GoKawiilObservations and data reduction
MoM-BH*-1 has been observed with JWST by three programmes. It was imaged in cycle 1 by the PRIMER survey14 (JWST-GO-1837; principal investigator: J. Dunlop) using the MIRI (5 January 2023 and 16 January 2023) and NIRCam (7 August 2023 and 9 August 2023) instruments. In cycle 2 (19 December 2023), the EXCELS survey15 (JWST-GO-3543) obtained 1.5 h of NIRSpec G395M spectroscopy. In cycle 3 (15 December 2024), we targeted MoM-BH*-1 as part of the ‘Mirage or Miracle’ NIRSpec prism survey (JWST-GO-5224). We included MoM-BH*-1 as a high-priority target second in importance only to luminous z > 10 sources in our UDS masks, because it appeared in several priority target lists—AGN/LRD candidates selected based on compact morphology and template fitting with EAZY47, extremely massive galaxy candidates, sources with peculiar red colours and the literature LRD candidates48.
We use the v.7.2 images of the PRIMER field released on the DAWN JWST archive (DJA) reduced using the grizli software49. PSF-matched photometric catalogues based on these images were produced in ref. 50. We use the public v.3 NIRSpec reductions of the EXCELS grating data from the DJA derived using the msaexp software37,51,52. The MoM data are reduced with the same pipeline following the same choices.
Although we found no relevant radio or ALMA archival data, MoM-BH*-1 has been observed with Chandra53. Similar to virtually all LRDs9,54,55, it remains undetected in the X-rays (L x < 44.5 erg s−1 (1σ) at rest frame 5–90 keV). Key empirical properties of the source are summarized in Extended Data Table 1.
Emission line fitting
We use a custom NIRSpec emission line fitting package56 to simultaneously fit emission lines in the grating and prism spectra. The advantage of this approach is that despite the low signal-to-noise ratio (SNR) in either mode, features may be robustly recovered because of their occurrence at the same wavelength across both dispersers.
We first fit the Hβ line and [Oiii] doublet, and then use the redshift as a prior to fit Hγ (Fig. 1). We model Hβ as a single emission line with three absorbers constrained to have negative flux (one at line-centre and two on either side of zero velocity) motivated by the symmetric absorption troughs on either side of the central double-peak (Extended Data Fig. 2). The systemic redshift is tied to [Oiii] and the broad Hβ component, with the absorbers allowed to range freely. The number of absorbers is decided based on the maxima reached in the reduced χ2, which is similar for three and four absorbers, but we opt for parsimony. This large number of absorbers may be merited to account for secondary peaks at ±2,400 km s−1 (Extended Data Fig. 2). Including narrow Hβ at the systemic redshift leads to completely unconstrained flux degenerate with absorption and no improvement, so we neglect this component. Resulting fits, in which we sample the posterior with the NUTS sampler implemented in numpyro57, are shown in Extended Data Fig. 1 and reported in Extended Data Table 2.
Although the formal errors on the derived fluxes and line widths of many of the components is significant, the key features relevant to this analysis are robustly recovered—extremely broad Hβ emission, and deep, broad absorption wiping out about 25% of the emission flux, including approximately 100% of the flux at line-centre. Another notable aspect of these fits is the location of the two absorbers—they are recovered at very similar velocities, but on either side of line-centre (approximately ±1,500 km s−1), albeit with significant uncertainties (\(-1,53{2}_{-113}^{+345}\,\mathrm{km}\,{{\rm{s}}}^{-1}\) and \(+1,55{6}_{-1,378}^{+232}\,\mathrm{km}\,{{\rm{s}}}^{-1}\)). We explore this symmetry, which extends not only to the location of the absorbers but also to the detailed structure of the entire emission line profile over a few 1,000 km s−1 in Extended Data Fig. 2.
Morphology
We use pysersic58 to fit a Sersic profile to the imaging. We focus on the F356W and F444W imaging, in which the source is well-detected. We follow the procedure described in ref. 59—we build an empirical PSF using stars in the field, and then use this PSF with pysersic to sample the posterior Sersic parameters with numpyro. The source is unresolved, and we are able to place a 99% upper limit on the effective radius of <117 pc consistent with the BH-dominated interpretation.
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