The Direct Measurement of A Black Hole Mass During the Era of Reionisation
The Λ-Cold Dark Matter (ΛCDM) model successfully explains many large-scale cosmological phenomena, but the early presence of supermassive black holes (SMBHs) still poses a major challenge. In ΛCDM, density fluctuations in the primordial universe create gravitational instabilities that collapse into structures over time. Because gravity is well understood, we can estimate how long this process should take and predict what structures ought to appear at different epochs. Within the past few years, partially thanks to the James Webb Telescope (JWST), we have seen an unprecedented rise in SMBH detections far before our models predict them to have existed.
A new study led by Ignas Juodžbalis reports the first direct, dynamical measurement of a black hole’s mass during the Universe’s Epoch of Reionization, just 700 million years after the Big Bang. The measurement comes from a peculiar type of galaxy known as a Little Red Dot (LRD), typically hosting an active galactic nucleus (AGN) and distinguished by its compact size and striking red colour. They are believed to make up between 15 and 30% of all AGNs in the early universe. Because of their odd properties, some researchers have questioned whether LRDs really host black holes at all.
Juodžbalis and his team investigated LRD Abell2744-QSO1 (QSO1), located at a redshift of Z=7.04. The team used JWST’s NIRSpec Integral Field Unit (IFU) and captured a combined 7.3-hour exposure. QSO1 was of particular interest due to debates surrounding its status as an AGN. Previous research has claimed that the mass may have been severely overestimated, as different approaches to theoretical mass estimation have yielded wildly conflicting figures. However, due to massive clusters in the foreground acting as a gravitational lens, naturally boosts the brightness of QSO1, allowing it to be investigated in far greater detail than would normally be possible at such redshifts. This makes it an ideal candidate for direct observation.
The team collected spectra at multiple points across QSO1’s circumgalactic disk. By tracking shifts in the Hα line, interpreted as Doppler shifts, they constructed a rotation curve mapping gas velocity versus distance from the centre. The team attempted to fit a multitude of internal mass distributions this curve, in an attempt to discern the internal structure. Juodžbalis et al. find that the source of mass could not be adequately described by a stellar cluster, or a diffuse gas. The team conclude that that their data must corresponded to gas swirling around a SMBH of around 5x107 M☉.
Surprisingly, the host galaxy is nearly absent. Gas dynamics set an upper limit on the stellar mass component of the system to be just 2x106 M☉, implying the black hole significantly dominates QSO1’s mass. For reference, in the local Universe, galaxies typically outweigh their central black holes by factors of a thousand or more. This demonstrates that in some cases, black holes can form before their host galaxies do.
The existence of QSO1 seems to favour a primordial black hole formation mechanism. This posits that during the early stages of the universe, certain density instabilities could have been so great that black hole collapse was already possible, even moments after the big bang. This could, in theory, permit enough time to pass for QSO1 to accumulate the mass it has today. However, primordial black holes remain controversial, as their stability and growth require several additional assumptions.
As a direct measure of mass, this study likely gives us the most reliable estimate of the mass and nature of QSO1. It also demonstrates that some previous models, which relied on the virial theorem, were able to accurately discern these properties, suggesting we can safely apply them to early type galaxies. Future research will focus on constraining the origins of QSO1 further by exploring more exotic formation mechanisms, as well as attempts to extend direct measurement approaches to additional candidates.
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Journal Source: I. Juodžbalis et al, A direct black hole mass measurement in a Little Red Dot at the Epoch of Reionization, (2025), arXiv: https://doi.org/10.48550/arXiv.2508.21748
Cover Image Credit: ESA