Size matters: are we witnessing super-Eddington accretion in high-redshift black holes from JWST?

¹ Dipartimento di Scienza e Alta Tecnologia, Università degli Studi dell’Insubria, via Valleggio 11, 22100 Como, Italy
² INFN, Sezione di Milano-Bicocca, Piazza della Scienza 3, 20126 Milano, Italy
³ INAF, Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monte Porzio Catone, Italy
⁴ Institut d’Astrophysique de Paris, UMR 7095, CNRS and Sorbonne Université, 98 bis boulevard Arago, 75014 Paris, France
⁵ Dipartimento di Fisica “G. Occhialini”, Università degli Studi di Milano-Bicocca, Piazza della Scienza 3, 20126 Milano, Italy
⁶ Instituto de Estudios Astrofísicos, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Avenida Ejercito Libertador 441, Santiago, Chile

Received: 25 June 2024
Accepted: 15 July 2024

Abstract

Observations by the James Webb Space Telescope of the Universe at z ≳ 4 have shown that massive black holes (MBHs) appear to be extremely overmassive compared to the local correlation for active galactic nuclei. In some cases, these objects might even reach half the stellar mass inferred for the galaxy. It has become a great challenging for theoretical models to understand how these objects formed and grew to these masses. Different ideas range from heavy seed to super-Eddington accretion phases. We take a different approach and try to infer how accurate these MBH mass estimates are and whether we really need to revise our physical models. By considering how the emerging spectrum (both the continuum and the broad lines) of an accreting MBH changes close to and above the Eddington limit, we infer a much larger uncertainty in the MBH mass estimates relative to that of local counterparts. The uncertainty is up to an order of magnitude. We also infer a potential preference for lower masses and higher accretion rates, which i) moves accreting MBHs closer to the local correlations, and ii) might indicate that we witness a widespread phase of very rapid accretion for the first time.