New class of galaxies could indicate early growth of black holes

In December 2022, less than six months after it began scientific operations, NASA’s James Webb Space Telescope revealed something never seen before: numerous red objects that appear small in the sky, which scientists quickly called “little red dots” (LRD). Although these dots are quite abundant, researchers are perplexed about their nature, the reason for their unique colors, and what they convey about the early universe.

A team of astronomers recently compiled one of the largest samples of LRDs to date, almost all of which existed in the 1.5 billion years after the Big Bang. They found that a large portion of the LRDs in their sample showed signs of growing supermassive black holes.

“We are baffled by this new population of objects discovered by Webb. We don’t see analogues of them at lower redshifts, which is why we didn’t see them before Webb,” said Dale Kocevski of Colby College of Waterville. , Maine, and lead author of the study published on the arXiv preprint server. “A lot of work is underway to try to determine the nature of these little red dots and whether their light is dominated by accumulating black holes.”

A potential insight into the early growth of black holes

The use of publicly available Webb data was a significant factor contributing to the team’s large LRD sample size. To begin, the team searched for these red sources in the Cosmic Evolution Early Release Science (CEERS) survey before expanding their scope to other legacy extragalactic domains, including the JWST Advanced Deep Extragalactic Survey (JADES) and the Next Generation Deep Extragalactic Exploratory Public Survey (NGDEEP).

The methodology used to identify these objects also differed from previous studies, allowing the census to cover a wide redshift range. The distribution they discovered is intriguing: LRDs emerge in large numbers about 600 million years after the Big Bang and undergo a rapid decline in quantity about 1.5 billion years after the Big Bang.

The team turned to the Red Unknowns: Bright Infrared Extragalactic Survey (RUBIES) to obtain spectroscopic data on some of the LRDs in their sample. They found that about 70 percent of the targets showed traces of gas orbiting fast at a speed of 2 million miles per hour (1,000 kilometers per second), a sign of an accretion disk around a black hole supermassive. This suggests that many LRDs accumulate black holes, also called active galactic nuclei (AGN).

“The most exciting thing to me is the redshift distributions. These really red and high redshift sources cease to exist at some point after the Big Bang,” said Steven Finkelstein, co-author of the paper. study at the University of Texas at Austin. “If they are growing black holes, and we think at least 70% of them are, that hints at an era of obscured black hole growth in the early universe.”

Contrary to headlines, cosmology is not broken

When LRDs were first discovered, some suggested that the cosmology was “broken.” If all the light from these objects came from stars, this implied that some galaxies had grown so large and fast that theories could not account for them.

The team’s research supports the argument that much of the light from these objects comes from accumulating black holes, not stars. Fewer stars means smaller and lighter galaxies, which can be understood by existing theories.

“This is how you solve the problem of the universe breaking up,” said study co-author Anthony Taylor at the University of Texas at Austin.

More and more curious

There is still plenty of room for debate as LRDs seem to raise even more questions. For example, it remains an open question why LRDs do not appear at lower redshifts. One possible answer is growth in reverse: as star formation in a galaxy expands outward from the core, less gas is deposited by supernovas near the accreting black hole, and that -it becomes less obscured. In this case, the black hole sheds its gas cocoon, becomes more blue and less red, and loses its LRD status.

Additionally, LRDs are not bright in X-ray light, in contrast to most black holes at lower redshifts. However, astronomers know that at certain gas densities, X-ray photons can be trapped, thereby reducing the amount of X-ray emission. Therefore, this quality of LRDs could support the theory that it is highly obscured black holes.

The team takes several approaches to understanding the nature of LRDs, including examining the mid-infrared properties of their sample and globally searching for clustering black holes to see how many match the LRD criteria. Obtaining further spectroscopy and some follow-up observations will also be beneficial in resolving this current “open case” regarding LRDs.

“There are always two or more potential ways to explain the confounding properties of the little red dots,” Kocevski said. “It’s a continuous exchange between models and observations, finding a balance between what aligns well between the two and what conflicts.”

These results were presented at a press conference at the 245th meeting of the American Astronomical Society in National Harbor, Maryland, and have been accepted for publication in The Astrophysics Journal.