Little red dots, or LRDs, are one of the most intriguing classes of objects JWST has revealed since it began scientific operations in 2022. They are extremely small on cosmic scales yet shine with a characteristic red color in the infrared, and researchers interpret them as black holes that are collecting matter from their surroundings and growing at a furious pace. Because their light has traveled for more than twelve billion years before reaching the telescope, each dot offers a snapshot of a supermassive black hole in the act of assembling itself during the universe’s first billion years or so of cosmic history.

The central puzzle motivating the study is one of the biggest open questions in modern astrophysics. Astronomers agree that a supermassive black hole sits at the center of essentially every large galaxy in today’s universe, with masses ranging from millions to billions of times that of the Sun. These objects are orders of magnitude more massive than ordinary stellar-remnant black holes, yet no one knows for certain how they grew so large so quickly. One possible explanation is that black holes merge with one another over time, building up mass in jumps rather than through steady feeding alone. Testing that idea requires finding black holes caught in the act of approaching one another, something that had never been convincingly achieved for LRDs until now.

The obstacle, ironically, may have been the search methods themselves. When two LRDs lie very close together on the sky, conventional analysis techniques, which rely largely on the overall brightness and color of a source, can mistake the pair for a single, complex object. Two growing black holes on the verge of a merger could therefore be hiding in plain sight within telescope data, misclassified as one source rather than counted as two. Tanaka’s team suspected that this blending problem was masking exactly the population of close pairs that theorists most wanted to find.

To get around the limitation, the researchers developed a new pixel-by-pixel color selection method. Rather than treating each LRD as a single point of aggregated light, the technique examines the color of every individual pixel in the JWST infrared images. Because the two components of a close pair imprint subtly different color signatures across the image, a pixel-level analysis can separate sources that traditional photometry would smear together. By fine-tuning conventional search procedures with this approach, the team looked specifically for objects cataloged as single LRDs that were in fact two black holes in the process of closing in on one another.

The search paid off. The team uncovered four sets of dual LRDs in the universe between 12.5 and 12.8 billion years ago, with each member of every pair lying extremely close to its companion. The separations ranged from a few thousand to a few tens of thousands of light-years, a distance far smaller than the roughly 100,000 light-year span of our own Milky Way galaxy. In other words, these growing black holes are packed within a fraction of a single galaxy’s width of each other, precisely the configuration expected if two galactic nuclei are being drawn together by gravity.

Of course, apparent closeness on the sky is not the same as true closeness in three-dimensional space. Two objects at very different distances from Earth can happen to line up along the line of sight, creating an illusion of a pair. To rule out this possibility, the researchers calculated the probability that such close alignments would appear by chance, based on the number and distribution of LRDs in the surveyed area. The odds made it unlikely that all four pairs were mere coincidences of projection. Instead, the analysis pointed to something more interesting: LRDs appear to cluster strongly on scales of several thousand light-years, meaning these rapidly growing black holes genuinely prefer the same cosmic neighborhoods.

That clustering carries a deeper implication for how galaxies and their central black holes co-evolved. Galaxy mergers are thought to be one of the mechanisms that transport large amounts of gas toward the centers of galaxies, driving matter inward and stimulating the black hole at the core to feed and brighten. The new results could indicate that galaxy mergers are linked to the rapidly growing black holes seen in the early universe, with the dual LRDs representing the smoking gun of galactic collisions in progress. If each dot in a pair indeed harbors a growing supermassive black hole, then the two black holes themselves might eventually spiral together and merge once their host galaxies fully combine.

If those black hole mergers do occur, they would not stay hidden forever. The collision of two supermassive black holes would churn the fabric of spacetime itself, producing gravitational waves, ripples that could be detected by future gravitational-wave observatories now being planned and built. The four candidate pairs identified in this study thus represent not only a snapshot of black hole growth in the infant universe but also potential future sources for an entirely different kind of astronomy, one that listens to the universe rather than looks at it. Confirming the pairs and tracking their evolution could connect the optical and infrared picture of early black hole growth with the gravitational-wave signals that mergers are expected to leave behind.

The team’s next step is to push the analysis further. The researchers plan to study the dual LRDs in more detail and to improve their pixel-by-pixel method by applying it to a larger sample of objects, turning a first handful of candidate pairs into a statistically meaningful census. By continuing the search for black hole mergers with this technique and measuring what fraction of massive black holes form close pairs, astronomers hope to quantify the role that mergers played in building the first supermassive black holes during their earliest evolutionary stages. For now, the four close pairs tucked inside JWST’s infrared images stand as a reminder that some of the universe’s most dramatic events can remain invisible until someone looks at the data one pixel at a time.

Subject of Research: Discovery of dual little red dots, candidate merging supermassive black holes, in the early universe using JWST infrared imaging

Article Title: Researchers have uncovered a growing black hole heading toward a merger in the early universe

Article References: Researchers have uncovered a growing black hole heading toward a merger in the early universe. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: James Webb Space Telescope, little red dots, supermassive black holes, black hole mergers, early universe, galaxy mergers, Kavli IPMU, infrared astronomy, gravitational waves, Publications of the Astronomical Society of Japan, black hole growth, quasars