JWST Unveils the Secrets Behind the Enigmatic “Little Red Dots”

During the summer of 2022, our understanding of the remote Universe — and the narrative of how galaxies developed and matured — transformed permanently as the James Webb Space Telescope (JWST) initiated scientific operations. With its exceptional light-collecting capability, it could identify fainter objects than ever witnessed. Its remarkable resolution allowed it to distinguish single-source items from extended or multi-source entities more effectively than before. Furthermore, with its specialized infrared abilities, it was capable of detecting more distant, high-redshift objects than had ever been achieved previously. Thanks to the advanced technology aboard JWST, we were set to uncover insights about the earliest phases of cosmic evolution regarding stars, galaxies, and supermassive black holes.

Drawing from all we had previously discovered, we held anticipations for:

• the quantity of galaxies we would observe in the early stages of cosmic history,
• the masses and luminosities of those galaxies,
• and the relative contribution of that emitted light from stars compared to that arising from active supermassive black holes within those galaxies.

Surprisingly, a novel category of object — Little Red Dots (LRDs) — appeared in considerable numbers within the data. These LRDs were entirely at odds with our predictions: they were extremely bright, which implied significant stellar masses, yet their abundance at such early times rendered the overall picture inconsistent. Now, as we approach the start of 2025, we have finally unraveled the mystery, with active supermassive black holes playing a crucial role in elucidating the Universe.

This infographic illustrates the structure and lifecycle of an active galaxy, where two bipolar jets are emitted from accelerated matter that develops within an accretion disk encircling a supermassive black hole.

In the majority of galaxies that we observe and evaluate, there are primarily two significant explanations as to why a galaxy might appear to be intrinsically red in hue when we analyze it.

1. The galaxy may exhibit a red appearance because its stars are predominantly aged; the brightest, hottest, and bluest stars are also the most short-lived. These “red-and-dead” galaxies are generally devoid of gas and have not formed any new stars in billions of years.
2. The galaxy might appear red due to an abundance of cosmic dust, which obstructs the light emanating from objects (such as stars) situated behind the dust. This light-blocking dust, comprised of dust grains of certain finite sizes, is notably more efficient at obstructing blue light compared to red light, thus this “reddening” effect would influence all the light produced by the galaxy prior to its traversal across the Universe.

For galaxies in proximity to us, we frequently encounter galaxies that fit into both descriptions: those that are inherently red due to age, and those that are inherently red due to dust. However, during the early stages of cosmic history — merely a few hundred million years or possibly a billion years post-Big Bang — it is impossible for there to be “red galaxies” of that initial type. The Universe was unequivocally too youthful for the hot, blue, luminous stars within them to have perished.

This animation showcases the Bok globule Barnard 68 across different visible and infrared wavelengths. The longer wavelengths reveal, this is not a void in the Universe but simply…a dusty mass of gas, where the longer (redder) wavelengths of light can infiltrate and traverse through the dust. As these dust masses develop and dissipate, the density of dust can be uncovered by analyzing the light obstructed and transmitted by static, background elements.

When we observe galaxies that are situated at great distances, especially on cosmological scales, there’s an additional “reddening” phenomenon we must consider: the impact of cosmological redshift. The more distant an object is from us, the longer the light must travel through the Universe to reach our vision. Due to the Universe’s expansion, as light traverses it, the wavelength of that light becomes elongated — or redshifted — into progressively longer wavelengths. This can cause a remote galaxy to appear red in comparison to a similar galaxy that is perceived nearby, but we can precisely measure how much.

In relation to a nearby galaxy, the light from a far-off galaxy will experience elongation, yet if we introduce a “stretch factor” to the light that we observe, we can grasp that all wavelengths are elongated by the same multiplicative factor. By dividing the wavelength of the light from the distant galaxy by this “stretch factor” — which is simply (1 + z), where z represents the redshift of the object — you accurately ascertain how that galaxy would look if it were in close proximity. Consequently, you realize that “red and dead” is not a viable explanation, so you would naturally attribute any lingering “redness” that appears in that galaxy’s spectrum to dustiness, since dust obstructs blue light but permits red light to pass through much more easily.

This artistic representation of the dusty core of the galaxy-quasar hybrid object, GNz7q, illustrates a supermassive, expanding black hole at the heart of a dust-laden galaxy that’s birthing new stars at a pace of approximately ~1600 solar masses worth of stars annually: a rate about 3000 times that of the Milky Way. If the early JWST galaxies are “contaminated” by an active galactic nucleus, this could lead to a bias in the initially calculated masses for these galaxies.

In simpler terms, the straightforward narrative is that:

• stars produce light,
• dust obstructs a portion of the light (more of the shorter-wavelength, bluer light and less of the longer-wavelength, redder light),
• and then that light traverses the Universe, with all wavelengths of light experiencing redshift by the same “stretch factor” until we see them.

This straightforward narrative worked exceptionally well for the majority of galaxies, with only active galaxies — or galaxies with feeding supermassive black holes at their cores — such as AGNs or quasars, presenting a significant “extra” source of light in addition to stars.

Keeping all this in view, we are now prepared to examine what JWST first observed when it turned its sights on the distant Universe. In addition to enhanced, more detailed perspectives of objects we were already aware of, it uncovered a vast number of objects that were too remote — i.e., whose light was too long-wavelength, due to cosmological redshift — to have been pinpointed and analyzed before JWST’s distinctive infrared capabilities. Some of these objects aligned with our predictions: faint and intrinsically blue, populated with young stellar populations that were in the process of formation.

However, the most luminous objects discovered at immense cosmic distances turned out to be these Little Red Dot galaxies (LRDs), and they completely contradicted our expectations.

Three of the “little red dot” galaxies identified by JWST, illustrated with their brightness (y-axis) plotted against their wavelength (x-axis). At short wavelengths, the brightness remains independent of the wavelength, but at longer wavelengths, the brightness increases as the wavelength expands.

These LRDs resembled no other category of object we had encountered previously. Upon examining the light they emit — meaning when correcting for the impacts of redshift — you discover that the light’s spectrum is consistent in the ultraviolet, then ascends at long wavelengths in the optical and into the infrared. The spectrum of these galaxies features broad emission lines, suggesting some form of high-energy source capable of exciting vast numbers of atoms. Additionally, they are extraordinarily prevalent, representing 20% of all broad emission-line sources in the Universe discovered when the Universe was merely 1.2 billion years old or less.

This is why, at first, these entities were posited to potentially “revolutionize the Universe” as we understand it. If the following statements were accurate:

• stars generate light,
• dust obstructs some of that light,
• that light crosses through the Universe,
• and experiences redshift as it journeys,

then the only explanation for the luminosity of these LRDs would necessitate having extraordinarily large stellar masses within these galaxies. However, this poses a dilemma, because

it requires time for formations to emerge and for substances to condense in the Universe, and more substantial entities necessitate extended durations to accumulate. If these LRDs adhered to the principles we previously established, then there were:

• excessive objects,
• that exhibited excessive brightness,
• possessing an oversupply of stars,
• and containing excessive mass within them,

making it impossible for our current understanding of the Universe to account for. This explains the multitude of assertions, particularly during the early phases of JWST in 2022 and 2023, that these Little Red Dots had disrupted our conception of the Universe.

Including illumination from not only stars but also the central, supermassive black hole clarifies the unexpected additional brightness from these early galaxies. The inquiry about their observed abundances, which slightly diverges from theoretical models, remains unresolved.

So, was the Universe indeed disrupted? Or had we neglected some facet of astrophysics regarding our assumptions about these LRDs?

In science — generally — we must uphold skepticism towards any findings that contradict our anticipations. We owe it to ourselves and the broader scientific community to thoroughly investigate for potential influences we have yet to recognize. One viable interpretation for the “heightened brightness” of these LRDs suggested that only a portion of the light originated from stars, with the remainder stemming from a central supermassive black hole, akin to a quasar or an AGN.

Numerous astronomers were swift to dismiss such an idea for an elementary reason: we had already detected quasars and AGNs at later epochs in the Universe: when the Universe was approximately 1 billion years or older. We understood their abundances at various moments in cosmic history, enabling us to predict the frequency of quasar-like and AGN-like systems existing even earlier. However, upon receiving data, the density of the LRDs identified was nearly tenfold greater than the projections based on luminous quasars and even more significantly overabundant compared to the recognized population of AGNs.

By merging information from Pandora’s Cluster, Abell 2744, utilizing both infrared JWST and X-ray sensitive Chandra space observatories, researchers successfully identified several lensed galaxies, including one that emits significant amounts of X-ray light from the early stages of the Universe’s history, even while emitting minimal ultraviolet/optical/infrared light. This “overmassive” black hole contains crucial insights about black hole formation and growth.

However, this didn’t necessarily imply that there weren’t active supermassive black holes influencing the brightness of these LRDs; it could also indicate that something is amiss with our simplistic extrapolations. Indeed — based on an entirely separate line of evidence — there is a breakdown: the assumption that supermassive black holes constitute only about 0.1% of the total stellar mass within the galaxy hosting the supermassive black hole. As was first demonstrated in late 2023, supermassive black holes can possess far greater masses compared to the stellar mass within their galaxies, particularly in the first ~1 billion years of cosmic history, than merely 0.1% of the total stellar mass.

JWST unveiled, especially during the formative periods of cosmic history, the presence and extensive number of what we define as overmassive black holes: black holes that are considerably more massive than just 0.1% of the overall stellar mass of the galaxy. Many early galaxies harbor supermassive black holes around ~1% of the total stellar mass, some early galaxies have supermassive black holes nearing ~10% of the total stellar mass, and at least one galaxy contains a supermassive black hole with a mass equivalent to the combined stellar mass. In essence, compared to the stars in a galaxy, the supermassive black hole can significantly influence the overall luminosity of an object, even more so than in quasars or AGNs observed in more contemporary epochs.

When the information from various “little red dot” galaxies is disaggregated into their stellar mass contributions versus the contributions originating from an active supermassive black hole, the mass ratios comparing the galaxy’s total stellar mass to the mass of the supermassive black hole.can be established. Numerous, and likely the majority, of these black holes are observed to be notably overmassive: exceeding over 0.1% of the total mass of the stellar element.

It may be that these LRDs, or at least a portion of them, are not receiving all of their luminosity from stars, but rather that active black holes are amplifying their luminosity. By doing so, they ultimately deceive us — if we cling to our simplistic beliefs — into thinking that we inhabit a Universe that appears inconsistent.

This concept may seem reasonable to you, yet it could also seem absurd; how an idea “appears” is not necessarily a reliable determinant of its correctness. What is required instead is to examine a substantial sample of the pertinent entities — the LRDs, in this scenario — with enough accuracy to understand their characteristics and ascertain if this possible explanation holds water or not.

This is exactly what a research conducted by a broad coalition of scientists utilizing JWST data, spearheaded by Dale Kocevski, has recently achieved. They conducted a survey of LRDs from various JWST observation initiatives, including CEERS, JADES, NGDEEP, PRIMER-UDS, PRIMER-COS, and UNCOVER. From the initial 1.1 billion years of cosmic evolution, they pinpointed 341 LRDs and uncovered a vast amount of details about them: insights that could determine whether supermassive black hole activity might reconcile these entities with our framework of the Universe’s development.

This visual represents 15 out of the 341 “little red dot” galaxies that have been identified in the early Universe by JWST. Each of these galaxies shares similar characteristics but only exists during the earliest epochs of cosmic history; there are no recognized instances of such galaxies nearby or in later times.

What insights did they gain about these Little Red Dot entities? A significant amount, including:

• that approximately ~70-80% definitely house accreting black holes,
• that they nearly vanish in observations beyond 1.2 billion years after the Big Bang,
• but they reach their peak around 900 million years post-Big Bang,
• and then their numbers decline further back in time,
• but they are observable at least as far back as 400 million years after the Big Bang.

The LRDs that exhibit indications of supermassive black hole activity also provide some of the most compelling evidence for “overmassive” black holes: where the estimated mass of the supermassive black hole surpasses 0.1% of the total stellar mass of the galaxy.

Nevertheless, these LRDs seem to represent something distinct when contrasted with other known entities. Do they have dusty tori surrounding them similarly to known quasars and AGNs? We are uncertain; there is no observational data suggesting that the answer is “yes,” but we cannot discount it. Among the 341 LRD galaxies identified thus far, only two demonstrate X-ray emissions: marking them as the first LRDs recognized to emit such radiation. Furthermore, when you analyze the light they produce, the “flat, rest-frame ultraviolet” light must originate from stars within the host galaxy, while the “rising-at-long-wavelengths, rest-frame optical and infrared” light must be attributed to the inputs from the active supermassive black holes present within them.

In each of the “little red dot” galaxies identified in the early Universe (mainly by JWST), there seems to be a UV-rest-frame component dominated by stars, along with an optical/IR component driven by the activity from a supermassive black hole. By analyzing and quantifying the contributions from both, one can ascertain the stellar mass and black hole mass components of the galaxy.

This is the essential aspect of the narrative: the element that permits us to unravel the enigma. By understanding that these Little Red Dots receive input from stars within the galaxy as well as from the activity of the central black hole, one can quantify which portions of the light observed (the flat, rest-frame ultraviolet) originate from the stars in these LRDs compared to the segments (the rising, rest-frame optical and infrared) produced by the active black hole, enabling estimates for:

• the stellar mass of every galaxy,
• the central black hole mass for each galaxy,

and subsequently ascertain bothhow extensive the galaxy is and just how “overmassive” each black hole remains.

It is solely through collecting this information and carrying out such an examination that we can arrive at the definitive conclusion: regarding whether these LRDs do in fact “break the Universe,” or if the now more-precisely-measured characteristics of these LRDs align with the concordance cosmology utilized to depict the remainder of the Universe. As you may have anticipated, cosmology has persevered, and the Little Red Dot galaxies we observe can be accounted for by a stellar component alongside an active black hole component, instead of just stars-and-dust alone. With the Universe secure now, the forthcoming steps will involve deepening our understanding of the essence of these LRD entities, exploring their creation, evolution, attributes, and the characteristics of their vanishings as time progresses and the Universe expands: from its early days into cosmic maturity.