Â鶹ÒùÔº

The discovery of a large population of compact, red objects, now widely known as Little Red Dots (LRDs) is one of the most exciting results by the James Webb Space Telescope (JWST) to date. An international team of astronomers, led by Pierluigi Rinaldi (Postdoctoral Research Fellow at STScI in Baltimore) studied 99 LRDs to shed more light on their structure. Their paper will be published in The Astrophysical Journal, and is now on the arXiv preprint server.

Many of the Little Red Dots (LRDs) show signs of hosting actively growing supermassive black holes (AGNs), which appear overly massive compared to their host galaxies. This raises one of the central puzzles in modern astronomy: how could such giant black holes already exist when the universe was only a few billion years old? The problem was first highlighted with the discovery of rare luminous quasars in the distant universe, but their scarcity made it difficult to study them in detail.

"The discovery of LRDs is changing this," says Rinaldi. "The more abundant population JWST provides, allows us to explore these cosmic giants statistically for the first time."

The team studied 99 LRDs, and found that about 30% are not simply compact dots when observed in the ultraviolet. Instead, they reveal disturbed or clumpy structures, in stark contrast to their smooth, point-like appearance at optical wavelengths. Because these galaxies are so far away, their optical light is stretched, or "redshifted," into the long-wavelength channel of JWST, where the resolution is not sharp enough to see structure, so they look like simple dots.

Rinaldi says, "But their ultraviolet light is shifted into JWST's short-wavelength channel, where the telescope has much finer resolution, and there we suddenly see clumps, asymmetries, and signs of interaction. On top of this, in the spectra of some of our LRDs we directly detect the fingerprints of active black holes, with gas moving at thousands of kilometers per second."

This shows that at least part of this population is powered by growing black holes, while others seem to be dominated by star formation, making LRDs a mixed and diverse family of sources. This is a crucial clue, suggesting that mergers and galaxy interactions may be the trigger for the "LRD phase."

Co-author Karina Caputi (Kapteyn Institute, University of Groningen, the Netherlands) adds, "These findings have important consequences. They could help explain why current simulations struggle to reproduce LRDs, and they provide guidance for improving theoretical models, where interaction and merging are possibly key to explain such sources."

Still, the remaining 70% of the sample still appears compact. This may be due to their faintness or to the current limits of the data, which are not homogeneous in their depth.

Rinaldi says, "Future ultra-deep imaging and, for instance, observations through gravitational lensing clusters will be key to uncovering the full picture.

"Our results put interactions and mergers at the center of explanations for the "LRD phase," as they may be the very processes that trigger active black holes in young galaxies. In this way, LRDs offer a new window into the earliest stages of galaxy and black hole growth."

The astronomers combined several techniques: photometry to identify the sources, spectral energy distribution fitting to estimate their stellar properties, and non-parametric morphological analysis to reveal their complexity in the ultraviolet. Spectroscopic data further showed that some of them host a supermassive black hole. All of this was made possible by the ultra-deep observations from the JWST Advanced Deep Extragalactic Survey (JADES), the largest deep survey program carried out with JWST.