Free Astronomy Magazine November-December 2026

NOVEMBER-DECEMBER 2026 — 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 beau- tiful and bright host galaxy in high resolution and detail with Webb and Hubble,” said Zihao Wu of the Harvard-Smithsonian Center for As- trophysics, 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 character- istics of a prototypical LRD. In par- ticular, 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 care- fully disentangled the light emitted from the host and nucleus, and con- sidered the presence of X-ray emis- sion from the source. Although the majority of little red dots at high redshift are not de- tectable in X-ray light, NASA’s Chan- dra 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 ob- scured 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 emis- sion 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 struc- ture 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 ob- servational bias,” said Rinaldi. “We simply are not able to sample the S cientists 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 ap- pearance suggests that little red dots are a phase of highly active super- massive black holes. [NASA, ESA, CSA, Pierluigi Rinaldi (Steward Observa- tory); Image Processing: Alyssa Pagan (STScI); Illustration: Leah Hustak (STScI)] 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.” 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 be- tween the populous high-redshift little red dots seen by Webb and the local universe? While the Saguaro is not representa- tive of all LRDs, the team proposes that this is one phase of these com- pact red sources. To build more con- fidence, further study of the Saguaro is necessary, as well as seeking other Saguaro-like galaxies at lower red- shift. The team also intends to comb through Webb’s bountiful archival data to build a census of little red dots to study how their environ- ments may impact how they mature. These different approaches are all geared to helping uncover the fam- ily tree of little red dots. !