Title: UNCOVER: A NIRSpec Identification of a Broad-line AGN at z = 8.50

Authors: Vasily Kokorev, Seiji Fujimoto, Ivo Labbe, Jenny E. Greene, Rachel Bezanson, Pratika Dayal, Erica J. Nelson, Hakim Atek, Gabriel Brammer, Karina I. Caputi, Iryna Chemerynska, Sam E. Cutler, Robert Feldmann, Yoshinobu Fudamoto, Lukas J. Furtak, Andy D. Goulding, Anna de Graaff, Joel Leja, Danilo Marchesini, Tim B. Miller, Themiya Nanayakkara, Pascal A. Oesch, Richard Pan, Sedona H. Price, David J. Setton, Renske Smit, Mauro Stefanon, Bingjie Wang (王冰洁), John R. Weaver, Katherine E. Whitaker, Christina C. Williams, and Adi Zitrin

First Author’s Institution: Kapteyn Astronomical Institute, University of Groningen, Groningen, The Netherlands; Department of Astronomy, The University of Texas at Austin, Austin, Texas, USA (current)

Status: Published in the Astrophysical Journal Letters [open access]

There are cosmic monsters hiding throughout the universe. Enormous black holes, as massive as millions to billions of Suns put together, lurk in the centers of nearly all large galaxies. When they swallow their unfortunate prey of gas and dust, they form swirling masses of particles called accretion disks. These disks can glow bright enough to outshine their entire host galaxies, making one object brighter than potentially trillions of stars!

These supermassive black holes with accretion disks are called Active Galactic Nuclei (AGN). They act like galactic lanterns, illuminating dark corners of the universe. In previous years, only the very brightest AGN (known as quasars) could be observed at high redshifts (large distances). 

A major puzzle started surfacing regarding the nature of the earliest quasars. Black holes can grow by gulping down gas, but only at a certain rate. If they try to consume matter faster than the so-called Eddington limit, they (metaphorically) burp it back up. We know roughly when the first black holes should have formed, so we can simulate what happens if you constantly feed a baby black hole at the Eddington rate, supercharging its growth. As it turns out, even this extreme scenario can’t account for the immense masses of the earliest quasars. It’s like seeing a 1-month-old tree over 30 feet fall – no matter how much you water a sapling, there’s no conceivable way for it to grow that quickly! 

A new heavyweight enters the ring

The authors of today’s paper used data from the James Webb Space Telescope (JWST) UNCOVER program to discover one of the most distant AGN, named UNCOVER 20466. The observations targeted a huge cluster of galaxies known as Abell 2744. The immense mass of this cluster acts as a lens, magnifying any objects behind it.

UNCOVER 20466 was previously identified as an AGN candidate using images from JWST’s NIRCam instrument, which can take images over wavelengths from 0.6 to 5 microns. The authors analyzed follow-up spectroscopy from the PRISM on the JWST/NIRSpec instrument.

First off, they confirmed that UNCOVER 20466 is indeed an AGN by identifying broad emission lines. These features are generated by clouds of gas whipping around a massive black hole at breakneck speeds (although some recent papers have challenged this interpretation!). The authors derived the black hole’s mass from the width of the Hβ spectral line, obtaining a value of around 150 million solar masses. With a redshift of 8.5, this black hole had only existed for around half a billion years. This an extremely short time for it to have grown so massive!

Baking a supermassive black hole from scratch

One explanation for UNCOVER 20466’s confounding mass is that it may have grown from an exotic object known as a direct-collapse black hole instead of the usual stellar-mass seed. These hypothetical objects are theorized to form when a massive cloud of gas collapses directly into a black hole, without becoming a star at all. If UNCOVER 20466 started off as one of these direct-collapse seeds, it wouldn’t have had to grow quite so quickly to reach its current size. Figure 1 shows, as dashed lines, the tracks that a black hole can take as it grows starting as each kind of initial seed. The points are all observed black holes, and the fact that they are above the line from stellar-mass seeds suggests that they cannot have formed this way.

Another explanation is that conditions in early galaxies may have been just right for super-Eddington growth. Normally, the outward radiation pressure from an AGN prevents mass from being accreted faster than the Eddington rate, but theoretical work and simulations have shown that this limit can be exceeded under certain extreme conditions. It’s challenging to imagine a scenario where matter is shoveled into a black hole at such an alarming rate, especially since this pace would have to be kept for hundreds of millions of years. But the Universe has certainly cooked up stranger things in the past!

Another less climactic possibility is that our black hole mass estimates are too high. Whenever we calculate the masses of these distant black holes, we assume that their environments are similar to nearby ones. In the past, this seemed like a decent assumption, since quasars across billions of years of time looked remarkably similar. But the new kinds of AGN that JWST has uncovered appear truly unique, which might mean we have to come up with more sophisticated ways to weigh them accurately.

Putting a spotlight on the universe’s darkest secrets

Whatever the case may be, JWST is truly pushing the edges of our understanding of physics. This object, and other ones like it, are presenting us with a plethora of puzzles. Some of these black holes are almost half as massive as their entire host galaxies, a truly terrifying thought! Returning to the analogy of a tree, it’d be as if a single leaf somehow grew as large as the entire trunk, while the others had just started to sprout. Future work will be needed to untangle the true nature of UNCOVER 20466 and other similar objects across the sky. Observations using the high-resolution gratings on JWST’s NIRSpec instrument and data from across the electromagnetic spectrum, such as radio measurements from the Atacama Large Millimeter Array, might help us shine light on these mysterious cosmic horrors.

Astrobite edited by Sonja Panjkov and Bình Nguyễn

Featured image credit: ESO/M. Kornmesser