Scientists may have finally pinned down the nature of some of the most baffling objects in the night sky.
In a new study, researchers investigated the identity of “little red dots.” These mysterious objects from the early universe have characteristics of both galaxies and supermassive black holes but don’t quite fit the description of either.
Little red dots were first observed by the James Webb Space Telescope (JWST) shortly after the spacecraft began collecting data in 2022. They were initially thought to be compact, star-filled galaxies, but they were present too early in the universe to have formed so many stars — at least under our current understanding of galaxy evolution.
Instead, other researchers suggested that the unusual objects might be early supermassive black holes. Light emitted by energized hydrogen atoms around the dots suggests that the gas is moving at thousands of miles per second, tugged along by the gravitational pull of the object at the center.
“Such extreme speeds are a smoking gun of an active galactic nucleus,” meaning a hungry supermassive black hole at the center of a galaxy that’s pulling in matter, Rodrigo Nemmen, an astrophysicist at the University of São Paulo in Brazil, wrote in an accompanying article published in the journal Nature.
But unlike supermassive black holes, little red dots haven’t been observed emitting X-rays or radio waves. And regardless of whether the dots are black holes or early galaxies, they appear to have too much mass to have formed as early in the universe as they did.
In the new study, the researchers looked closely at the light emitted from these objects to better understand their nature. The scientists studied spectra from 30 little red dots, each one collected by JWST’s infrared instruments.
The light emitted from the little red dots closely matches the light that the team predicted would be emitted from a supermassive black hole surrounded by a dense cloud of gas. That gaseous cocoon could have trapped X-ray and radio emissions from the growing black holes, blocking them from reaching JWST.
When the team recalculated the masses of the little red dots under the new interpretation, they found that the dots were about 100 times less massive than previously thought. Together, the evidence suggests that little red dots are growing supermassive black holes that are accreting the surrounding gas.
“These are the lowest mass black holes at high redshift, to our knowledge, and suggest a population of young [supermassive black holes],” the researchers wrote in the study. (Redshift describes how light stretches toward the redder end of the electromagnetic spectrum as it crosses the expanding cosmos; a higher redshift signifies a more distant object.)
“With the corrected mass estimates, [little red dots] fit standard theories of cosmic evolution,” Nemmen wrote. Confirming the findings will involve studying more little red dots to explore whether this “cocoon” phase is common, and determining what role it plays in black hole growth.
