Direct Measurement of a Supermassive Black Hole in a Little Red Dot 700 Million Years After the Big Bang

Recent advancements in astronomical research have unveiled intriguing insights into the early universe, particularly through the study of Little Red Dots (LRDs). These faint, red active galactic nuclei were identified using the Webb Telescope, revealing their existence at significant redshifts, indicating their formation in the nascent stages of cosmic history. A team of astrophysicists has successfully conducted a direct and dynamic measurement of a black hole's mass within one such LRD, located at a redshift of 7.04, or approximately 700 million years after the Big Bang. This black hole is estimated to be around 50 million solar masses, surrounded by only about 20 million solar masses of stars, with the remainder composed of primordial gas, shedding light on the origins of supermassive black holes. The findings have been published in Nature.

Among the LRDs studied, Abell 2744−QSO1 stands out due to its unique appearance, being seen three times as a result of gravitational lensing caused by the foreground galaxy cluster Abell 2744. Ignas Juodžbalis from the University of Cambridge, alongside a diverse team of international collaborators, combined the effects of gravitational lensing with deep spectroscopic data from Webb. This analysis revealed a rotation curve inconsistent with that of a stellar nuclear cluster, suggesting instead a Keplerian rotation around a point mass of 50 million solar masses. The findings indicate a very low stellar component in the host galaxy, with a black hole to star mass ratio greater than 2. This scenario is markedly different from typical galaxies, as the black hole appears almost "naked" in a nearly primordial environment. Juodžbalis and his team propose that this LRD represents a seed of a massive black hole in the early stages of accretion.

Previous studies had estimated the black hole's mass at approximately 40 million solar masses based on virial relations, assuming that these calibrations were applicable at a redshift of z = 7. The direct measurement of 50 million solar masses by Juodžbalis and his collaborators provides evidence that virial mass calibrations are indeed relevant for this prototypical high-redshift emission line.

The astrophysicists further calculated the Eddington luminosity (LEdd) of the black hole to be 7.6 × 10^45 erg s^-1. Utilizing standard scaling relations between broad Hα emission lines and bolometric luminosity (L), they deduced that the black hole is accreting material well below its Eddington limit, with L / LEdd approximately equal to 0.02. If the relationship between broad Hα and bolometric luminosity is higher than locally estimated, L / LEdd could be around 0.01 or even less, suggesting that the black hole might be in a near-dormant state. This discrepancy highlights why mass estimates based on bolometric luminosity assuming L / LEdd = 1 do not align with dynamic measurements.

Additionally, the researchers established a dynamic upper limit on the stellar mass of the galaxy Abell 2744−QSO1, which stands at 20 million solar masses. This finding makes QSO1 the most "naked" massive black hole discovered to date. This observation aligns with previous findings indicating that the black hole exists in a chemically nearly pristine environment. Juodžbalis and his collaborators argue that this supports the notion of black hole primacy, suggesting that black holes may form and grow prior to their host galaxies.

They contend that the only scenarios capable of explaining such a system involve either the existence of "heavy seeds," like direct collapse black holes (formed from the direct collapse of massive primordial gas clouds) or primordial black holes (which may have formed in the first seconds after the Big Bang). However, most direct collapse scenarios would require a significant source of ultraviolet radiation nearby, which is not observed in this case, as there is no galaxy that could have produced such radiation in the past. Furthermore, models of direct collapse black holes suggest that their initial growth is limited by the baryonic fraction in a cooling halo, imposing an upper limit on the black hole to dynamical mass ratio of about 0.1, which is more than an order of magnitude lower than the lower limit deduced by Juodžbalis et al.

The researchers note that the extremely low metallicity of this system provides independent support for the primordial black hole scenario regarding its origin. However, the observed mass of 50 million solar masses significantly exceeds the preferred mass scale of about 1 million solar masses for primordial black holes, a value derived from the electron-positron annihilation era in the early universe. Thus, they propose that the observed mass would require either substantial accretion or rapid mergers of multiple primordial black holes.

Regardless of the specific model, the substantial mass of the black hole in Abell 2744−QSO1, set against the backdrop of such a distant cosmic epoch, along with its extremely high mass ratio and nearly pure gas environment, confirms that this is indeed a massive black hole seed in its early phases of accretion.