The mysterious "little red dots" revealed by the James-Webb telescope might be less exotic than they appear. The detailed study of a galaxy nicknamed Saguaro indicates that some correspond to a temporary phase of highly active supermassive black holes, whose surrounding galaxy becomes invisible at great distance.
Discovered in Webb images starting in 2022, these objects are compact, red, and very distant. They appear numerous in the young universe, then much rarer in more recent epochs. Several teams suspect that they host active galactic nuclei, that is, central black holes fueled by gas, but their properties resist simple explanations.
Thousands of galaxies of various shapes and colors against the black background of space.
At the top right, a box displays a magnified view of the image. This detail reveals a spiral galaxy named "WISEA J123635.56+621424.2". This galaxy has a bright yellow center and faint brown arms dotted with blue regions that appear to spiral clockwise around its core.
Image: NASA, ESA, CSA, STScI, Pierluigi Rinaldi (Steward Observatory), Alyssa Pagan (STScI)
Pierluigi Rinaldi's team studied WISEA J123635.56+621424.2, a spiral galaxy whose arms earned it the nickname Saguaro. It is observed at a redshift of 2. Its light therefore shows the object about 3.3 billion years after the Big Bang, much later than the first known little red dots.
This less extreme distance offers a decisive advantage. Hubble's ultraviolet images and Webb's infrared, spectroscopic, and archival data make it possible to distinguish the host galaxy from its compact center. The nucleus shines more brightly in ultraviolet and infrared than in visible light, a profile comparable to that of distant little red dots.
The Chandra space telescope also detected faint X-ray emission from Saguaro. For the researchers, this signal reveals an active galactic nucleus heavily obscured by matter. Such a combination could explain why most very distant little red dots escape X-ray detectors.
The astronomers then artificially moved Saguaro to a higher redshift in their simulations. Its spiral arms and surroundings then become too faint to be visible with current instruments. Only the compact red center remains, giving a structured galaxy the deceptive appearance of a simple point of light.
The result does not demonstrate that all these objects follow the same evolution. Saguaro is a case study, and the authors stress the likely diversity of the population. Their scenario instead indicates that an observational bias can hide the galaxy surrounding certain active black holes.
The team now wants to study Saguaro further, search for other comparable galaxies at intermediate distances, and establish a census in Webb's archives.