The most overlooked fact about JWST’s Little Red Dots

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When JWST opened its eyes, it spied a huge number of Little Red Dots. What we saw inside was a puzzle, but what's missing could solve it.

An astrophysics column on big questions and our universe.

When JWST opened its eyes on the Universe, it glimpsed the Universe at farther distances, fainter magnitudes, and higher resolutions than ever before. One of the more profound surprises that emerged was a bright class of ultra-young, distant objects known as Little Red Dots. If all of the light from these objects were coming from their stars, they’d have to be ultra-massive at such early times: a colossal difficulty for our consensus cosmology to explain.

The history of astronomy is all about using what we know and combining it with what we see to infer a full picture of what’s out there. We see the light from these Little Red Dots, make inferences about the populations of stars that must be inside, and wonder how things could’ve come to be the way we observe them to be. But if we have too many objects emitting too much light too early on in cosmic history, then we have a puzzle: how did these objects get so abundant and so massive so quickly?

We often use what we know nearby — what we can see in great detail, inside and out — and use what we can measure much farther away to draw conclusions about all the rest of what might be inside: including what’s beyond our observing limits at such distances. We can see the proverbial “tip of the iceberg,” but what if the “underwater” part of the iceberg is different, smaller, or even absent compared to the full iceberg we can see nearby? That might be precisely what’s going on with Little Red Dots, and here’s what we typically overlook that, just possibly, is essential to solving the puzzle of what they fully are.

The big questions surrounding Little Red Dots involve explaining what’s causing such an abundance of them, at such great brightnesses, so early on. If the light that JWST was seeing were all due to stars — what provides the light here in the local Universe — they’d be way too massive for our current model of cosmology to explain. Several contributions, however, have already been identified:

It’s possible that all of these factors, when taken together, could explain the brightnesses and abundances of the Little Red Dot populations that we’ve seen. However, even with all of these factors taken into account, we still might not have the full story. After all, puzzles still remain.

One such puzzle is that Little Red Dots, despite being all lumped together, aren’t across-the-board similar in all ways. There are two species of Little Red Dots, as some of them appear point-like, with all of their light concentrated into the same few pixels that a true point source would create, while others appear extended: with emissions that cannot be explained by a point-like source alone, but rather require a large volume of space to be illuminated at these great cosmic distances. A single explanation would not work for all of these Little Red Dots.

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Another puzzle is that these Little Red Dots, despite their overwhelming appearance in the ultra-distant Universe, cannot possibly represent pristine objects. They come with a wide variety of metallicities, or heavy element abundances, present among the material within them, but absolutely none of them are metal-free, or pristine. This is normally interpreted as implying that previous generations of stars have already formed inside of them, making them perhaps more mature objects than whatever seeds they grew from.

And for yet another, even if we have explanations for why there are so many Little Red Dots at such great distances, they’re still producing an awful lot of light. Even though we aren’t sure what the rest of the story is for producing that light, it makes sense to infer “starlight” for much of it, but if we infer that all of the light comes from stars and stellar populations that are similar to the stellar populations we see nearby, there’s way too much stellar mass for the early times we’re probing.

That’s where the story really gets interesting.

One idea is that there aren’t stars — at least, not enough of them to produce a significant fraction of the light we see — from these Little Red Dots. If it isn’t starlight that’s causing these Little Red Dots to shine, it must be due to energy coming from a different source. There are a few options for what that source could be.

It could be a huge cloud of gas that’s gravitationally contracted down, turning potential energy into kinetic energy, so that it shines brightly at thousands of degrees, but over a volume of space much larger than any star. To support this huge amount of mass over such an enormous volume, however, a source of energy is required.

One option is that the source could be an active black hole at the center of this cloud, accreting and accelerating matter, and injecting it back into the surrounding environment. Active black holes typically accrete matter in a disk, and at the high temperatures found in that disk, atoms are fully separated into their two components: electrons and ions. As they swirl around in that disk, they create magnetic fields, which accelerate matter and inject it back into the surrounding material. However, that should make a disk with two bi-directional jets pointing perpendicular to the disk. Because of that configuration, this explanation has difficulty explaining the symmetric, point-like nature of most of the Little Red Dots.

Another scenario involves an actively accreting black hole at the center of a large cloud of gas, except this time, the energy doesn’t come from matter getting accelerated from within the black hole’s accretion disk, and then re-injected into the material of the surrounding medium. Instead, because the cloud is so large and massive, the temperatures rise into the millions of degrees in the cloud’s interior: near the black hole itself, but over a much larger volume. Because the temperatures get so high in this large region, nuclear fusion occurs, and that energy can indeed get ejected spherically outward: explaining the configuration observed.

This has another attractive feature: if fusion is occurring within this cloud, heavy elements are getting created. With such high masses, there could be not just hydrogen fusing into helium, but helium fusing to create elements like carbon, neon, oxygen, and more. This scenario could then potentially provide heavy elements without the need for prior generations of stars at all: explaining the significant and varying metallicities within Little Red Dots without the need to invoke large numbers of stars.

However, this model also has a drawback: once you make a seed black hole at the center of this cloud, it should begin to devour the cloud relatively quickly. Yet we don’t see evidence for these Little Red Dots shrinking, evolving, or varying over time; they seem to persist in an unchanged state for at least many millions of years or more. It isn’t clear that a supermassive cloud of gas surrounding a black hole can support itself by fusion without rapidly growing the black hole in a way that would show up observationally.

Or, perhaps most exotically of all, there could potentially be no black hole at all, but rather a single, supermassive star surrounded by an enormous envelope of matter: supported by the intense nuclear fusion going on at the core. If the envelope expands enough to occupy a large enough volume, it could provide the right temperatures and brightnesses: explaining what we observe. However, the more massive a star is, the greater its core rate of fusion and the shorter its lifetime is. For a supermassive star — not of hundreds or even thousands of solar masses, but millions — its lifetime would be incredibly short: so short that we should see Little Red Dots evolving in ways that are inconsistent with observations.

The puzzle is so deep, and so important to how our Universe grew up, that people are resorting to progressively wilder and wilder scenarios to try to explain their brightness, abundance, spectra, and other physical features. However, there is still hope for perhaps the most basic explanation of all: that these Little Red Dots are simply powered by stars. There may or may not be active black holes along with those stars — a potential explanation for the two types of (point-like and extended) Little Red Dots — but the stars themselves could still provide most of the light without implying that these galaxies are too massive too early on in cosmic history.

In our modern Universe, whenever we form stars from a large, massive cloud of matter, the stars form with a wide variety of masses: from tiny, low-mass red dwarf (M-class) stars up to huge, massive blue supergiant (O-class) stars. They range in mass from just 0.075 solar masses (the lower-limit of mass required to form a star) up to a maximum of 260 solar masses, which is the estimated mass of the most massive single star presently known: R136a1. The masses of these stars form throughout this mass range in a spectrum, where the specific shape of that spectrum is called the IMF: the initial mass function.

What’s kind of wonderful about the IMF is that, in all the places we’ve ever observed stars, the IMF doesn’t seem to vary all that much. Whether you have:

everything is consistent with that same IMF. The only exception should be the very first stars of all — stars made of matter with no heavy elements at all — where the stars that form continue to gain mass without limit: forming a top-heavy initial mass function, theorized but not yet observed.

However, there’s a limitation to this: we can only observe the full spectrum of stars — from bright, blue, ultra-luminous giant stars to faint, red, low-mass, dimly-shining dwarf stars — nearby. A star that’s hundreds of solar masses is going to shine millions of times as brightly as the Sun, while a star that’s only 10% of the Sun’s mass might emit just 0.1% of the Sun’s total energy, and emit just 0.005% of the visible light that the Sun emits. When we see populations of stars that are far away, the light that we observe is dominated by the output from the brightest stars of all. We cannot see the fainter ones at all; we can only infer that they’re there based on what we see nearby.

It’s like seeing the tip of the iceberg: we know how ice works, and so when you see the tip of an iceberg, you can infer that the rest of the iceberg ought to be there, even if we can’t see it. We do the same when it comes to the light from distant populations of stars:

It’s a straightforward method, but it relies on an assumption: that both the physics and the physical conditions that lead to the formation of a mass spectrum of stars are the same back then as they are today. But there was a key difference that’s often overlooked, or assumed to be negligible: the Universe, as it was back then, was smaller, denser, and hotter.

The “hotter” part is normally ignored. After all, today’s Universe is very cold: at 2.725 K, with a photon density of a few hundred photons per cubic centimeter, as dictated by measurements of the CMB. But the Universe was hotter and denser early on.

At hotter temperatures, it takes more and more mass to cause a clump of matter to gravitationally collapse. Early on in the Universe, when temperatures were much hotter, this should change the IMF: not by making it top-heavy, but rather by making it bottom-light: by forbidding the formation of stars below a certain mass threshold. It’s as though we’re still seeing the tip of the iceberg as far as the full mass spectrum of stars go, but when we look to great cosmic distances and earlier cosmic times, the “underwater” part of the iceberg could be much less than it is today, and in extreme cases, may even be absent entirely.

In other words, the brightest, most massive stars that emit the majority of the light — but that don’t contain the majority of a stellar population’s mass — might represent the majority or even the entirety of the stellar population that’s out there in the distant Universe.

If these Little Red Dot galaxies do indeed have bottom-light mass distributions within them, it could provide an important caveat to the question at the core of their very existence. Normally, we ask, “how did there get to be so many of these objects, with such large masses and so many stars, so early on in cosmic history?” The overlooked part of the story, if the higher temperatures early on would instead imply a “bottom-light” mass distribution in these Little Red Dots, could then imply that they don’t have such large masses and they don’t have so many stars. Rather, we’ve assumed a modern-day IMF (and a modern-day “iceberg”) when instead, the IMF early on (and the “underwater” component of the “iceberg”) could lead to radically different conclusions.

However, even at the early times where many of these Little Red Dot galaxies are found, largely from the first few hundred million to two billion years of cosmic history, we do have local relics that formed at around the same time: the oldest globular clusters found in our own Local Group and Milky Way. Using JWST observations, astronomers were able to identify stars all the way down to just 10% of the mass of the Sun, demonstrating that even when the Universe was much hotter than it is today, even the lowest-mass stars could still form. Assuming we’ve got the ages of the stars within these globular clusters correct, and that there wasn’t a second, much more recent star-forming episode within them that created the lower-mass stars we observe, these observations would disfavor the bottom-light IMF hypothesis.

However, we ignore the physics of a hotter Universe — and its potential impact on what’s actually inside these Little Red Dots — at our own peril. These objects remain luminous, abundant, and not yet fully explained. We don’t know why there are so many, of seemingly such great mass and definitely such great brightness, at such early times. Until we definitively solve the puzzle, all options that are consistent with the full suite of physics we know should be included, and that means no more overlooking the effects of a hotter, denser Universe on the early formation of stars at early cosmic times!

Theoretical astrophysicist and science writer

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