The James Webb Space Telescope has been making waves in the astronomy community with its discovery of enigmatic 'little red dots' in the early universe. These dots, found at redshift values suggesting they existed between 13.2 and 12.2 billion years ago, have been a perplexing cosmological mystery. As their name suggests, they appear small, red, and surprisingly bright, with light mostly in infrared and some ultraviolet, but no X-rays, which would be expected from an active black hole. This has led researchers to propose various hypotheses, including that they are 'black hole stars' or vast clouds of gas heated from within by a concealed but growing supermassive black hole.
One recent hypothesis is that little red dots turned into large globular clusters. Now, an alternative possibility has emerged. A team led by Pierluigi Rinaldi has identified a descendent of a little red dot, in the form of a distant galaxy called WISEA J123635.56+621424.2, found at a redshift of 2, meaning we see it as it was 10.5 billion years ago. This galaxy, nicknamed the Saguaro, displays neat spiral arms around a red core, with its core having all the hallmarks of a little red dot.
The Saguaro's core shines bright in infrared as seen by the James Webb Space Telescope and emits in ultraviolet as detected by the Hubble Space Telescope. However, NASA's Chandra X-ray Observatory has also detected faint X-rays originating from the Saguaro. This combination of X-ray emission and infrared brightness fits the puzzle of little red dots nicely.
This is not the first time X-rays have been seen coming from a little red dot. Earlier this year, scattered X-rays were seen breaking through from a little red dot called 3DHST-AEGIS-12014, given credence to the hypothesis that little red dots are black hole stars. The Saguaro, which existed 1.3 billion years later, is a little bit further on in its development.
Rinaldi's team simulated how the Saguaro would appear to us if it existed a billion years after the big bang. They found that much of its galactic structure would either be less developed or too faint to be seen at such a distance, leaving only the nucleus, which would look very much like all the other little red dots. This implies that little red dots are not a unique population of their own, but are simply a phase in the development of galaxies with supermassive black holes.
The findings of Rinaldi's team, reported in The Astrophysical Journal, provide a crucial link in the story of little red dots and how they connect with modern galaxies. While there is still much to learn, such as the beginning of their story and how they form, the middle and end of their story is now beginning to take shape. This discovery raises a deeper question: what does the future hold for these enigmatic little red dots?
