Some of the most mysterious objects spotted in the early universe may be supermassive black holes sitting inside surprisingly small and dense galaxies, according to new research.
Known as “little red dots,” these objects appear as tiny reddish points of light in observations of the distant universe.
Astronomers have been trying to understand what they are since the James Webb Space Telescope began revealing large numbers of them.
A new study published in Nature Astronomy provides evidence that little red dots are not simply isolated points of light. Instead, many may lie at the centers of compact galaxies filled with young stars.
Because these objects are extremely distant, astronomers see them as they existed during the first billion years after the Big Bang. Understanding them could therefore provide important clues about how the earliest galaxies and supermassive black holes formed.
Previous studies have detected material surrounding some little red dots in ultraviolet observations, suggesting that galaxies might exist around them. But astronomers have found little evidence of this surrounding material at optical wavelengths.
Detecting optical light is particularly useful because it can help scientists estimate the amount of material contained in stars and better understand the galaxies hosting these mysterious objects.
To search for this extremely faint light, Xuheng Ding, Lilan Yang and their colleagues combined James Webb Space Telescope images of 217 little red dots.
Rather than studying each object separately, the researchers combined information from many observations. This allowed very faint features that were essentially invisible around individual little red dots to become detectable in the combined data.
The researchers discovered faint optical light extending beyond the bright central objects. Computer modeling suggested that this surrounding light most likely comes from stars forming inside small galaxies.
The results indicate that the little red dots themselves may represent actively feeding supermassive black holes located at the centers of these galaxies.
The galaxies appear to be remarkably compact. Their average radius is only about 210 parsecs, or roughly 685 light-years. That makes them approximately 2.5 times more compact than other star-forming galaxies with similar masses from the same period of cosmic history.
Despite their small size, the galaxies may contain substantial amounts of material. The researchers estimate that their total masses are equivalent to around 1 billion suns.
The discovery could help explain why little red dots have been so difficult to understand. Much of the light astronomers see may come from activity around their central black holes, while the surrounding galaxies are so faint and compact that they are difficult to detect individually.
However, the researchers caution that their findings describe the average properties of the 217 objects studied. The extended light remains too faint to clearly identify around most individual little red dots.
More observations will therefore be needed to determine whether all of these mysterious objects have similar surroundings.
The researchers also want to use spectroscopy, which separates light into different wavelengths, to confirm the distances of the objects and learn more about their physical properties.
If confirmed, the findings could provide another piece of the puzzle surrounding how galaxies and enormous black holes emerged so rapidly in the young universe.
