<\/p>\n
MoM-BH*-1 has been observed with JWST by three programmes. It was imaged in cycle 1 by the PRIMER survey14<\/a><\/sup> (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<\/a><\/sup> (JWST-GO-3543) obtained 1.5\u2009h of NIRSpec G395M spectroscopy. In cycle 3 (15 December 2024), we targeted MoM-BH*-1 as part of the \u2018Mirage or Miracle\u2019 NIRSpec prism survey (JWST-GO-5224). We included MoM-BH*-1 as a high-priority target second in importance only to luminous z<\/i>\u00a0>\u00a010 sources in our UDS masks, because it appeared in several priority target lists\u2014AGN\/LRD candidates selected based on compact morphology and template fitting with EAZY<\/span>47<\/a><\/sup>, extremely massive galaxy candidates, sources with peculiar red colours and the literature LRD candidates48<\/a><\/sup>.\u00a0<\/p>\n We use the v.7.2 images of the PRIMER field released on the DAWN JWST archive (DJA) reduced using the grizli\u00a0software49<\/a><\/sup>. PSF-matched photometric catalogues based on these images were produced in ref.\u200950<\/a><\/sup>. We use the public v.3 NIRSpec reductions of the EXCELS grating data from the DJA derived using the msaexp\u00a0software37<\/a>,51<\/a>,52<\/a><\/sup>. The MoM data are reduced with the same pipeline following the same choices.<\/p>\n Although we found no relevant radio or ALMA archival data, MoM-BH*-1 has been observed with Chandra53<\/a><\/sup>. Similar to virtually all LRDs9<\/a>,54<\/a>,55<\/a><\/sup>, it remains undetected in the X-rays (L<\/i>x<\/i><\/sub>\u00a0<\u00a044.5\u2009erg\u2009s\u22121<\/sup> (1\u03c3<\/i>) at rest frame 5\u201390 keV).\u00a0Key empirical properties of the source are summarized in Extended Data Table 1<\/a>.<\/p>\n We use a custom NIRSpec emission line fitting package56<\/a><\/sup> 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.<\/p>\n We first fit the H\u03b2 line and [Oiii<\/span>] doublet, and then use the redshift as a prior to fit H\u03b3 (Fig. 1<\/a>). We model H\u03b2 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<\/a>). The systemic redshift is tied to [Oiii<\/span>] and the broad H\u03b2 component, with the absorbers allowed to range freely. The number of absorbers is decided based on the maxima reached in the reduced \u03c7<\/i>2<\/sup>, 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 \u00b12,400\u2009km\u2009s\u22121<\/sup> (Extended Data Fig. 2<\/a>). Including narrow H\u03b2 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\u00a0numpyro57<\/a><\/sup>, are shown in Extended Data Fig. 1<\/a> and reported in Extended Data Table 2<\/a>.<\/p>\n 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\u2014extremely broad H\u03b2 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\u2014they are recovered at very similar velocities, but on either side of line-centre (approximately \u00b11,500\u2009km\u2009s\u22121<\/sup>), albeit with significant uncertainties ((-1,53{2}_{-113}^{+345},mathrm{km},{{rm{s}}}^{-1})<\/span> and (+1,55{6}_{-1,378}^{+232},mathrm{km},{{rm{s}}}^{-1})<\/span>). 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\u2009km\u2009s\u22121<\/sup> in Extended Data Fig. 2<\/a>.<\/p>\n We use\u00a0pysersic58<\/a><\/sup> 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.\u200959<\/a><\/sup>\u2014we build an empirical PSF using stars in the field, and then use this PSF with\u00a0pysersic to sample the posterior Sersic parameters with\u00a0numpyro. The source is unresolved, and we are able to place a 99% upper limit on the effective radius of <117\u2009pc consistent with the BH-dominated interpretation.<\/p>\n That extremely dense gas might blanket the LRDs was already inferred in the work that defined this class of sources9<\/a><\/sup>\u2014about 10% of these LRDs showed clear signs of Balmer absorption, with this being a lower limit due to resolution and SNR11<\/a><\/sup> as opposed to the \u226a0.1% in pre-JWST AGN60<\/a><\/sup>, with only a handful examples9<\/a><\/sup>. Local AGN61<\/a><\/sup> do not display Balmer breaks anywhere as strong as MoM-BH*-1 (about 8), with the strongest breaks (\u22722.5) occurring in quiescent galaxy AGN that reach the stellar population maximum shown in Fig. 2b<\/a>. Inspired by Balmer absorption in LRDs, and using Cloudy\u00a0models for continuum absorption5<\/a><\/sup>, demonstrated that the very gas producing strong Balmer absorption likely also produces Balmer breaks. However, note the breaks in these models are weaker than what we require to explain MoM-BH*-1 and are abrupt instead of the smooth rollover seen in this source. Here we build on this work.<\/p>\n The grid of parameters we explore is summarized in Extended Data Table 3<\/a>. This grid spans a wide range to capture the extreme spectral shape at hand. We start with an intrinsic AGN continuum SED that is parametrized using a series of power laws and with a \u2018big bump\u2019 temperature32<\/a><\/sup>. This AGN continuum is passed through gas surrounding the central source that is defined in terms of its gas density, column density, metallicity and turbulent velocity. The irradiation of the cloud is modulated by an ionization parameter (log(U<\/i>)). A large turbulent velocity of the absorbing gas 500\u2009km\u2009s\u22121<\/sup> is motivated by the width of the absorption lines we observe (see Fig. 1<\/a>). This implies a high Mach number, which is consistent with recent models of AGN disks62<\/a><\/sup>\u2014whether such high turbulence can be sustained across a large envelope remains to be seen. The turbulence is important in producing a smooth rollover instead of a sharp break6<\/a><\/sup>. Finally, we apply a uniform dust screen63<\/a><\/sup> with A<\/i>V<\/sub>\u00a0=\u00a00\u22123 in the post-processing\u2014this is the only post-processing step we apply.<\/p>\n To select models that are consistent with the data, we require: (1) H\u03b2 in net emission with 30\u00a0<\u00a0EW [\u00c5]\u00a0<\u00a045; (2) H\u03b3 in absorption with \u22125\u00a0<\u00a0EW [\u00c5]\u00a0<\u00a00\u2009\u00c5; (3) a strong Balmer break and high optical-to-UV ratio such that ({f}_{{rm{4.5mu m}}}^{lambda }\/{f}_{{rm{1.8mu m}}}^{lambda } > 6)<\/span> and ({f}_{{rm{4.5mu m}}}^{lambda }\/{f}_{{rm{2.8mu m}}}^{lambda } > 3)<\/span>; and (4) fluxes in MIRI bands within 2\u03c3<\/i> of the observations. The few thousand models that satisfy these constraints are re-simulated at higher resolution, retaining only hydrogen that is relevant to the key features for simplicity and speed. Of these, we select the one that closely follows the detailed shape of the continuum. Furthermore, we experiment with the covering factor and distance between the gas and central source, and find these quantities to be degenerate with the AGN SED and ionization parameter. We find the \u2018net transmitted\u2019 flux (that is, the sum of the attenuated incident continuum and diffuse continua or lines) produces a good match to the data, not the \u2018total\u2019 flux (which also includes reflected continua or lines).<\/p>\n First, we emphasize that this model (Fig. 3<\/a> and Extended Data Fig. 3<\/a>) is a model that matches various features of interest based on our limited grid. The relevant parameter space that we have done our best to sample is high-dimensional, degenerate and much remains unknown (for example, the intrinsic SEDs of early AGN with effects such as photon trapping). Therefore, we caution against detailed inferences beyond the feasibility of the broad physical picture that we present here (an accretion disk embedded in dense gas).<\/p>\n Notably, the selected model features an extreme column density (about 1025.8<\/sup>\u2009cm\u22122<\/sup>) comparable to the most enshrouded systems observed64<\/a><\/sup> and a high gas density (1011<\/sup>\u2009cm\u22123<\/sup>) conducive to Balmer absorption. The turbulent velocity of 500\u2009km\u2009s\u22121<\/sup> is commensurate with the width of the central H\u03b2 absorber. Metal-poor gas expected of a dwarf galaxy at z<\/i>\u00a0\u2248\u00a08 is preferred. The AGN slope parameters are within the range of literature SEDs6<\/a>,65<\/a><\/sup>.<\/p>\n An important feature of the fiducial model is that it is virtually dust-free (A<\/i>V<\/sub>\u00a0=\u00a00.15\u2009mag). The UV-weakness arises entirely because of the extreme hydrogen opacity. This is a crucial constraint on LRD models that typically invoke significant amounts of dust to suppress strong UV emission from classical AGN21<\/a>,24<\/a>,66<\/a><\/sup> or to explain the weakness of H\u03b2 relative to H\u03b112<\/a><\/sup>. Although significant A<\/i>V<\/sub>\u00a0\u22482\u20133 helps dense gas AGN models produce a smooth Balmer break in the rest-optical6<\/a><\/sup>, this is ruled out by longer wavelength constraints in our source (Extended Data Fig. 3<\/a>) and more generally by stringent infrared constraints on the LRDs34<\/a>,35<\/a><\/sup>. We note that given the very low A<\/i>V<\/sub>\u00a0\u2248\u00a00, the exact details of the dust geometry (for example, screen compared with clumps) do not have a bearing on our results.<\/p>\n The H\u03b2 profile of MoM-BH*-1 bears a remarkable resemblance to double-peaked Ly\u03b167<\/a>,68<\/a>,69<\/a><\/sup>. This motivates us to explore whether H\u03b2 is behaving like a resonant line under the high densities in MoM-BH*-1. It may be the case that this is a common phenomena in the LRDs, but that a symmetric double-peaked line with large separation is easier to observe in MoM-BH*-1 because of negligible H\u03b2 emission from the sub-dominant host galaxy.<\/p>\n Although H\u03b2 normally has an easy cascade escape route, saturating the 2p<\/i> state can effectively trap H\u03b2 photons, which may be plausible through Ly\u03b1 pumping in these optically thick environments70<\/a><\/sup>. We test this idea with the\u00a0COLT71<\/a><\/sup> radiative transfer code with which we implement a simple shell model (directly analogous to Ly\u03b1 shell models) for H\u03b2. The dominant parameters in these calculations are the thermal velocities of the inner and outer shells, the relative velocities between the shells and the optical depth encountered by H\u03b2. Strong turbulence is not accounted for in these toy models.<\/p>\n In Extended Data Fig. 4<\/a>, we show how a relatively narrow Gaussian H\u03b2 line manifests as a double-peaked profile. We confirm the basic trends seen with the Ly\u03b1 line hold up for Balmer resonant scattering and can serve as a powerful guide to interpret MoM-BH*-1. With this simple toy model, we are able to generate a match for the velocity separation and intensity of the strongest peaks seen in the MoM-BH*-1 H\u03b2 profile.<\/p>\n If scattering is underway, this has some important implications for the physics of the situation. The observed width of Balmer lines (especially when fitted as an absorption system to a broad Gaussian line; that is, the most common baseline model9<\/a>,21<\/a>,72<\/a><\/sup>) may not trace the kinematics of the broad-line region but are a consequence of resonant scattering. Therefore, SMBH masses in these systems that are based on line widths of the Balmer lines may be severely overestimated by up to 2\u2009dex (Extended Data Table 4<\/a>). More sophisticated treatments of resonant scattering effects in LRDs73<\/a><\/sup> differ in assumptions and details, but make the same basic point.<\/p>\n Given the unique conditions (for example, unusual gas density) in this source, we must exercise care in applying the local scaling relations typically used to derive SMBH properties. Here we explore implications for a variety of approaches to back out the SMBH properties that are summarized in Extended Data Table 4<\/a>. Given the significant systematic uncertainties, these calculations must be seen as order-of-magnitude estimates to bracket the range of possibilities.<\/p>\n First, we simply apply H\u03b2-based local scaling relations39<\/a><\/sup> to the absorption-corrected and dust-corrected H\u03b2 line. The absorption correction is typically done by emission line fitting assuming an underlying Gaussian or Lorentzian9<\/a>,21<\/a><\/sup> (as in Extended Data Fig. 1<\/a>). For the dust correction, note that the H\u03b1 and H\u03b2 line ratio13<\/a><\/sup> is challenging to use given that their observed fluxes reflect radiative transfer in the dense gas. The standard approaches used in the literature at the moment infer a significant A<\/i>V<\/sub> of a few magnitudes either from continuum slope fitting or through SED modeling6<\/a>,10<\/a>,12<\/a>,13<\/a><\/sup>. Going by the optical continuum slope10<\/a><\/sup> would imply an A<\/i>V<\/sub>\u00a0\u2248\u00a02 for MoM-BH*-1 implying a BH mass of about 108.3<\/sup>M<\/i>\u2299<\/sub>. This is comparable to the stellar mass of the host galaxy (<108.5<\/sup>M<\/i>\u2299<\/sub> at 95% confidence), that is, this is an apparently \u2018overmassive\u2019 BH relative to local scaling relations between host galaxy mass and BH mass13<\/a>,41<\/a><\/sup>.<\/p>\n However, a key insight from the SED of MoM-BH*-1 is that the A<\/i>V<\/sub> required to explain the LRDs is likely negligible as the SED is intrinsically UV-weak below the Balmer break. That is, the LRDs are red not because of obscuration from dust, but because of opacity from gas. Accounting for this, with A<\/i>V<\/sub>\u00a0=\u00a00, the SMBH mass derived now is about 5\u00a0\u00d7\u00a0107<\/sup>M<\/i>\u2299<\/sub> with a luminosity of approximately 15% the Eddington limit.<\/p>\n Next, we note that the observed line width (after correcting for absorbers) may not be faithfully tracing the broad-line region, violating the basic ansatz for scaling relations (Extended Data Figs. 4<\/a> and 5<\/a>). The underlying BH emission line profile in LRDs may be as complex as the one shown in Extended Data Fig. 2<\/a>, but is perhaps difficult to disentangle from strong emission from the host. The complex structure may signify radiative processes that are modifying the emission arising from the broad-line region. As an example of one such process, if H\u03b2 is undergoing resonant scattering through the atmosphere around the BH, then the intrinsic BLR width before scattering could be as low as around 600\u2009km\u2009s\u22121<\/sup> (Extended Data Fig. 5<\/a>). This would yield an SMBH mass of approximately 106<\/sup>M<\/i>\u2299<\/sub>.<\/p>\n Finally, we are able to constrain the BH properties using our Cloudy\u00a0model. The bolometric luminosity directly follows from integrating the SED shown in Extended Data Fig. 3<\/a> (about 1044.5<\/sup>\u2009erg\u2009s\u22121<\/sup>). We note that this is an order of magnitude lower than the value expected from local calibrations, further underscoring their inapplicability. We can convert this luminosity into a mass by observing that theoretical models of similar scenarios74<\/a>,75<\/a>,76<\/a><\/sup> predict close to Eddington or super-Eddington accretion to sustain these convective envelopes. Furthermore, the ubiquity of OI emission thought to arise from Ly\u03b2 fluorescence in LRDs21<\/a>,66<\/a>,77<\/a><\/sup> is thought to be a hallmark of super-Eddington accretion78<\/a><\/sup>. Inspired by these lines of evidence, requiring L<\/i>\/L<\/i>bol<\/sub>\u00a0\u2248\u00a01, therefore, allows us to estimate the BH mass as approximately 106.3<\/sup>M<\/i>\u2299<\/sub>. Correcting the potentially overestimated literature BH masses by about 10\u2212100\u00d7 would bring the typical M<\/i>BH<\/sub>\/M<\/i>\u22c6<\/sub> of JWST AGN (about 1\u201310%; refs.\u200979<\/a>,80<\/a>,81<\/a><\/sup>) closer to the ratio in the local Universe (0.01%; ref. 82<\/a><\/sup>).<\/p>\n To constrain the host mass, we can leverage the insight from clustering analyses that the rest-UV light in the typical LRD (FWHM\u2009=\u20091,000\u22122,000\u2009km\u2009s\u22121<\/sup>), on average, originates almost entirely from the host41<\/a><\/sup>. Of course, this is not true for all LRDs, particularly the most luminous sources with higher FWHM broad lines that display AGN signatures even in the rest-UV21<\/a><\/sup>, but so far they seem the exception. To construct an empirical M<\/i>\u22c6<\/sub>\u2212M<\/i>UV<\/sub> relation, we use the compilation of low-luminosity galaxies at z<\/i>\u00a0=\u00a03\u22127 from the All the Little Things (ALT) survey40<\/a><\/sup> in the Abell-2744 field. This is the largest spectroscopic sample of M<\/i>UV<\/sub>\u00a0<\u00a0\u221215 galaxies at these redshifts. The ALT stellar masses are derived with the Prospector<\/span> SED fitting code applied to 27 bands of NIRCam\u2009+\u2009HST photometry, including all JWST medium and broad bands83<\/a>,84<\/a><\/sup>. We estimate that a source with M<\/i>UV<\/sub> between \u221217.9 and \u221218.3 has a median stellar mass of (log ({M}_{star }\/{M}_{odot })=7.{4}_{-0.3}^{+0.6})<\/span> and a 95% upper limit of (log ({M}_{star }\/{M}_{odot }) < 8.5)<\/span>.<\/p>\n It is of particular interest to test for signs of variability in MoM-BH*-1 as an independent constraint on the physics of the source. In LRDs, the SED is a summation of the host and BH at all wavelengths to varying degrees, but in this case, the stark outshining of the host galaxy means an \u2018undiluted\u2019 variability signal may be stronger and more easily detected. One of the handful reports of variability in an LRD to date has been in a source with a large Balmer break implying a high BH fraction30<\/a><\/sup>.<\/p>\n Three sets of observations covering 3\u22125\u2009\u03bcm exist for MoM-BH*-1 separated by\u00a0\u2248\u00a060 days in the rest-frame (Extended Data Fig. 6<\/a>). It is certainly not ideal to test for variability across three different observing modes with distinct systematics. Nevertheless, it is notable that the source appears to have brightened by 30\u00a0\u00b1\u00a07% between the first epoch (NIRCam) and third epoch (NIRSpec prism, no post-processing renormalization to photometry). The prism spectrum suffers from slit losses, and yet this source appears brighter than expected from the NIRCam photometry. It is the only source across the 136 sources with high-SNR photometry (SN\u00a0>\u00a010) and spectra (50th<\/sup> percentile of SN\u00a0>\u00a05) observed in two masks as part of the MoM program that shows this degree of brightening. We also note that typically, the NIRSpec\/G395M flux is systematically \u00a0\u2248\u00a010\u201320% lower than the prism flux due to calibration uncertainties85<\/a><\/sup> \u2013 accounting for this, both the prism and NIRSpec fluxes for MoM-BH*-1 appear to be in agreement and \u00a0\u2248\u00a030% brighter than the NIRCam flux. The shape of the SED (and the depth of the Balmer break) is consistent across the NIRCam and prism data (for example, ({f}_{{rm{F410M}}}^{nu }\/{f}_{{rm{F356W}}}^{nu }=2.0pm 0.2)<\/span> in the prism vs. 2.2\u00a0\u00b1\u00a00.2 in NIRCam). The difference is in the absolute brightness.<\/p>\n Taken at face value, these measurements imply a \u22484\u03c3<\/i> detection of variability over a mere two months, marking MoM-BH*-1 as an excellent target for future monitoring campaigns. This may be independent evidence not only for the AGN nature of the source, but also for the volatile environment that exists around it.<\/p>\n<\/div>\n Mais Not\u00edcias<\/a><\/p>\n Home Page – In\u00edcio<\/a><\/p>\nEmission line fitting<\/h3>\n
Morphology<\/h3>\n
Cloudy\u00a0modelling<\/h3>\n
Key features of fiducial Cloudy\u00a0model: blanketed by gas, not by dust<\/h3>\n
Resonant Balmer scattering and insights from Ly\u03b1-like shell models<\/h3>\n
SMBH properties and ramifications for LRD parameters<\/h3>\n
Host galaxy properties<\/h3>\n
Hints of variability<\/h3>\n