Since their discovery by NASA’s James Webb Space Telescope in 2022, small red dots (LRDs) have been the subject of great interest to astronomers. Understanding the nature of these compact and extremely distant red sources has been a perplexing scientific endeavor.

One popular theory is that the small red dots are supermassive black holes known as active galactic nuclei, although they show different characteristics than nearby active galactic nuclei. While they appear abundant in the early stages of the universe at high redshift, their numbers decline rapidly at lower redshifts. (The greater the redshift, the greater the distance the light has traveled through the universe.) This puzzling change in number raises the question: What happens to the 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, built on their previous research in a new study published July 29 in The Astrophysical Journal and proposed a path that LRDs may follow as the universe ages: Although they may appear to be a single population of galaxies, these spots are affected by observational bias; some features simply do not appear at higher redshifts with current technology.

Their conclusions are based on their analysis of the 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 small red dot-shaped center reminiscent of the ruby ​​red fruit produced by the desert plant.

“Everything created in the early universe must evolve into something around us. We had little idea what LRDs became, but these results finally show us how to find their progeny,” said co-author George Rieke of the University of Arizona. Earlier studies by NASA’s retired Spitzer Space Telescope provided the first indication of the population of compact, dust-obscured galaxies in the lower-redshift universe to which Saguaro belongs, paving the way for high-resolution analyzes by NASA’s Hubble and James Webb space telescopes.

“The Saguaro is important because it is a prototypical small red dot and is one of the few we have found with a lower redshift. It can be used to study the path of these dots through cosmic time,” said Fabio Pacucci of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and co-author of the study.

Among the thousands of sources Rinaldi examined in various studies, 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 at the right time, at a lower redshift. To get the broadest possible view of the spiral galaxy across the electromagnetic spectrum, the team used Hubble’s ultraviolet images and Webb’s infrared images and archival spectroscopic data, respectively.

“Because Saguaro has a lower redshift, we can see the beautiful, bright host galaxy in high resolution and detail with Webb and Hubble,” said Zihao Wu of the Harvard-Smithsonian Center for Astrophysics and co-author of the study. “Webb’s observations can help us understand how the galaxy and its small red dot-shaped nucleus are connected.”

The team took multiple approaches to verify that Saguaro’s compact red core 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, as are distant LRDs. The team also carefully disentangled the light emitted by the host and the nucleus, and considered the presence of X-ray emission from the source.

Although most small red dots at high redshift are not detectable with 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 co-author of the paper. “Not only is it obscured but it’s also weak to X-rays. That kind of combination could explain the lack of

In addition to demonstrating how Saguaro’s central compact red source fits the small red dot criteria, the team synthetically shifted the galaxy to a higher redshift to explore how this galactic environment would appear to observers if it were located in the early universe. As expected, 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 observation bias,” Rinaldi said. “We simply cannot sample the immediate surroundings of small, high-redshift red dots because their surroundings are too faint to be observed even with Webb. Small red dots are much more complex than simply being a dot. They are just the tip of the iceberg: of a supermassive black hole interacting with its nearby environment.”

Taking the Saguaro case study into account, the team believes that LRDs may not be a single population of galaxies, but rather a temporary phase of highly active supermassive black holes. Could this theory be the link between the small, populous, high-redshift red dots seen by Webb and the local universe?

While Saguaro is not representative of all LRDs, the team proposes that this is one phase of these compact red sources. To build more confidence, more studies on Saguaro are needed, as well as searching for other galaxies similar to Saguaro at a lower redshift. The team also aims to sift through Webb’s rich archival data to create a census of tiny red dots to study how their environments may affect their maturation. All of these different approaches are geared toward helping you discover the family tree of the 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 it to distant worlds around other stars, and exploring 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 more than three decades and continues to make groundbreaking discoveries that shape our fundamental understanding of the universe. Hubble is an international cooperation project between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope and mission operations. Denver-based Lockheed Martin Space also supports mission operations at Goddard. The Space Telescope Science Institute in Baltimore, operated by the Association of Universities for Research in Astronomy, conducts Hubble science operations for NASA.

For more information about Webb, visit:

https://science.nasa.gov/webb

For more information about Hubble, visit:

https://science.nasa.gov/hubble

The following sections contain links to download the images and videos in this article in all available resolutions, followed by links to related information, media contacts, and, if available, links to research papers and translations into Spanish.