Since their discovery by NASA’s James Webb Space Telescope in 2022, little red dots (LRDs) have been the subject of great interest to astronomers. Understanding the nature of these extremely distant, compact red sources has been a puzzling scientific endeavor.
One popular theory is that little red dots are supermassive black holes known as active galactic nuclei, although they display characteristics unlike nearby active galactic nuclei. While they appear abundant at high redshift early in the universe, they rapidly decrease in number at lower redshifts. (The higher the redshift, the greater the distance the light has traveled across the universe.) This perplexing shift in number raises the question: What happens to little red dots as the universe matures?
A team of researchers led by Pierluigi Rinaldi of the University of Arizona’s Steward Observatory, now at the Space Telescope Science Institute (STScI) in Baltimore, has built upon their previous research in a new study published on July 29 in The Astrophysical Journal and proposed one pathway LRDs can follow as the universe ages: Though they may look like a unique galaxy population, these dots are affected by observational bias — some features just don’t appear at higher redshifts with current technology.
Scientists have proposed one pathway little red dots can follow as the universe matures based on their analysis of spiral galaxy WISEA J123635.56+621424.2, nicknamed the “Saguaro.” They suggest little red dots may be a temporary phase of highly active supermassive black holes.
Image: NASA, ESA, CSA, STScI, Pierluigi Rinaldi (Steward Observatory); Image Processing: Alyssa Pagan (STScI)
Their conclusions are based on their analysis of lower-redshift spiral galaxy WISEA J123635.56+621424.2, nicknamed the “Saguaro” for its prominent arms, like the cactus native to the Sonoran Desert in the Southwestern United States. A particularly intriguing feature of this redshift 2 galaxy, which corresponds to approximately 3.3 billion years after the big bang, is its little red dot-like center that is reminiscent of the ruby red fruit produced by the desert plant.
“Everything created in the early universe must evolve into something around us. We have had little idea of what LRDs become, but these results finally show us how to find their progeny,” said co-author George Rieke of the University of Arizona. Previous studies by NASA’s retired Spitzer Space Telescope provided the first hint of the dust-obscured, compact galaxy population in the lower-redshift universe that the Saguaro belongs to, paving the way for NASA’s Hubble and James Webb space telescopes’ high-resolution analyses.
“The Saguaro is important because it’s a prototypical little red dot and is one of the few we have found at lower redshift. It can be used to study the pathway of these dots throughout cosmic time,” said Fabio Pacucci of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and a co-author of the study.
Among the thousands of sources Rinaldi looked at across several surveys, the Saguaro was an example of the right place — with one of Webb’s microshutter arrays perfectly framed over the galaxy’s core to take spectroscopic data — and right time — being at lower redshift. To get as broad a view of the spiral galaxy as possible across the electromagnetic spectrum, the team used Hubble’s ultraviolet- and Webb’s infrared-imaging and spectroscopic archival data, respectively.
“Because the Saguaro is at lower redshift, we can see the very beautiful and bright host galaxy in high resolution and detail with Webb and Hubble,” said Zihao Wu of the Harvard-Smithsonian Center for Astrophysics, and a co-author of the study. “Webb’s observations can help us understand how the galaxy and its little red dot-like nucleus are connected.”
The team took multiple approaches to verify that the Saguaro’s compact red nucleus matched the characteristics of a prototypical LRD. In particular, the Hubble and Webb data showed that the nucleus is brighter in both ultraviolet and infrared light than in visible light, just like distant LRDs. The team also carefully disentangled the light emitted from the host and nucleus, and considered the presence of X-ray emission from the source.
Although the majority of little red dots at high redshift are not detectable in X-ray light, NASA’s Chandra X-ray Observatory detected weak X-ray emission from the Saguaro.
“What the X-ray light observations show is that this galaxy has an active galactic nucleus, and a very obscured one at that,” said Carys Gilbert, a Master’s student at the University of Cape Town in South Africa and a co-author of the paper. “It’s not only obscured but also X-ray weak. That kind of combination could explain the lack of X-ray emission that we see from all other little red dots. It fits the puzzle of little red dots nicely.”
In addition to demonstrating how the Saguaro’s central compact red source fits the little red dot criteria, the team synthetically shifted the galaxy to a higher redshift to explore how this galactic environment would appear to observers if located in the early universe. As expected, the Saguaro’s surrounding galactic structure fades so that only the bright, LRD-like source at its center is visible.
“Our theory is that most of these distant sources are affected by this cosmological effect, creating an observational bias,” said Rinaldi. “We simply are not able to sample the immediate environment of high-redshift little red dots because their surroundings are just too faint to be observed even with Webb. Little red dots are far more complex than just being a dot. They’re just the tip of the iceberg — of a supermassive black hole interacting with its nearby surroundings.”
Scientists synthetically shifted the Saguaro, a lower-redshift spiral galaxy, to a higher redshift to find out how it would appear if it was in the early universe. Its compact red appearance suggests that little red dots are a phase of highly active supermassive black holes.
Image: NASA, ESA, CSA, Pierluigi Rinaldi (Steward Observatory); Image Processing: Alyssa Pagan (STScI); Illustration: Leah Hustak (STScI)
Considering the Saguaro case study, the team believes that LRDs may not be a unique galaxy population, but instead a temporary phase of highly active supermassive black holes. Could this theory be the link between the populous high-redshift little red dots seen by Webb and the local universe?
While the Saguaro is not representative of all LRDs, the team proposes that this is one phase of these compact red sources. To build more confidence, further study of the Saguaro is necessary, as well as seeking other Saguaro-like galaxies at lower redshift. The team also intends to comb through Webb’s bountiful archival data to build a census of little red dots to study how their environments may impact how they mature. These different approaches are all geared to helping uncover the family tree of little red dots.
The James Webb Space Telescope is the world’s premier space science observatory. Webb is solving mysteries in our solar system, looking beyond to distant worlds around other stars, and probing the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency) and CSA (Canadian Space Agency).
The Hubble Space Telescope has been operating for over three decades and continues to make ground-breaking discoveries that shape our fundamental understanding of the universe. Hubble is a project of international cooperation between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope and mission operations. Lockheed Martin Space, based in Denver, also supports mission operations at Goddard. The Space Telescope Science Institute in Baltimore, which is operated by the Association of Universities for Research in Astronomy, conducts Hubble science operations for NASA.
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