When a new space telescope is launched, it’s designed to address specific issues in astronomy and provide critical answers to important questions. The JWST was built with four overarching science goals in mind. However, when anticipating new telescopes, astronomers are quick to point out that they’re also excited by the unexpected discoveries that new telescopes make.
There has been no shortage of unexpected discoveries regarding the JWST, especially regarding the very early Universe.
In December 2022, after the JWST had been performing science operations for just six months, the telescope revealed the presence of small red objects in the high-redshift sky. Astronomers called them Little Red Dots (LRDs). The nature of these objects wasn’t obvious, but they’re abundant, and astronomers are curious about what they can tell us about the early Universe.
Recently, a team of researchers compiled a large sample of these LRDs. Most of them existed only 1.5 billion years after the Big Bang. According to observations, a large number of the LRDs may contain growing supermassive black holes (SMBHs). There is no class of corresponding objects at lower redshifts, which only deepens their mysterious nature.
The new research is “The Rise of Faint, Red AGN at z>4: A Sample of Little Red Dots in the JWST Extragalactic Legacy Fields.” It’s been accepted for publication in The Astrophysical Journal, and the lead author is Dale Kocevski of Colby College in Waterville, Maine. The paper is available on the pre-print server arxiv.org.
“We’re confounded by this new population of objects that Webb has found. We don’t see analogs of them at lower redshifts, which is why we haven’t seen them prior to Webb,” said lead author Kocevski. “There’s a substantial amount of work being done to try to determine the nature of these little red dots and whether their light is dominated by accreting black holes.”
The JWST has generated an enormous amount of data during its observations. The astronomers behind the sample of LRDs used publicly available data from the CEERS, PRIMER, JADES, UNCOVER and NGDEEP surveys. While they’re not the first to probe datasets for LRDs, the team used a different methodology that identified LRDs over a wider redshift range. They identified 341 LRDs spanning from about redshift 2 to 11. The researchers found that LRDs emerged in large numbers only 600 million years after the Big Bang, before their number rapidly around 1.5 billion years after the Big Bang.
Fortunately, some spectroscopic data are already available for a portion of the LRDs in the Red Unknowns: Bright Infrared Extragalactic Survey (RUBIES), which contains JWST/NIRSpec spectroscopy of red sources. The spectroscopic data showed that about 70% of the LRDs show evidence of rapidly rotating gas. The gas is moving at about 1,000 km per second, indicating that these could be accretion disks around supermassive black holes.
The conclusion seems pretty clear: LRDs are active galactic nuclei (AGN), which are black holes that are actively feeding. But why do they peter out after about 1.5 billion years after the Big Bang?
“The most exciting thing for me is the redshift distributions. These really red, high-redshift sources basically stop existing at a certain point after the big bang,” said Steven Finkelstein, a co-author of the study at the University of Texas at Austin. “If they are growing black holes, and we think at least 70 percent of them are, this hints at an era of obscured black hole growth in the early universe.”
Most of us remember when some overeager headlines claimed that the JWST had “broken cosmology.” The discovery of LRDs was responsible for some of this thinking. If the light coming from the LRDs was from stars, then some galaxies had to have grown very large very fast. Our theories couldn’t account for them.
If these results are true, then the light is coming from active galactic nuclei rather than large, rapidly growing galaxies. In this case, our theories are safe (for now).
“This is how you solve the universe-breaking problem,” said Anthony Taylor, a co-author of the study at the University of Texas at Austin.
In their paper, the authors highlight some of their important points.
“One of our primary findings is that the red compact objects that have come to be known as little red dots appear in large numbers at z > 4,” they write. “The redshift distribution that we observe for our sample of LRDs may provide insight into the nature of their obscuration and the mechanisms fueling their nuclear activity.”
Their obscuration could result from what’s called “inside-out growth.” In that model, stars begin forming in a galaxy’s central regions first, and they are created more rapidly in those regions. Eventually, star birth moves outward to a galaxy’s periphery. This is because gas collapses inward due to gravity, fuelling star birth in the center. That same collapsing gas could also trigger the concurrent growth of the galaxy’s SMBH. This can also explain the red colour we see. “The rapid accumulation of metals in the proto-bulge then provides the reddening we observe,” the authors write.
As star birth moves outward, less dust is deposited near the AGN, meaning that over time, there are fewer LRDs.
However, inside-out growth is only one potential explanation for LRDs, though it’s a good fit with much of the existing data. The team intends to follow up on this work with mid-infrared imaging and spectroscopy. That will help them understand the number density of these faint AGN and shed more light on what’s obscuring them.
“There’s always two or more potential ways to explain the confounding properties of little red dots,” said Kocevski. “It’s a continuous exchange between models and observations, finding a balance between what aligns well between the two and what conflicts.”
Whatever exactly is going on with LRDs, the issue shows how powerful unexpected discoveries can be. It also shows, again, how valuable the JWST is.
“While much remains to be determined about the nature of LRDs, the prevalence of broad emission lines in their spectra suggests this population is shedding light on a phase of obscured black hole growth in the early universe that was largely undetected prior to the JWST era,” the authors conclude.