Tracing the rise of supermassive black holes

A panchromatic search for faint, unobscured quasars at z  ≳ 6 with COSMOS-Web and other surveys^⋆

¹ Technical University of Munich, TUM School of Natural Sciences, Department of Physics, James-Franck-Str. 1, 85748 Garching, Germany
e-mail: irham.andika@tum.de
² Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85748 Garching, Germany
³ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁴ Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU, WPI), The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8583, Japan
⁵ Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, PR China
⁶ Department of Astronomy, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan
⁷ National Astronomical Observatory of Japan, 2-21-1, Osawa, Mitaka, Tokyo 181-8588, Japan
⁸ INAF, Osservatorio Astronomico di Roma, Via di Frascati 33, 00078 Monte Porzio Catone, Italy
⁹ Cosmic Dawn Center (DAWN), Copenhagen, Denmark
¹⁰ DTU-Space, Technical University of Denmark, Elektrovej 327, 2800 Kgs. Lyngby, Denmark
¹¹ Department of Astronomy, The University of Texas at Austin, 2515 Speedway Blvd Stop C1400, Austin, TX 78712, USA
¹² Caltech/IPAC, MS 314-6, 1200 E. California Blvd., Pasadena, CA 91125, USA
¹³ Department of Computer Science, Aalto University, PO Box 15400 Espoo 00 076, Finland
¹⁴ Department of Physics, Faculty of Science, University of Helsinki, 00014 Helsinki, Finland
¹⁵ Center for Computational Astrophysics, Flatiron Institute, 162 Fifth Avenue, New York, NY 10010, USA
¹⁶ Institute for Physics, Laboratory for Galaxy Evolution and Spectral Modelling, EPFL, Observatoire de Sauverny, Chemin Pegasi 51, 1290 Versoix, Switzerland
¹⁷ INAF, Osservatorio Astronomico di Trieste, Via Tiepolo 11, 34131 Trieste, Italy
¹⁸ Department of Physics and Astronomy, Colby College, Waterville, ME 04901, USA
¹⁹ Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218, USA
²⁰ Kapteyn Astronomical Institute, University of Groningen, PO Box 800 9700 AV Groningen, The Netherlands
²¹ NASA-Goddard Space Flight Center, Code 662, Greenbelt, MD 20771, USA
²² Department of Physics and Astronomy, University of California, Los Angeles, CA 90095-1547, USA
²³ School of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel
²⁴ Department of Physics, Yale University, PO Box 208120 New Haven, CT 06520-8120, USA
²⁵ Astronomy Centre, University of Sussex, Falmer, Brighton BN1 9QH, UK
²⁶ Institute of Space Sciences and Astronomy, University of Malta, Msida MSD 2080, Malta

Received: 19 December 2023
Accepted: 25 January 2024

Abstract

We report the identification of 64 new candidates of compact galaxies, potentially hosting faint quasars with bolometric luminosities of L_bol = 10⁴³–10⁴⁶ erg s⁻¹, residing in the reionization epoch within the redshift range of 6 ≲ z ≲ 8. These candidates were selected by harnessing the rich multiband datasets provided by the emerging JWST-driven extragalactic surveys, focusing on COSMOS-Web, as well as JADES, UNCOVER, CEERS, and PRIMER. Our search strategy includes two stages: applying stringent photometric cuts to catalog-level data and detailed spectral energy distribution fitting. These techniques effectively isolate the quasar candidates while mitigating contamination from low-redshift interlopers, such as brown dwarfs and nearby galaxies. The selected candidates indicate physical traits compatible with low-luminosity active galactic nuclei, likely hosting ≈10⁵–10⁷ M_⊙ supermassive black holes (SMBHs) living in galaxies with stellar masses of ≈10⁸–10¹⁰ M_⊙. The SMBHs selected in this study, on average, exhibit an elevated mass compared to their hosts, with the mass ratio distribution slightly higher than those of galaxies in the local Universe. As with other high-z studies, this is at least in part due to the selection method for these quasars. An extensive Monte Carlo analysis provides compelling evidence that heavy black hole seeds from the direct collapse scenario appear to be the preferred pathway to mature this specific subset of SMBHs by z ≈ 7. Notably, most of the selected candidates might have emerged from seeds with masses of ∼10⁵ M_⊙, assuming a thin disk accretion with an average Eddington ratio of f_Edd = 0.6 ± 0.3 and a radiative efficiency of ϵ = 0.2 ± 0.1. This work underscores the significance of further spectroscopic observations, as the quasar candidates presented here offer exceptional opportunities to delve into the nature of the earliest galaxies and SMBHs that formed during cosmic infancy.