Astronomers have verified the earliest and most distant black hole discovered so far - and, for such an early era, it is astonishingly huge.
CAPERS-LRD-z9 supermassive black hole: a giant in the infant Universe
The object sits inside a galaxy known as CAPERS-LRD-z9. Just 500 million years after the Big Bang - when the young Universe was only 3 per cent of its present age - the black hole had already grown to roughly 300 million times the mass of the Sun.
Little Red Dots (LRDs) and JWST’s view of Cosmic Dawn
The find also offers new, literal illumination on an ancient and puzzling family of objects called Little Red Dots (LRDs). These are intensely bright, compact, reddish sources seen in the early Universe. They begin to show up around 600 million years after the Big Bang, and then fade from view less than a billion years later.
LRDs have only come into focus recently thanks to JWST’s unprecedented infrared capability for probing Cosmic Dawn, the Universe’s earliest epochs. Those epochs also appear the reddest because the light that reaches JWST has been stretched to longer, redder wavelengths during its long passage through the expanding fabric of spacetime.
An active galactic nucleus (AGN) hidden in a red cocoon
At the centre of CAPERS-LRD-z9 is the newly confirmed supermassive black hole, classified as an active galactic nucleus (AGN): a luminous, rapidly feeding black hole at a galaxy’s centre. Its reddish appearance is linked to a radiant shroud of gas and dust surrounding it - a glowing cocoon that could make it resemble a science-fiction-sounding “black hole star”.
Spectroscopy and winds racing at 3,000 kilometres per second
The immense gravity of this supermassive black hole drives surrounding gas to extraordinary speeds of about 3,000 kilometres (1,864 miles) per second, or 1 per cent of the speed of light. Astronomers use these fast, gaseous outflows to infer the presence of black holes through spectroscopy.
"There aren't many other things that create this signature," explains lead author Anthony Taylor, an astrophysicist at the University of Texas at Austin.
Spectroscopy separates incoming light into its component wavelengths, producing a spectrum that contains clues about the source. Here, light emitted by gas near the black hole is stretched and shifts to redder wavelengths when the gas is moving away from an observer. When gas is moving towards an observer, the light is compressed and becomes bluer. From these shifts, an object’s velocity can be determined.
What CAPERS-LRD-z9 reveals about LRD black holes
Crucially, the spectroscopic confirmation of CAPERS-LRD-z9 reinforces the idea that LRDs host supermassive black holes - with “supermassive” arguably not doing justice to their scale. Some appear to reach 10 million solar masses within their first billion years. By comparison, the supermassive black hole at the centre of the Milky Way is about 4 million solar masses.
The black holes powering LRDs may be not only supermassive but “overmassive”, with black-hole-to-galaxy mass ratios approaching 10 per cent to 100 per cent of the host galaxy’s stellar mass.
In CAPERS-LRD-z9 specifically, the supermassive black hole - at up to around 300 million solar masses - amounts to roughly half the combined mass of all the stars in its galaxy. In contrast, in more nearby galaxies the central black hole may represent only about 0.1 per cent of the galaxy’s stellar mass.
A remarkably compact galaxy beyond JWST’s resolving power
To provide another sense of scale, CAPERS-LRD-z9 is so compact that even JWST cannot resolve it. The galaxy appears to be no more than 1,140 light-years across - comparable to the dwarf galaxies that orbit the Milky Way.
Two growth routes in 500 million years: Eddington and super-Eddington
The team says there are two possible pathways for a black hole to reach such a mass within only 500 million years of cosmic time, each beginning with a large, heavy “seed” black hole that grows at a different rate.
If the black hole is increasing in mass at the theoretical maximum, known as the Eddington rate, the initial seed could have been about 10,000 solar masses.
Alternatively, the seed might have been much smaller - only 100 solar masses - but in that case it would need to gain mass even more rapidly, at the super-Eddington rate, effectively force-fed by gravity along with a thick, dense envelope of gas.
Where the seed black holes may come from
The seeds themselves could arise as primordial black holes formed when the Big Bang, well, banged. They might also be created through the collapse of Population III stars (the elusive first stars to light up the cosmos), via “runaway collisions” within dense star clusters, or through the direct collapse of enormous, primordial gas clouds.
Pushing observations to the practical limit
Looking much farther back through spacetime is challenging: "When looking for black holes, this is about as far back as you can practically go. We're really pushing the boundaries of what current technology can detect," Taylor adds.
Implications for early galactic evolution
Lastly, the findings contribute evidence that LRDs were an ephemeral phenomena in the early Universe, and may represent an initial phase of galactic evolution that could ultimately have produced the Milky Way itself.
This research is published in Astrophysical Journal Letters.
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