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Inset shows the JWST image of the galaxy in infrared, along with the X-rays from the black hole seen by the Chandra. While the X-ray source is far smaller than the galaxy, X-rays are much harder to remove.
Researchers analyzing the earliest galaxies in the Universe have discovered the presence of an actively feeding central black hole. Based on its radiation emissions, it is estimated that this black hole accounts for approximately fifty percent of the galaxy’s mass. This is an astonishingly high fraction compared to modern galaxies, and it presents significant challenges in terms of its formation without the involvement of intermediate steps and star-related processes.
The identification of these early galaxies was made possible through the James Webb Space Telescope, which used gravitational lensing to magnify distant galaxies. In this case, the researchers examined 11 galaxies imaged by the Webb less than a billion years after the Big Bang. To determine the presence of supermassive black holes in these galaxies, the astronomers employed the Chandra X-ray Observatory to compare the positions of X-ray sources with those identified by the Webb.
Within this study, a clear match was found with a galaxy named UHZ1, which is magnified by a factor of four due to gravitational lensing. The X-rays emitted by this location were significantly higher than the background levels, indicating the presence of an active galactic nucleus powered by a supermassive black hole. Further analysis suggests that the black hole is shrouded in a dust and gas environs within its host galaxy.
Understanding these results requires familiarity with the Eddington Limit, which determines the rate at which a black hole incorporates material from its surroundings. Exceeding this limit would result in a radiation-induced reduction in the black hole’s food supply. By applying this concept, the researchers were able to estimate the size of the black hole in UHZ1, linking it to at least a mass equivalent to 107 times that of the Sun.
Based on assessments of star masses in UHZ1, it is inferred that the central black hole accounts for half of the galaxy’s total mass. This is in contrast to the present Universe, where central supermassive black holes only contribute to about 0.1 percent of their galaxies’ mass. Thus, UHZ1 represents an early stage in the evolution of supermassive black holes, which is expected due to its age.
This research also bears implications for theories on the formation of supermassive black holes. One hypothesis suggests that the first stars were exceptionally large, resulting in the formation of unusually large black holes. These black holes would then rapidly grow through mergers and feeding on the dense gas present in the early galaxies. Alternatively, proponents argue that supermassive black holes were always excessively large, forming early in the Universe’s history through the direct collapse of extremely dense gas clouds.
The analysis performed in this study suggests that a black hole formed through the direct collapse of a gas cloud would need to consistently feed at the Eddington Limit to reach the mass observed in UHZ1. In contrast, a black hole originating from the supernova of one of the first stars would have to feed at twice the Eddington Limit throughout its history. While the study does not consider the possibility of mergers, the gravitational pull of a smaller black hole would limit its ability to merge with neighboring black holes. Furthermore, sustaining super-Eddington feeding for the hundreds of millions of years required to build a black hole of this magnitude is unlikely.
Although caution should be exercised in interpreting this result, as it represents the first supermassive black hole to undergo this level of analysis, future observations with the Webb may provide a more comprehensive understanding of the formation and evolution of early black holes.
Nature Astronomy, 2023. DOI: 10.1038/s41550-023-02111-9 (About DOIs).
