JWST Finds Possible 36-Million-Sun Black Hole 470 Million Years After Big Bang
The James Webb Space Telescope may have exposed a feeding black hole only 470 million years after the Big Bang, but astronomers cannot yet agree on how much it weighs. Two accepted calibration methods place the mass of MoM-BL-z10 at either 2.1 million or 36 million Suns a 17-fold difference that produces sharply different accounts of how the object formed and grew.
An international team of 15 astronomers led by Xingyao Cai found the object by searching a public archive of existing Webb observations, rather than conducting a new, dedicated observing campaign. The team posted its findings online on October 20 ahead of scientific-journal review, so MoM-BL-z10 remains a candidate broad-line active galactic nucleus rather than a settled discovery.
Fast gas reveals a feeding black hole
The strongest signal is broadened hydrogen emission. Gas associated with MoM-BL-z10 is moving at about 3,480 kilometers per second, or 7.8 million mph. Material moving toward and away from an observer shifts the emitted light in opposite directions, smearing a narrow spectral line into a broader one. Around an actively feeding black hole, that velocity can be used to estimate the central object’s mass.
Webb also detected emission from highly excited neon and oxygen. The gas is heated beyond what young stars alone could produce, supporting the interpretation that matter is accreting around a massive black hole. Cosmic expansion made the measurement possible in Webb’s operating range by stretching the relevant hydrogen light from visible blue-green wavelengths into infrared wavelengths more than 10 times longer.
MoM-BL-z10 is seen when the universe was roughly 3% of its current age. The team describes it as one of the earliest black holes detected and the most distant object of its type. That timing makes the mass disagreement more than a measurement detail: each result demands a different early growth history.
One spectrum, two very different masses
A conventional broad-line calculation gives MoM-BL-z10 a mass of about 36 million Suns. At that size, its observed output suggests it was feeding at roughly one-tenth of the maximum rate normally associated with the balance between gravity pulling gas inward and radiation pushing it outward.
A second calibration, adapted for the compact red galaxies uncovered by Webb, produces a mass of about 2.1 million Suns. The same observed luminosity would then put the black hole’s feeding rate near that physical limit. Astronomers have not settled on which calibration best describes this newly observed class, so the researchers report both results rather than selecting one preferred mass.
The uncertainty changes the object’s likely origin. A relatively small seed left by an early star could reach the observed state if it experienced exceptionally rapid growth, including a period above the usual feeding limit. Alternatively, a much heavier seed could have formed through the collapse of a huge gas cloud and begun with a substantial head start. The present spectrum does not choose between those paths.
A Little Red Dot with a large implication
MoM-BL-z10 has the compact shape and color associated with the objects astronomers call Little Red Dots. These sources began appearing in Webb observations in late 2022 and had nearly disappeared by about 2 billion years after the Big Bang. A January 2026 Nature study concluded that the brightest examples are most likely black holes enclosed by thick gas rather than unusually crowded star clusters.
That broader interpretation supports the case that MoM-BL-z10 contains an accreting black hole, but it does not resolve its mass. For these compact, gas-obscured objects, the conversion from line width and luminosity to mass remains the controlling uncertainty.
Webb has therefore delivered an unusually early opportunity to weigh a feeding black hole directly from the speed of nearby gas. Whether the answer is 2.1 million or 36 million Suns will determine if MoM-BL-z10 mainly records near-limit growth from a smaller beginning or the early appearance of a much heavier seed.
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By David Whitaker — Associate editor for AMI’s aerospace and drone systems desk, translating flight systems, aircraft programs, spaceflight, and UAV developments into accessible technical stories.
