A weak Lyα halo for an extremely bright little red dot

Indications of enshrouded supermassive black hole growth

¹ Institute of Science and Technology Austria (ISTA) Am Campus 1 3400 Klosterneuburg, Austria
² Kapteyn Astronomical Institute, University of Groningen Landleven 12 9747 AD Groningen, The Netherlands
³ Leibniz-Institut für Astrophysik, Potsdam An der Sternwarte 16 Potsdam 14482, Germany
⁴ Max Planck Institute for Astrophysics Karl-Schwarzschild-Str. 1 85748 Garching, Germany
⁵ Kavli Institute for Cosmology, University of Cambridge Madingley Road Cambridge CB3 0HA, United Kingdom
⁶ Cavendish Laboratory – Astrophysics Group, University of Cambridge 19 JJ Thomson Avenue Cambridge CB3 0HE, United Kingdom
⁷ MIT Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology Cambridge MA 02139, USA
⁸ Department of Astrophysical Sciences, Princeton University Princeton NJ 08544, USA
⁹ Institute of Physics, Laboratory of Astrophysics, Ecole Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny 1290 Versoix, Switzerland

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 20 May 2025
Accepted: 4 November 2025

Abstract

The abundant population of little red dots (LRDs), compact objects with red UV to optical colors and broad Balmer lines at high redshift, is revealing new insights into the properties of early active galactic nuclei (AGN). Perhaps the most surprising features of this population are the presence of Balmer absorption and ubiquitous strong Balmer breaks. Recent models link these features to an active supermassive black hole (SMBH) cocooned in very dense gas (N_H ∼ 10²⁴ cm⁻²). We present a stringent test of such models using VLT/MUSE observations of A2744-45924, the most luminous LRD known to date (L_Hα ≈ 10⁴⁴ erg s⁻¹), located behind the Abell-2744 lensing cluster at z = 4.464 (μ = 1.8). We detect a moderately extended Lyα nebula (h ≈ 5.7 pkpc), spatially offset from the point-like Hα seen by JWST by ≈1.6 pkpc. The Lyα emission is narrow (FWHM = 270 ± 15 km s⁻¹), and faint (Lyα = 0.07Hα) compared to Lyα nebulae typically observed around quasars of similar luminosity. We detect compact N IV]λ1486 emission, spatially aligned with Hα, and a spatial shift in the far-UV continuum matching the Lyα offset. We discuss that Hα and Lyα have distinct physical origins: Hα originates from the AGN, while Lyα is powered by star formation. In the environment of A2744-45924, we identified four extended Lyα halos (Δz < 0.02, Δr < 100 pkpc). Their Lyα luminosities match the expectations based on Hα emission, and show no evidence for radiation from A2744-45924 affecting its surroundings. The lack of strong, compact, and broad Lyα and the absence of a luminous extended halo, suggest that the UV AGN light is obscured by dense gas cloaking the SMBH with a covering factor close to unity.