Astronomers analyzing data from the James Webb Space Telescope have discovered that mysterious little red dots in the early universe hide compact galaxies containing roughly a billion solar masses, suggesting supermassive black holes formed before their host stars.
In 2023, the James Webb Space Telescope revealed something strange: the distant reaches of the universe are filled with little red dots
that astronomers still can’t quite explain. The distant reaches of the cosmos contain an abundance of compact, intensely red objects that have baffled researchers since their discovery. The James Webb Space Telescope is the most powerful telescope ever made and has enabled the discovery of things we had never imagined. This is the case for little red dots. Their name is a simple derivation of their appearance: they appear as very small compact dots that are “red” in colour (noting the James Webb Space Telescope sees in the infrared). These objects are extremely distant, and are seen at an epoch when the universe is only around one billion years old (based on the time it takes the light to reach us). Their compact size and extreme distance mean that most of their light comes from an area smaller than 2% of the size of our Milky Way galaxy.
Despite finding many of these distant objects, astronomers are still unclear what they might be. The current favoured theory is that most little red dots are young supermassive black holes, with some consuming matter at such a feverish rate that their “surface” acts like the surface of a star (termed black hole stars
). Another theory is that some are regions of extreme star formation that form the most massive galaxies in the universe. JWST images reveal that mysterious little red dots
are surrounded by extremely compact galaxies containing roughly a billion solar masses in stars. The faint host galaxies contribute only about 10% to 20% of the redder light, suggesting a brilliant central source, potentially a growing supermassive black hole, dominates.
The universe is full of strange ‘little red dots
Peeling Away the Central Glare in COSMOS-Web Data
To help unravel the mystery of little red dots, astronomers have been looking to the host galaxies they reside in for answers, aiming to understand how galaxies and supermassive black holes grow in tandem with each other. Our new paper, led by Yiyang Zhang from Wuhan University in China, provides key insights astronomers have been looking for. A team led by Yiyang Zhang of Wuhan University tackled the observational challenge by analyzing more than 200 little red dots from the COSMOS-Web field, a broad survey covering an area about three times the apparent size of the Moon. We used high-definition James Webb Space Telescope imaging of the distant universe over a region of space about three times the size of the Moon, known as the COSMOS-Web region (which you can view online). More than 400 little red dots are found in this region, a large enough number that astronomers can now unveil the secrets of the galaxies they reside in. The primary hurdle in studying these objects is that the brilliant central source overwhelms everything surrounding it, making the faint host galaxy appear far dimmer in the red wavelengths measured by the telescope.
Compact Host Galaxies and Early Supermassive Black Holes
Universe-shaking collision of black hole and neutron star could
Measuring the size of little red dot host galaxies is not a simple task. Imagine trying to see the colour details in a painting with a flashlight at its centre that shines bright, red light directly into your eyes. This work uses precise modelling of the optics of the James Webb Space Telescope to remove the bright central light in each image. The findings, published in Nature Astronomy, come from an analysis led by Yiyang Zhang of Wuhan University. The team examined 217 little red dots using high-resolution images from the James Webb Space Telescope, or JWST. These objects sit at redshifts of about 5 to 9, meaning their light comes from an era when the universe was roughly a billion years old or younger. Their apparent size is remarkably small. Most of their light comes from a region less than 2% the size of the Milky Way. That compact appearance is part of what has made them so difficult to explain.
One leading idea holds that many little red dots contain actively feeding supermassive black holes. In some models, a black hole sits inside dense material that produces something resembling the glowing surface of a star, sometimes described as a black hole star.
Other explanations place more emphasis on extreme star formation. Finding faint galaxies around the bright central objects could help astronomers distinguish among those possibilities. The problem is that the central source overwhelms almost everything around it. At the red wavelengths measured by JWST, the host galaxy can be far dimmer than the center. The findings suggest black holes may have gained much of their mass before their host galaxies finished building their stars in the early universe.
Rethinking Cosmic Evolution and Binary Formation Pathways
JWST’s mysterious ‘little red dots’ may hide early black
The universe-shaking collision of a black hole and a neutron star just led astronomers to a strange type of orbital interaction never seen before, and it’s forcing them to rethink their theories. Before the two extremely dense objects crashed and combined, they first swooped around each other in an eccentric, oval shape resembling the swirls of a Spirograph, scientists reported March 11 in The Astrophysical Journal Letters. The new discovery challenges the prevailing assumption about how black holes and neutron star systems form, and whether they must fall into perfectly circular orbits before they die, according to the study authors.
The fact that this system is still eccentric at the very end of its life is essentially a smoking‑gun signal that at least some neutron star-black hole binaries must form differently [than theory predicts],
study co-author Patricia Schmidt, an associate professor of physics and astronomy at the University of Birmingham in the U.K., told Live Science in an email. This observation forces us to rethink where, and under what conditions, these systems arise.
In January 2020, scientists detected the first compelling evidence of a black hole swallowing a neutron star — the ultradense, collapsed core of a once-massive star — resulting in the creation of a new black hole with roughly 13 times the mass of Earth’s sun. Although the event occurred roughly a billion light-years from Earth, the researchers measured the properties of the two objects using a pair of gravitational waves. These ripples in space-time are released by extreme cosmic collisions and were first predicted by Einstein’s relativity. Researchers detected the two waves, which arrived 10 days apart, using the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the United States, comprising two gravitational wave detectors separated by 1,900 miles (3,000 kilometers). The first wave, labeled GW200105, is the focus of the new study. The two LIGO gravitational wave observatories in Washington and Louisiana are separated by a distance of roughly 1,880 miles (3030 km), which allows scientists to better localize the location of gravitational waves on the sky.
Using a new model developed by the University of Birmingham’s Institute of Gravitational Wave Astronomy, as well as complementary data from the Virgo interferometer gravitational wave detector in Italy, the team refined their measurements of the space-time ripple and found that some initial assumptions were wrong. For example, the earlier studies of GW200105 underestimated the black hole’s mass while overestimating the neutron star’s mass. The highly oval, Spirograph-like trajectory discovered in GW200105 forces scientists to reconsider the environmental conditions where these systems arise, pointing to gravitational influences from nearby objects long before the final cataclysmic crash.
