Five hundred million years following the Big Bang, when the universe was but three percent as old as it is now, a black hole in the CAPERS-LRD-z9 galaxy already had an estimated 300 million times the mass of the Sun. Its identification, made with the aid of James Webb Space Telescope (JWST) spectroscopy, represents the oldest and most ancient black hole ever confirmed and it is much larger than current models indicate should exist at such an early point in history.
The host galaxy is a member of a mysterious population of compact, luminous, reddish objects called Little Red Dots (LRDs). First unveiled by JWST’s infrared capability, LRDs abound starting at about 600 million years after the Big Bang and disappear from the cosmic archive in a few hundred million years. The red coloration is not due to aging stars, as originally assumed, but in most instances from an optically thick cocoon of gas and dust around a central active galactic nucleus. In CAPERS-LRD-z9, that veil could be reprocessing the high ultraviolet and X-ray radiation from the accreting black hole into longer, redder wavelengths, in effect concealing its high-energy signature.
Spectroscopic analysis was central. As University of Texas at Austin’s Anthony Taylor described, There aren’t many other things that create this signature. Gas is whipped to 3,000 kilometers per second a fraction, roughly one percent, of light speed by the black hole’s gravity, and its Doppler shifts are characteristic: red when it recedes from us, blue when it approaches. This velocity profile is a matter of record for matter accreting into a deep gravitational well.
The size of the black hole compared to its galaxy is rare. As massive as about half the stellar mass of CAPERS-LRD-z9, it is “overmassive” by today’s standards, when central black holes generally make up only about 0.1 percent of a galaxy’s stars. The galaxy itself is so small no larger than 1,140 light-years in diameter that even the JWST can’t resolve its structure, putting it into the size range of dwarf galaxies within the Milky Way.
This kind of acceleration so early on compels astrophysicists to reexamine formation routes. One theory is that the black hole formed as a massive seed of approximately 10,000 solar masses, accreting continuously at the Eddington limit theoretical point of balance where outward radiation pressure equals inward gravity. Alternatively, it might have formed as a smaller, ~100-solar-mass remnant of a Population III star and expanded at super-Eddington rates, where dense infalling gas dominates radiation pressure and accumulates at the black hole at rates in excess of classical limits.
Super-Eddington growth is complicated. Hydrodynamical models indicate that such accretion will frequently power strong winds and jets, which can shut off the gas supply in a few million years. Nevertheless under some conditions say, when inflows are channeled through an equatorial plane as feedback escapes along the poles long-term super-Eddington episodes may be possible, allowing a black hole to accrue mass more quickly than Eddington-limited theory suggests.
The source of the first seed is a mystery. Suggested pathways are the direct collapse of massive early gas clouds, runaway collisions between stars in dense clusters, or collapse of the first metal-poor Population III stars. New simulations of Population III star clusters indicate that collisions and accretion of gas can lead to the formation of black holes with masses ranging from several thousand solar masses and would thus have a head start toward supermassive growth. A second, and more speculative, avenue is the production of primordial black holes in the Big Bang itself.
The LRD connection introduces another level. Other researchers suggest that such objects are “black hole stars” quasistellar envelopes of gas fueled from the inside by an accreting black hole. In this case, the ambient gas both supplies the black hole and emits as a supergiant star, obscuring high-energy emission. If common in the early universe, such an object may be a normal phase in the co-evolution of galaxies and central black holes.
For the moment, CAPERS-LRD-z9 is a unique, spectroscopically verified instance of this effect. When searching for black holes, this is roughly as far back as you can go practically, said Taylor. We’re really pushing the boundaries of what current technology can detect. Further JWST observations, with higher resolution and broader wavelength coverage, may reveal whether this galaxy is an outlier or the first known member of a vast, hidden population that rewrites the timeline of cosmic growth.
