, several others with a 1-to-10 ratio, and even one remarkable system known as UHZ1 that appears to have a black hole of between 10-to-100 million solar masses, but only 10-to-100 million solar masses worth of stars: the first system with a 1-to-1 ratio, giving it the most “overmassive” black hole in the known Universe at the time of its discovery. It suggest that, somehow, at least one massive black hole formed prior to and independent of the stars within a given galaxy.

Instead of a black hole-stellar mass ratio of about 1:1000, as seen in modern times, the JWST galaxies from the first 1.5 billion years of cosmic history exhibit a black hole-stellar mass ratio more consistent with 1:10 or 1:100, indicating that more massive seed black holes were required to explain the observations. This has severe implications for the seeds and origins of supermassive black holes, suggesting a “heavy seed” formation mechanism.

That brings us up to the present day. What we’d really love to be able to do is find a very early system where we had a supermassive black hole within an active galaxy: one emitting a large amount of light, whose radiation would be detectable even after traveling through the expanding Universe for 13 billion years or more. But ideally, that galaxy would have very few stars, would be small and low in mass, and would have its gas oriented at least partially inclined to us: edge-on, rather than face-on. If the galaxy weren’t exactly face-on, then we’d be able to measure its rotation, at least, if our telescopes were good enough.

For galaxies in the very early Universe, even JWST isn’t a large enough, high-resolution enough telescope to do that: at least, under normal circumstances. However, if the early, active galaxy were instead located behind, but along the line-of-sight, of an intervening massive object (like a galaxy cluster), then its light could be bent, stretched, and magnified, potentially enabling us — if the configuration were just right — to measure the properties of not just the central black hole to great precision, but of the gaseous material surrounding the black hole as well.

For a few years, this has only been a hope. But a recently discovered object, Abell 2744−QSO1 (or QSO1 for short), has just brought that hope to fruition for the first time.

This image shows JWST’s NIRCam view of the broad galaxy cluster Abell 2744, also known as Pandora’s Cluster, with three inset boxes labeled A, B, and C. These boxes all contain gravitationally lensed images of the same background object, Abell 2744-QSO1, magnified, time-delayed, and distorted by different amounts. QSO1 is about 1300 light-years across, with its light coming to us from a time when the Universe was merely 750 million years old.

In a series of two papers, this tiny galaxy is found to be located behind the iconic galaxy cluster Abell 2744: the main target of the GLASS survey with JWST and one of the most scientifically prolific JWST deep fields so far. It’s triply lensed, meaning that there are three different light-paths that bend the galaxy’s light towards our eyes. The images that we see are, to varying degrees, magnified and distorted, but also stretched: appearing far larger in angular size than its actual physical size dictates. That means it doesn’t just appear as a single pixel to JWST’s eyes, but rather appears extended, with its light spread out across many pixels in JWST’s detectors.

This provides a great opportunity to conduct an unprecedented form of science.

Most importantly, it means that instead of relying on proxies to estimate the black hole’s mass, you can now use equipment aboard JWST to determine the mass of the black hole directly. You can do this by using the integral field unit aboard JWST’s NIRSpec instrument, measuring the surrounding gas’s motion relative to the black hole itself. Based on how the gas rotates at a variety of distances from the central black hole, you can construct a rotation curve, estimating the total mass of the black hole and also determining whether it behaves as a point-source as far as mass goes, or whether the mass follow an extended distribution.

This detailed picture of galaxy Abell 2744-QSO1, from JWST’s NIRCam imager, has the inferred gas speeds of the material surrounding it overlaid in the panel at the right. Material in orange is moving away from us; material in blue is moving towards us. The data shows that the rotation is Keplerian: as though practically all of the mass were located at a single point in the center.

That’s exactly what the team that just put out two recent papers worked hard to do. Before their work, this object had been identified and had the mass of its central black hole estimated by conventional means: by looking at the light coming from this “Little Red Dot.” Objects like this are plentiful in the early Universe and show up quite frequently in deep-field JWST images, appearing red in color, with broad hydrogen emission lines, and looking a lot like an active galactic nucleus (AGN). These little red dots, collectively, make up about 15-30% of all high-redshift, broad-line AGNs. Based on traditional, standard modeling, a black hole mass estimate of 40 million solar masses was inferred, but with huge uncertainties.

Led by Ignas Juodžbalis and Roberto Maiolino, respectively, the two papers were able to actually put together a rotation profile, based on observations of the relative motion of the gas to the central galactic nucleus, which allowed them to determine both the mass of the central object and whether the mass was all concentrated in the center, or whether it was distributed throughout the galaxy. The object itself, QSO1, was determined to be only 1300 light-years across, but was shown to have a central nucleus whose mass is right around 50 million solar masses (pretty close to the initial 40 million solar mass estimate), and where practically all of the mass was concentrated in the central core.

Using a combination of spectroastronomy for the portions of the galaxy QSO1 close to the central black hole and kinematic observations of the rotating gas for greater distances, astronomers were able to put together a rotation curve for the inner 500 light-years of the galaxy, showing that nearly all of the mass is concentrated towards the center, and that very little exists in a disk or halo surrounding it.

This, already, is remarkable. It tells us that this galaxy does indeed house a large, supermassive black holes at extremely early cosmic times. In fact, its light comes to us from a redshift of z = 7.04, giving it a cosmic age of just 750 million years after the Big Bang. As you move farther away from the galactic center, the study authors found that the motion of the gas around the galactic center was very high at short distances, but fell off very quickly: just as Newton’s force law, with a ~1/r² profile, predicts. That tells us that, compared to the mass in the central core, there’s only a negligible amount of mass in a volume of space hundreds of light-years in radius surrounding it: strongly disfavoring the presence of a large mass of stars, gas, or dark matter in that region.

It also tells us that our indirect mass estimates, made using methods that were unconfirmed to apply at these great distances, ought to actually be valid, at least in some instances (like here, in the case of QSO1). According to study coauthor Francesco D’Eugenio:

“Before now, all of the mass measurements of black holes in the early Universe have been indirect, based on assumptions from what we know about them in the local Universe. We didn’t know if those assumptions really apply to the distant Universe.”

But now, we know that they not only do apply, but that they point to a central black hole dominating the mass and the light coming from this galaxy.

This graph shows the ratios of the signal strengths of doubly ionized oxygen compared to the Balmer emission line known as Hβ (y-axis), with a fit to the implied metallicity on the x-axis. Although the metallicity is slightly lower at greater distances from the galactic center of QSO1, as expected, the extremely low overall metallicity (as inferred from the top panel) of just 0.4% of the Sun’s metallicity is extremely low, making this a galaxy that has not undergone very much star-formation at all overall.

Additionally, the researchers were able to use their spectroscopic observations to search for the presence heavy elements within the gas: a measure of the cumulative amount of past star-formation that’s occurred in that material. Initially, the Universe consisted only of hydrogen and helium prior to the formation of the first stars. Today, about 1-2% of the normal matter found in a galaxy (like our galaxy) is made up of heavier elements: oxygen, carbon, neon, nitrogen, iron, and a variety of others. By comparing the heavy element content (or metallicity) of an astronomical reservoir of material and comparing it to the heavy element content of our Sun and Solar System, we can learn something about the cumulative amount of star-formation that’s already occurred inside.

They found an extremely low metallicity: of only about 0.4% of the total fraction of heavy elements found in the Sun. This indicates only a small level of previous cumulative star-formation, and — for the first time — a ratio of the black hole mass to the stellar mass of the overall galaxy that was greater than 1-to-1, where the central black hole itself is more massive than all the stars in the galaxy surrounding it combined. Even though the individual stars themselves are not measurable or resolvable, the authors clearly state that there’s practically no room for a substantial stellar mass component at all. They infer that the black hole is at least double (and probably more) the mass of all the stellar mass in the entire galaxy, and interpret it as a massive black hole seed caught in the early phases of growth and accretion.

This figure shows the mass of the central supermassive black hole (y-axis) as a function of the stellar mass of the galaxy that the black hole is present within (x-axis), for a variety of different galaxies. The new object, QSO1, is shown with a red star: the single most overmassive black hole yet discovered in the entire Universe.

All in all, it’s the strongest clue yet that black holes, not stars, dominate not only the mass of early galaxies, but the light coming from these Little Red Dots in the distant Universe. It points towards the black holes being the answer to the “what came first” question when it comes to black holes and stars in early galaxies. And it gives us our most severe example of an overmassive black hole ever: with the greatest black hole-to-stellar mass ratio in the entire Universe so far. Even more remarkably, it’s in an extremely pristine environment: with few stars, a small amount of mass, and only very mild chemical enrichment.

What we still don’t know, however, is tremendous. We don’t know whether QSO1 is a prototype for most-to-all of the Little Red Dots and early galaxies that we’ve found so far, or whether it’s just one example, and perhaps not a typical example, of how galaxies form and grow up. We don’t know what the stellar mass and stellar population of this object is: how the stars within it are distributed and when they formed. And, perhaps most importantly, we don’t know whether this black hole arose from a scenario with no new physics, like direct collapse of converging, cold streams of gas, or from a scenario involving something exotic, like primordial black holes or something even perhaps stranger.

As always, no matter how far we come in science, there’s always farther to go. May we continue to invest in seeking the greatest of all cosmic truths in the only way that’s ever been successful: by asking the Universe, in quantitative ways, questions about itself.

This article JWST proves that black holes really do come before galaxies is featured on Big Think.

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