New observations of galaxy UHZ1 challenge claims of an outsize black hole in the early universe. A study published on Oct. 14 in The Astrophysical Journal showed no signs of hidden black hole activity. Researchers now suggest UHZ1 is likely a low-mass, metal-poor galaxy instead of a massive cosmic giant.
A recent reanalysis of the outsize black hole galaxy UHZ1 using existing X-ray data and new infrared observations challenges a previous groundbreaking claim. Three years ago, this galaxy made news for hosting an unusually massive and heavily dust-obscured black hole formed through direct collapse. Led by Fan Zou of the University of Michigan, the new study reports no signs of hidden black hole activity. The paper, published in The Astrophysical Journal on Oct. 14, suggests UHZ1 could be a low-mass, metal-poor, star-forming galaxy.
An early-universe mammoth
UHZ1 was first spotted in the deep NIRCam imaging of the James Webb Space Telescope. Later, astronomers estimated that the galaxy was seen when the universe was around 480 million years old, at a redshift of 10. Using the Chandra X-ray telescope, researchers saw a small excess of high-energy X-rays coming from the spot. They interpreted this signal as a black hole feeding on gas, making UHZ1 an active galactic nucleus (AGN). Some features also hinted at the black hole being buried in thick gas.
They estimated it weighed 10 million to 100 million solar masses, comparable to the mass of all stars in the whole galaxy. This scenario was consistent with the idea of a "heavy seed" black hole that formed directly from collapsing gas—a direct collapse black hole (DCBH), rather than growing slowly from a small star remnant.
This interpretation was somewhat shaky, as it heavily depended on certain assumptions. The JWST's spectrum showed no sign of a black hole, which they thought could be because of heavy dust blocking the emission. The key diagnostic features were outside the instrument's wavelength range, so this could not be tested at the time. "Independent confirmation was therefore urgently needed," the new team writes in the paper.
Chandra and Webb join hands
In their work, the team reprocessed all the old Chandra data along with 0.95 million seconds of new observations. They also analyzed new mid-infrared data from JWST that extend the wavelength range.
The elaborate reanalysis revealed a weaker X-ray detection, and the findings changed depending on the choice of alignment of the X-ray image with the JWST image. They simulated many plausible alignments, and only 0.1% of them gave a signal as strong as the original claim. When they added 0.95 million seconds of new Chandra data, a real and stable X-ray source should have gotten brighter with more observing time. Instead, the new data added almost nothing.
Using these JWST data, the team looked for the kind of emission expected when a powerful black hole buried in dust heats the dust. The mid-infrared instrument (MIRI) did not find any glow of that type. A non-detection does not prove that nothing is there, but it sets an upper limit for the glow. Anything brighter than MIRI's detection limit would have been seen.
Using two templates for what a hidden black hole's light looks like, they scaled each one up until it just reached that limit. That gave a maximum possible brightness for any hidden black hole. But that maximum turned out to be 10 to 1,000 times lower than the original X-ray claim required. So, a black hole as powerful as originally proposed was ruled out.
Escape routes
Next, they checked whether a black hole could still be hiding. There were two ways its infrared glow might be too faint for MIRI to see. First, if the gas had no dust, there would be no glow, in which case the black hole's visible and ultraviolet light would shine through, but that was not seen in JWST data. Second, if the dust were far away and cold, it would glow only in the far-infrared, but ALMA found very little dust, and the galaxy's light was barely dimmed. Neither scenario fit the data, so a luminous hidden "outsize" black hole is very unlikely.
"Our principal conclusion is that no compelling evidence for such an AGN exists," the team notes. Instead, the emission lines are consistent with UHZ1 being a metal-poor, star-forming galaxy of stellar mass around 100 million solar masses, and no black hole is required to explain them.
The team says that since the original claim was extraordinary, they need even stronger proof to support it. In their view, the evidence for a luminous, buried black hole does not meet that bar, as the newly reported X-ray signal is far less significant than first claimed. The infrared data also rule out a black hole as powerful as proposed.