A weak Ly$α$ halo for an extremely bright Little Red Dot: Indications of enshrouded SMBH growth

Alberto Torralba, Jorryt Matthee, Gabriele Pezzulli, Tanya Urrutia, Max Gronke, Sara Mascia, Francesco D'Eugenio, Claudia Di Cesare, Anna-Christina Eilers, Jenny E. Greene, Edoardo Iani, Yuzo Ishikawa, Ruari Mackenzie, Rohan P. Naidu, Benjamín Navarrete, Gauri Kotiwale

First listed 2025-05-14 | Last updated 2025-11-17

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 unveiling 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 ($N_{\rm H}\sim10^{24}\,\rm cm^{-2}$). We present a stringent test of such models using VLT/MUSE observations of A2744-45924, the most luminous LRD known to date ($L_{\rm Hα}\approx10^{44}~\rm erg\,s^{-1}$), located behind the Abell-2744 lensing cluster at $z=4.464$ ($μ=1.8$). We detect a moderately extended Ly$α$ nebula ($h\approx5.7$ pkpc), spatially offset from the point-like H$α$ seen by JWST. The Ly$α$ emission is narrow ($\rm FWHM=270\pm 15~km\,s^{-1}$), spatially offset to H$α$, and faint ($\rm Lyα=0.07Hα$) compared to Ly$α$ nebulae typically observed around quasars of similar luminosity. We detect compact N$\,$IV]$λ$1486 emission, spatially aligned with H$α$, and a spatial shift in the far-UV continuum matching the Ly$α$ offset. We discuss that H$α$ and Ly$α$ have distinct physical origins: H$α$ originates from the AGN, while Ly$α$ is powered by star formation. In the environment of A2744-45924, we identify four extended Ly$α$ halos ($Δz<0.02$, $Δr<100$ pkpc). Their Ly$α$ luminosities match expectations based on H$α$ emission, indicating no evidence for radiation from A2744-45924 affecting its surroundings. The lack of strong, compact, and broad Ly$α$ and the absence of a luminous extended halo, suggest that the UV AGN light is obscured by dense gas cloaking the SMBH with covering factor close to unity.

Short digest

Deep VLT/MUSE mapping around the ultra-luminous LRD A2744-45924 (z=4.464, μ=1.8; L_Hα≈10^44 erg s^-1) reveals a modest, spatially offset Lyα halo with exponential scale length ≈5.7 pkpc and a narrow line (FWHM 270±15 km s^-1) whose flux is only 7% of Hα. Compact N IV] λ1486 coincides with the point-like Hα seen by JWST, while the far-UV continuum centroid shifts with the Lyα offset. The authors argue Hα traces the AGN whereas Lyα is powered by star formation; four neighboring Lyα halos (Δz<0.02, Δr<100 pkpc) show no luminosity excess indicative of AGN illumination. Altogether this points to a dense, near-unity covering-factor cocoon that shrouds the SMBH, suppressing UV/ionizing escape and signaling enshrouded early black-hole growth.

Key figures to inspect

• Fig. 1 (NIRCam F070W vs F090W): Inspect the residuals—F070W shows an extended UV component that includes Lyα and a centroid shift relative to redder F090W, foreshadowing the Lyα/Hα spatial offset.
• Fig. 2 (MUSE Lyα morphology + spectrum): Use the core+halo fit to read off the exponential scale length ≈5.7 pkpc and check residuals for asymmetries; the optimally extracted 1D profile shows a single narrow red peak typical of SF-powered Lyα.
• Fig. 3 (Velocity-space Lyα vs Hα): Directly compare the narrow Lyα to the prominently broad Hα and note the suppressed Lyα/Hα ≈ 0.07, quantifying the weak, offset Lyα relative to the AGN-powered recombination line.
• Fig. 4 (Rest-UV emission-line maps): Verify that N IV] λ1486 is compact and co-spatial with Hα, while C IV/He II/O III] are weak or extended differently—evidence separating the AGN-dominated core from the SF-powered Lyα halo.