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

TL;DR

This paper investigates whether a dense gas envelope can fully enshroud the SMBH in a luminous high-redshift Little Red Dot by mapping rest-frame UV and Lyα emission with JWST and VLT/MUSE. The Lyα halo around A2744-45924 is relatively weak and compact (h ≈ 5.7 pkpc) and is offset from the AGN-tracing UV core, with Lyα likely powered by star formation in the extended component rather than AGN photoionization or resonant scattering. A strong detection of N IV] 1486, narrow Lyα, and a lack of broad Lyα emission support a high gas covering factor (f_cov ≳ 1) and dense neutral gas around the SMBH, compatible with BH* scenarios, though bolometric luminosity estimates may be uncertain. The results imply that LRDs can host rapidly growing SMBHs that are heavily obscured, with feedback and ionizing photon escape strongly suppressed along the line of sight, while the surrounding CGM is shaped by star formation in nearby components. Overall, the work provides key empirical constraints on SMBH growth and AGN-galaxy co-evolution at z > 4, informing models of BH enshrouding and Lyα radiative transfer in dense environments.

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.

Figures (15)

• Figure 1: Morphological model of NIRCam rest-frame UV of A2744-45924 (PSF FWHM $\approx 0.03\arcsec$). The left panels display the F070W and F090W stamps, corresponding to central rest-frame wavelengths of approximately 1290 Å and 1651 Å, respectively. The center panels show the best-fit models obtained with imfit, while the right panels present the residuals. The F070W image, probing the bluest part of the rest-frame UV spectrum of A2744-45924, including Ly$\alpha$, reveals a prominent extended component. In contrast, this component is less clear in F090W. An alternative view of the models can be found in Fig. \ref{['fig:NIRCam_UV_model_contours']}.
• Figure 2: Surface brightness of the Ly$\alpha$ halo in the wavelength interval 6638.75--6665.00 Å. A 2D Gaussian smoothing kernel is applied across the spatial directions of the cube with $\sigma=1.5$ px. The centroid of the NIRCam H$\alpha$ emission is marked with a red cross. The dotted, dashed, and solid lines show the surface brightness contours corresponding to a $\rm S/N = 1$, 5, and 10, respectively. The 1$\sigma$ SB limit corresponds to $2\times 10^{-19}$ erg s$^{-1}$$\rm cm^{-2}$$\rm arcsec^{-2}$.
• Figure 3: Ly$\alpha$ spatial morphology and 1D spectrum. Top: MUSE pseudo-narrowband of the Ly$\alpha$ emission (left), core+halo model (center), and residuals of the fit (right). The morphology of the Ly$\alpha$ halo is fitted to a two-component model composed of a 2D Gaussian and exponential. The scale length of the exponential component (halo) is $h=5.7\pm 0.7$ pkpc, correcting for magnification. Bottom: Optimally extracted 1D spectrum of Ly$\alpha$. We show the fitted skewed Gaussian model (red line). The Ly$\alpha$ line shows a narrow single-peaked profile, typical of star-forming galaxies. The bottom axis shows velocities with respect to the systemic redshift of the H$\alpha$ emission.
• Figure 4: Flux density in velocity space of the H$\alpha$ and Ly$\alpha$ lines of A2744-45924, centered on the systemic velocities of the two lines at $z=4.464$. The ratio of the line fluxes is ${\text{Ly}\alpha}{}/{\text{H}\alpha}{} = 0.066$. In addition, H$\alpha$ has a prominently broad profile (${\rm FWHM} \approx 4500~\rm km\,s^{-1}$; labbe2024), while Ly$\alpha$ presents the classical shape of a single narrow red peak (${\rm FWHM} = 270~\rm km\,s^{-1}$; see Fig. \ref{['fig:monster_lya']}).
• Figure 5: Rest-frame UV emission lines of A2744-45924. The left column shows the 1D spectrum around the wavelength of selected UV emission lines: Niv] ($\lambda\lambda$1483,1486), Civ ($\lambda\lambda$1548,1551), Heii ($\lambda$1640), and Oiii] ($\lambda\lambda$1661,1666). In the case of Niv] and Oiii] the best-fitting two-component Gaussian model with fixed width is also shown. The right column shows images obtained collapsing the MUSE datacube in an interval of 10 Å centered on the selected lines (for doublets, both components stacked), and subtracting the continuum, measured 1000 Å around the position of the line (masking other emission lines and skylines).