A big red dot at cosmic noon

¹ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, I-40129 Bologna, Italy
² Dipartimento di Fisica e Astronomia, Università di Bologna, Via Gobetti 93/2, I-40129 Bologna, Italy
³ Space Telescope Science Institute for the European Space Agency (ESA), ESA Office, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁴ The William H. Miller III Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA
⁵ INAF – Arcetri Astrophysical Observatory, Largo E. Fermi 5, I50125 Florence, Italy
⁶ Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona (IEEC-UB), Martí i Franquès, 1, 08028 Barcelona, Spain
⁷ ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Spain
⁸ Department of Physics and Astronomy, Clemson University, Kinard Lab of Physics, Clemson, SC 29634-0978, USA
⁹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD, USA
¹⁰ Eureka Scientific, 2452 Delmer Street, Suite 100, Oakland, CA 94602-3017, USA
¹¹ Department of Physics, Yale University, PO Box 208120 New Haven, CT 06520, USA
¹² Dipartimento di Matematica e Fisica, Univeristà di Roma 3 Via della Vasca Navale, 84, 00146 Roma, Italy
¹³ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 OHA, UK
¹⁴ Cavendish Laboratory – Astrophysics Group, University of Cambridge, 19 JJ Thomson Avenue, Cambridge CB3 OHE, UK
¹⁵ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
¹⁶ INAF – Istituto di Radioastronomia, Via Gobetti 101, I-40129 Bologna, Italy
¹⁷ Max-Planck-Institut für extraterrestrische Physik (MPE), Gießenbachstraße 1, 85748 Garching, Germany

^⋆ Corresponding author: federica.loiacono1@inaf.it

Received: 13 June 2025
Accepted: 16 August 2025

Abstract

We report the discovery of a little red dot (LRD), dubbed BiRD (‘big red dot’), at z ∼ 2.33 in the field around the z ∼ 6.3 quasar SDSS J1030+0524. Using JWST/NIRCam images, we identified it as a bright outlier in the F200W − F356W color versus F356W magnitude diagram of point sources in the field. The NIRCam/WFSS spectrum reveals the emission from He Iλ 10830 and Paγ line, both displaying a narrow and a broad (FWHM ≳ 2000 km s⁻¹) component. The He I line is affected by an absorption feature, tracing dense gas with He I column density in the 2³S level N(He I) ∼ 0.5 − 1.2 × 10¹⁴ cm⁻², depending on the location of the absorber, which is outflowing at a speed of Δ_v = −830₋₁₄₈⁺¹³¹ km s⁻¹. As observed in the majority of LRDs, BiRD does not exhibit any X-ray or radio emission down to 3.7 × 10⁴² erg s⁻¹ and 3 × 10³⁹ erg s⁻¹, respectively. The black hole mass and the bolometric luminosity, both inferred from the Paγ broad component, amount to M_BH ∼ 10⁸ M_⊙ and L_bol ∼ 3 × 10⁴⁵ erg s⁻¹, respectively. Intriguingly, BiRD presents strict analogies with other two LRDs spectroscopically confirmed at cosmic noon, namely, GN-28074 (nicknamed Rosetta Stone) at z ∼ 2.26 and RUBIES-BLAGN-1 at z ∼ 3.1. The blueshifted He I absorption detected for all three sources suggests that gas outflows could be common in LRDs. We derived a first estimate of the space density of LRDs at z ∼ 2 − 3 based on JWST data, as a function of the bolometric luminosity and black hole mass. The space density Φ(L) = 4.0_−2.4^+4.0 × 10⁻⁶ Mpc⁻³ dex⁻¹ is only a factor of ∼2 − 3 lower than that of UV-selected quasars with comparable bolometric luminosity and redshift, meaning that the contribution of LRDs to the broader AGN population is also relevant at cosmic noon. A similar trend has also been observed with respect to black hole masses. As suggested by recent theories, if LRDs can indeed serves as probes of the very first and rapid growth of black hole seeds, our finding suggests that the formation of black hole seeds continues to be efficient at least up to cosmic noon.