Dating N-loud AGNs at high redshift

GS_3073 as a snapshot of a ω Centauri-like evolution of a nuclear star cluster

¹ Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monte Porzio Catone, Italy
² Istituto Nazionale di Astrofisica – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, Padova 35122, Italy
³ Dipartimento di Fisica e Astronomia “Galileo Galilei”, Univ. di Padova, Vicolo dell’Osservatorio 3, Padova 35122, Italy
⁴ Department of Astronomy, Indiana University, Swain West, 727 E. 3rd Street, Bloomington, IN 47405, USA
⁵ Dipartimento di Fisica e Astronomia Augusto Righi, Università degli Studi di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
⁶ INAF – OAS, Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, via Gobetti 93/3, 40129 Bologna, Italy
⁷ Istituto Nazionale di Astrofisica – Istituto di Astrofisica e Planetologia Spaziali, Via Fosso del Cavaliere 100, 00133 Roma, Italy

^★ Corresponding author.

Received: 1 April 2025
Accepted: 8 July 2025

Abstract

We address two major questions raised by recent observations of the young Universe made with the James Webb Space Telescope. These questions are first, the amount of the seed initial masses, and the speed of growth of supermassive black holes (BHs) with masses of 10⁶–10⁸ M_⊙ in active galactic nuclei (AGN) that are hosted by very young galaxies. The second question is an explanation for the supersolar abundances of nitrogen in a fraction of young galaxies at high redshift, with and without evidence of a massive central black hole. While the redshift in the still-forming high-redshift galaxies with AGN is an upper limit to the ages of the seed black holes, any independent age determination provides a more stringent constraint on the BH accretion modalities and on the initial seed mass. We mainly focused on the system GS_3073. This system shows an exceptionally high log(N/O)=+0.42_−0.10^+0.13 in the gas close to the AGN. We show here that this abundance is consistent with the composition of gas ejected from massive asymptotic giant branch stars. Moreover, the chemical properties of this system match those expected at a specific point in the evolution of the abundances in the extreme populations of the former nuclear star cluster ω Centauri (ωCen). This analogy, along with the N/O, C/O, and Fe/O abundances in GS_3073, lead to an estimate of an age range of 270–440 Myr for this object, which is much younger than the redshift (z=5.5) age of ∼1 Gyr. We also adopted the same criteria to estimate an age for GN-z11. These two determinations constrain the BH mass versus age relation: Accretion onto the BH must proceed at intermittent super-Eddington rates in the first phases and at a much lower rate after the first half gigayear of life of the Universe. The intermittency of accretion is also a fundamental requirement to allow the formation of the extreme (N-rich, O-depleted, He-rich) populations that are observed today in ω Cen for a wide range of metallicities.