Three years ago, the galaxy UHZ1 attracted attention: astronomers claimed that a black hole was hiding in it whose mass was comparable to the mass of all the stars in the galaxy. It was supposedly concealed by dense dust, and it formed not from the remnants of a star but directly from collapsing gas. Such a scenario is called "direct collapse."
However, a reanalysis by Fan Zou's group at the University of Michigan found no convincing traces of such a black hole. The work was published on October 14 in The Astrophysical Journal.
UHZ1 was first noticed in images from the James Webb Space Telescope. It is estimated that we see the galaxy when the Universe was about 480 million years old. At that time, the Chandra telescope detected a faint excess of X-rays from the same location. This was interpreted as the activity of a black hole absorbing gas - a so-called active galactic nucleus (AGN).
But the interpretation rested on assumptions. The James Webb spectrum did not show a black hole, and this was attributed to dust. The hypothesis could not be tested then: the needed wavelengths lay outside the range of the instruments.
Now the researchers have reprocessed all the old Chandra data and added new observations. They also brought in new James Webb infrared data.
The X-ray signal turned out to be weaker, and its significance depended on how the X-ray image was aligned with the infrared one. The scientists tried many alignment variants: only in 0.1% of cases was the signal as strong as in the first work. If the source existed, the new Chandra observations would have made it brighter. This did not happen - the addition contributed almost nothing.
The MIRI infrared instrument on James Webb did not detect the glow that would arise if a powerful black hole were heating the surrounding dust. The absence of a signal does not in itself prove that the object does not exist, but it sets an upper limit on brightness. This limit turned out to be 10-1000 times lower than required for the black hole from the original claim.
The authors also checked workarounds: for example, if there were no dust at all, the black hole would shine in the visible and ultraviolet ranges, but this is not seen. If the dust were cold and distant, it would shine in the far infrared, but the ALMA telescope found very little of it. Neither scenario fits.
"Our main conclusion: there is no convincing evidence for the existence of such an AGN," the researchers write. The observed emission lines can be explained without a black hole as well - as by a galaxy with active star formation that is poor in heavy elements.
The scientists emphasize: the original claim was extraordinary, so it requires especially strong evidence. The new data do not meet that bar. This does not mean that there are no black holes in the early Universe - it is only about the fact that in UHZ1 there apparently is no powerful hidden black hole.