When the JWST came to life and began observations, one of its first jobs was to gaze back in time at the early universe. The Assembly of Galaxies is one of the space telescope’s four main science themes, and when it observed the universe’s first galaxies, it uncovered a mystery. Some of them appear to have supermassive black holes (SMBH) in their centers that are fueling active galactic nuclei (AGN). However, they’re not emitting X-rays, which is one of the hallmarks of AGN.

Little Red Dot (LRD) galaxies are small, red galaxies that formed about 600 million years after the Big Bang. The JWST has found more than 300 of them, but they remain a mystery collectively. Their brightness indicated they’re more massive and swollen with stars than they should be at an early age. Our models suggest there wasn’t enough time for them to grow so massive.

Astronomers then discovered AGN signatures that could explain the excess light. Rather than only stars, the LRD’s excess light came from AGN. That would mean that the LRDs wouldn’t need to be so massive to emit all that light, and their size wouldn’t challenge our galaxy evolution models.

Unfortunately, that potential conclusion causes another problem. AGN emit powerful X-rays as the material swirling around in their accretion disks heats up. However, according to new research, LRDs appear to emit no X-rays.

The new research, titled “Chandra Rules Out Super-Eddington Accretion For Little Red Dots,” has been submitted to The Astrophysical Journal. The authors are Andrea Sacchi and Akos Bogdan, both from the Harvard and Smithsonian Centers for Astrophysics. The paper is currently available on the arXiv preprint server.

“A key feature of LRDs is their extreme X-ray weakness: analyses of individual and stacked sources have yielded non-detections or only tentative, inconclusive X-ray signals, except for a handful of individual cases,” the authors write.

The lack of X-rays winds everything backward. If there are no X-rays, there can’t be AGN with accretion disks. If there are no accretion disks, then LRD’s powerful brightness can’t come from SMBHs. If it can’t come from SMBHs, it has to come from stars. Then we’re back to square one: trying to explain how early galaxies were so massive and swollen with stars.

Some researchers have suggested another solution. They say that the SMBHs are experiencing super-Eddington accretion rates.

SMBH black hole accretion is governed by the Eddington limit. The Eddington limit is a fundamental concept in astrophysics that explains the maximum brightness and accretion rates for astrophysical objects like SMBH. An object reaches the Eddington limit when two forces are balanced: outward radiation and inward gravitation. If one of these forces is too powerful, the object either expels its outer layers or ceases further accretion.

Astrophysicists know that the Eddington limit influences SMBH growth. However, they’ve proposed what’s called super-Eddington accretion to explain how these massive objects became so massive so early in the universe. Objects can exceed the Eddington limit for periods of time and experience super-Eddington accretion. Can that explain why LRDs are so bright while also being so weak in X-rays?

The authors point out that the only other explanation for the lack of X-rays is obscuration, and that explanation hasn’t held up.

“As the most natural explanation, high obscuration, is disfavored by JWST spectroscopic evidence, several authors have suggested that the X-ray weakness of LRDs is intrinsic, due to super-Eddington accretion rates,” the authors write. “In this work, we test that scenario by stacking X-ray data for 55 LRDs in the Chandra Deep Field South, accumulating a total exposure time of nearly 400 Ms.”

400 megaseconds is the cumulative observing time for the 55 LRDs combined, not the total telescope observing time. That’s an impressive depth of observation for the 55 objects. If super-Eddington accretion were occurring, that would explain the lack of X-rays.

Super-Eddington accretion still creates X-rays. However, those photons can get trapped in the accretion flow. They can also be absorbed or scattered by outflows and winds, or obscured by the thick disk or envelope around the SMBH. Current models show that super-Eddington accretion still emits X-rays, but as lower-energy soft X-rays. 400 megaseconds of stacked X-ray observations should detect them.

However, they didn’t.

“Despite reaching unprecedented X-ray depths, our stack still yields a non-detection,” the authors write. “The corresponding upper limits are deep enough to rule out current super-Eddington accretion models, and are compatible only with extremely high levels of obscuration.”

The authors say we’re left with only one explanation: “To explain the X-ray weakness of LRDs, we therefore speculate that the SMBHs in these systems are neither as massive nor as luminous as currently believed. ” Other researchers have also suggested this.

So what’s going on if observations show no X-rays, and if the JWST shows that dust obscuration is responsible?

“If the bolometric luminosities are overestimated by an order of magnitude, much lower levels of obscuration can hide the X-ray emission from accreting SMBHs without invoking super-Eddington accretion,” the authors conclude.

The JWST has fulfilled its promise by revealing the universe’s earliest galaxies. That the results go against our models isn’t surprising. Every new mission and telescope delivers some surprises, and scientists often look forward to surprising results.

For now, the LRD galaxies are unexplained. In fact, the mystery has deepened.