JADES

The diverse population of infant black holes at 4 < z < 11: Merging, tiny, poor, but mighty

¹ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
² Cavendish Laboratory, University of Cambridge, 19 JJ Thomson Avenue, Cambridge CB3 0HE, UK
³ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
⁴ Centre for Astrophysics Research, Department of Physics, Astronomy and Mathematics, University of Hertfordshire, Hatfield AL10 9AB, UK
⁵ Scuola Normale Superiore, Piazza dei Cavalieri 7, I-56126 Pisa, Italy
⁶ Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
⁷ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
⁸ Centro de Astrobiología (CAB), CSIC–INTA, Cra. de Ajalvir Km. 4, 28850- Torrejón de Ardoz Madrid, Spain
⁹ Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
¹⁰ Sorbonne Université, CNRS, UMR 7095, Institut d’Astrophysique de Paris, 98 bis bd Arago, 75014 Paris, France
¹¹ Center for Astrophysics - Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA
¹² Steward Observatory, University of Arizona, 933 N. Cherry Avenue, Tucson, AZ 85721, USA
¹³ European Space Agency (ESA), European Space Astronomy Centre (ESAC), Camino Bajo del Castillo s/n, 28692 Villafranca del Castillo Madrid, Spain
¹⁴ Department of Astronomy and Astrophysics, University of California, Santa Cruz, 1156 High Street, Santa Cruz, CA 95064, USA
¹⁵ National Astronomical Research Institute of Thailand, Don Kaeo, Mae Rim, Chiang Mai 50180, Thailand; Department of Physics, Faculty of Science, Chulalongkorn University, 254 Phayathai Road, Pathumwan, Bangkok 10330, Thailand
¹⁶ NSF’s National Optical-Infrared Astronomy Research Laboratory, 950 North Cherry Avenue, Tucson, AZ 85719, USA
¹⁷ NRC Herzberg, 5071 West Saanich Rd, Victoria, BC V9E 2E7, Canada

^⋆ Corresponding author; rm665@cam.ac.uk

Received: 2 August 2023
Accepted: 23 August 2024

Abstract

Spectroscopy with the James Webb Space Telescope has opened the possibility of identifying moderate-luminosity active galactic nuclei (AGNs) in the early Universe, at and beyond the epoch of re-ionisation, complementing previous surveys of much more luminous (and much rarer) quasars. We present 12 new AGNs at 4 < z < 7 in the JADES survey (in addition to the previously identified AGN in GN-z11 at z = 10.6) revealed through the detection of a broad-line region (BLR) seen in the Balmer emission lines. The depth of JADES, together with the use of three different spectral resolutions, enables us to probe a lower-mass regime relative to previous studies. In a few cases, we find evidence for two broad components of Hα, which suggests that these could be candidate merging black holes (BHs), although a complex BLR geometry cannot be excluded. The inferred BH masses range from 8 × 10⁷ M_⊙ down to 4 × 10⁵ M_⊙, interestingly probing the regime expected for direct collapse BHs. The inferred AGN bolometric luminosities (∼10⁴⁴ − 10⁴⁵ erg/s) imply accretion rates that are < 0.5 times the Eddington rate in most cases. However, small BHs, with M_BH ∼ 10⁶ M_⊙, tend to accrete at Eddington or super-Eddington rates. These BHs at z ∼ 4–11 are over-massive relative to their host galaxies’ stellar masses when compared to the local M_BH − M_star relation, even approaching M_BH ∼ M_star, as was expected from heavy BH seeds and/or super-Eddington accretion scenarios. However, we find that these early BHs tend to be more consistent with the local relation between M_BH and velocity dispersion, as well as between M_BH and dynamical mass, suggesting that these are more fundamental and universal relations. On the classical, optical narrow-line excitation-diagnostic diagrams, these AGNs are located in the region that is locally occupied by star-forming galaxies, implying that they would be missed by the standard classification techniques if they did not display broad lines. Their location on the diagram is consistent with what is expected for AGNs hosted in metal-poor galaxies (Z ∼ 0.1 − 0.2 Z_⊙). The fraction of broad-line AGNs with L_AGN > 10⁴⁴ erg/s among galaxies in the redshift range of 4 < z < 6 is about 10%, suggesting that the contribution of AGNs and their hosts to the re-ionisation of the Universe is > 10%.