Mysterious red dots spotted in the young Universe conceal small galaxies around their very luminous cores.
The James Webb Space Telescope has been observing these objects since its first campaigns. In images, they often look like simple red dots, hence their nickname, “little red dots.” They existed during the first billion years after the Big Bang.
The stacking approach reveals extended emission
Until now, one limitation made it difficult to learn more about the matter surrounding these dots of light. They are extremely small and distant.
The team overcame this difficulty by combining images of 217 little red dots from the COSMOS-Web survey. The principle is simple: by superimposing many comparable objects, shared and very faint features become easier to detect. The researchers used four infrared bands from James Webb’s NIRCam.
A faint extended glow then appears around the center in the longest infrared band studied. It corresponds to a galaxy with a characteristic size of about 200 parsecs, or roughly 650 light-years. That is extremely compact.
The researchers also estimated the total mass. On average, they measured about one billion times the mass of the Sun. This figure corresponds to the average galaxy revealed by stacking the 217 objects.
Why does this measurement matter? Many little red dots appear to harbor highly active black holes. To understand their rapid emergence, it is necessary to know which galaxy surrounds them. Until now, the light from the center had largely obscured this information. The new detection provides a direct measurement of the stars that accompanied these objects in the young Universe.
The authors now have a way to compare the growth of galaxies with that of their black holes during the first billion years. However, their method relies on an average: it does not yet reveal the precise shape and mass of each galaxy. Deeper observations will have to determine how much these hosts differ from one little red dot to another.