Black Hole Seeds and Little Red Dots: Connecting Formation and Growth with a JWST Multi-Cycle Treasury Program

abstract · abstract

JWST has revealed an abundant population of faint active galactic nuclei and a new class of compact, high-redshift sources known as Little Red Dots (LRDs), whose physical nature remains debated, with accreting massive black holes among the leading explanations. The emerging demographics of early black holes, with their high inferred number densities and, in some systems, the apparent excess of black hole mass relative to stellar mass, challenge simple scenarios of black hole seeding and growth. Despite these observational breakthroughs, the origin of early massive black holes remains uncertain, because "black holes have no memory": current samples cannot separate the imprint of seeding from subsequent growth. This white paper identifies the multi-cycle JWST observations needed to revolutionize our understanding of early black hole growth by: (i) distinguishing seeding pathways, (ii) constraining early black hole growth mechanisms, (iii) resolving the physical nature of broad-line emitters and LRDs, and (iv) testing black hole-galaxy co-evolution. These tasks require uniformly selected samples extending to low black hole and host-galaxy masses, allowing self-consistent measurements of black hole mass functions, Eddington-ratio distributions, and host properties. Grating spectroscopy, deep MIRI imaging, and variability analysis through repeated observations of the same fields would provide complementary tests of black hole formation and growth, their connection to host-galaxy evolution, and the nature of newly discovered populations of LRDs. In this white paper, we argue that these joint tests require a coordinated JWST Multi-Cycle Treasury program; such a program could provide the depth, uniformity, and time baseline needed to place substantially stronger, complementary constraints on early black hole formation, growth, and their connection to galaxy evolution.

Sx2.SSx5: Executive summary / block 1 · html

The four science cases detailed above converge on one requirement: obtaining black hole masses, accretion diagnostics, and host-galaxy properties for the same sources, drawn from a uniformly selected parent population and revisited across multiple cycles. No combination of independently conceived and executed GO programs can provide these data. With such a uniform sample, the community could make significant advances in understanding the nature of the LRDs, while the Eddington-ratio distribution would show how the first massive BHs grew, and the M ∙ M_{\bullet} – M ⋆ M_{\star} relation would rest on a well-characterized population. The high- z z community would then have the data to measure black hole growth over the first billion years in a complete, systematic way.

Sx2.SSx2: Early black hole growth and accretion physics / block 1 · html

Growth at λ Edd ≲ 1 \lambda_{\rm Edd}\lesssim 1 shifts the black hole mass function smoothly in mass with time, whereas episodic super-Eddington growth produces, at any epoch, a majority of dormant BHs and a minority caught in a burst [ 77 , 93 , 85 , 91 , 104 ] . For example, the 4 × 10 8 ​ M ⊙ 4\times 10^{8}\,{\rm M_{\odot}} black hole at z = 6.68 z=6.68 accreting at 2 % 2\% of the Eddington rate [ 38 ] is the first clear example of what several studies define as the dormant majority of high-redshift BHs. The Eddington-ratio distribution, and its link to electromagnetic properties, is also an important prediction. Super-Eddington flows are expected to be X-ray weak [ 73 , 48 , 59 ] and, if the inflow is chaotic, to keep the spin low [ 40 ] , thus lowering ϵ \epsilon and accelerating growth further. Theoretical models also differ in their predictions for the faint-end AGN luminosity function and on how the active fraction depends on host mass [ 46 , 10 ] .

Sx2.SSx3: The physical nature of Little Red Dots / block 1 · html

Broad Balmer lines (FWHM ∼ 1000 \sim 1000 – 4000 4000 km s -1 ) in many spectroscopically confirmed LRDs suggest accreting BHs [ 62 , 28 ] , yet their X-ray weakness and debated hot-dust signatures challenge conventional AGN interpretations [ 103 , 4 , 59 , 11 ] .

Sx2.SSx3: The physical nature of Little Red Dots / block 1 · html

Remarkably, LRD demographics provide independent constraints. LRDs occur in roughly 1 % 1\% of galaxies and are most common at z ≈ 4.5 − 8 z\approx 4.5-8 [ 43 , 72 ] . Their abundance is the product of two unknowns: the duration of the LRD phase and the fraction of galaxies that go through it. With LRDs at ∼ 1 % \sim 1\% of galaxies over the ∼ 700 \sim 700 Myr between z = 8 z=8 and z = 4.5 z=4.5 , a supermassive-star phase of ∼ 3 \sim 3 Myr [ 102 ] would have to recur ≈ 2 − 3 \approx 2-3 times in every galaxy; a ∼ 20 \sim 20 Myr quasi-star phase [ 6 ] would need one galaxy in three; and ∼ 230 \sim 230 Myr of Eddington-limited growth, only a few percent of them. The apparent cutoff of the LRD luminosity function near L bol ∼ 10 45 L_{\rm bol}\sim 10^{45} erg s -1 further constrains black hole or envelope masses [ 53 ] .

Sx2.SSx4: Black hole–galaxy co-evolution / block 1 · html

In the local Universe, BHs weigh ≈ 0.1 % \approx 0.1\% of their host’s stellar mass [ 45 , 83 ] . At z = 4 z=4 – 7 7 , some JWST-detected broad-line AGN appear to lie 10 10 – 100 100 times above that relation, with offsets exceeding 3 ​ σ 3\sigma in some analyses [ 75 ] . Remarkably, the offset grows with redshift [ 29 ] and, in some sources, the black hole may outweigh the stars [ 24 , 39 ] .

Sx1: Scientific Justification / block 1 · html

JWST discoveries have challenged our understanding of how the first black holes formed, grew, and began co-evolving with their host galaxies. For example, the detection of broad Balmer lines with full-width at half maximum (FWHM) of ≳ 1000 ​ kms − 1 \gtrsim 1000\,\rm kms^{-1} revealed accreting black holes of ≲ 10 6 − 10 8 ​ M ⊙ \lesssim 10^{6}-10^{8}\textrm{M}_{\odot} at z = 4 − 7 z=4-7 , hosted by galaxies with stellar masses of at most 10 8 − 10 10 ​ M ⊙ 10^{8}-10^{10}\textrm{M}_{\odot} [ 44 , 31 , 60 , 90 ] . In addition, the Little Red Dots (LRDs), compact sources with V-shaped spectral energy distributions and broad lines, outnumber UV-selected quasars of comparable luminosity by 1 − 2 1-2 orders of magnitude, and they represent > 1 % >1\% of all bright sources at high redshift [ 62 , 43 , 2 ] . Remarkably, in several of these newly uncovered systems, the black hole mass seems to exceed the local M ∙ − M ⋆ M_{\bullet}-M_{\star} relation by factors of ∼ 10 − 100 \sim 10-100 [ 75 , 37 , 36 , 29 ] . The current black hole frontier sits at z ≈ 10.6 z\approx 10.6 [ 60 ] , less than 500 500 Myr after the Big Bang, where GN-z11 may host a ∼ 10 6 ​ M ⊙ \sim 10^{6}\textrm{M}_{\odot} black hole.

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Black Hole Seeds and Little Red Dots: Connecting Formation and Growth with a JWST Multi-Cycle Treasury Program

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F. Pacucci 1,† , R. Schneider 2,3,† , H. Katz 4 , B. Robertson 5 , R. Somerville 6 , Abdurro’uf 7 , K. Aditya 8 , J. Afonso 9,10 , J. Arita 11 , G. Barro 12 , A. Basu 13 , R. Begley 14 , S. Bose 15 , M. Bradač 16,17 , G. Bryan 18 , C. Cain 19,20,21 , R. Caleno 2,3,22 , A. Cameron 23,24 , S. Carniani 25 , H. Choi 26 , N. Choustikov 19,20 , E. Curtis-Lake 27 , A. D’Angelo 3,28,2,22 , R. Dave 29 , R. Davies 30 , K. Davis 31,32 , C. Donnan 33 , Q. Duan 19 , Y. Dubois 34 , A. Faisst 35 , M. Farcy 36 , S. Finkelstein 37,38 , S. Gillman 39 , L. Graziani 2 , A. Gupta 37,38 , T. Gutcke 40 , M. Haehnelt 20,19 , N. Hathi 41 , E. Hatziminaoglou 42,43 , K. Heintz 39 , M. Hirschmann 36 , L. Ivey 19 , K. Iyer 40 , J. Jeon 37,38 , B. Jones 44 , I. Juodžbalis 45 , A. Koekemoer 46 , C. Kragh Jespersen 47 , D. Langeroodi 19 , A. Lapi 48 , M. Latif 49 , Y. Liu 15 , C. Lovell 19 , K. Nagamine 50 , P. Natarajan 51,52,53 , Z. Pan 54 , R. Pascalau 19 , P. G. Pérez-González 55 , E. Pizzati 1 , E. Regos 56 , P. Rinaldi 37,38 , D. Roberts 57 , D. Schleicher 2 , F. Shankar 57 , X. Shen 1 , T. Stanton 29 , G. Sun 58 , Y. Sun 59 , A. Trinca 3 , J. Trump 31 , J. Turner 60 , H. Übler 61 , R. Valiante 3 , F. van Donkelaar 20,19 , A. Vijayan 60 , F. Vito 62 , T. Wang 63 , S. Wilkins 60 , J. Wise 64 , J. Witstok 23,24 , Z. Wu 1 , L. Yung 65 , T. Zana 2,3,22 , H. Zhang 66 , S. Zhang 38 , M. Zhuang 54

Sx2.SSx1: The origin of massive black hole seeds / block 1 · html

These pathways lead to different seed properties, especially regarding their initial mass function, occupation fractions, and host-galaxy environments. However, subsequent accretion and mergers can partially erase the initial signatures of the seed population. Semi-analytical models and cosmological simulations, including CAT, DELPHI, BRAHMA, SEEDZ, and AESOPICA, have explored how different seeding prescriptions translate into the evolving BH and host-galaxy properties [ 95 , 93 , 92 , 20 , 9 , 8 , 46 , 81 , 63 ] .

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Our results demonstrate that heavy-seed formation is a natural outcome of early structure formation in overdense regions, with profound implications for the observed abundance of early quasars and their gravitational-wave signatures. The overmassive BHs produced in these simulations represent plausible progenitors of the first quasars, bridging the gap between the birth of seed BHs and the luminous active nuclei observed by JWST.

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The LW background is primarily contributed by Pop II stars, and its amplitude depends on assumptions regarding the IMF and the stellar models adopted. We note that uncertainties in the LW-intensity modeling have only a minor impact on our results, as we later test explicitly. A halo begins to form Pop III stars once its mass exceeds the critical threshold determined by the local LW intensity ( Kulkarni et al., 2021 ) . Under strong LW irradiation, star formation is delayed until the halo virial temperature reaches ∼ 8000 \sim 8000 K, at which point Ly α \alpha cooling triggers rapid collapse. Such halos typically experience high mass accretion rates and host more massive Pop III stars, extending to supermassive stars (SMSs) with M ∗ ∼ 10 5 ​ M ⊙ M_{\ast}\sim 10^{5}~M_{\odot} , which subsequently collapse into heavy black-hole (BH) seeds. Indeed, Ishiyama and Hirano (2025) identified more than ∼ 10 4 \sim 10^{4} SMSs with the masses larger than 10 5 ​ M ⊙ 10^{5}~M_{\odot} as heavy seed candidates in their simulation volume.

S4: 4 Summary / block 1 · html

We have conducted a systematic search for low-redshift analogues to the high-redshift LRD population within the DESI DR1 spectroscopic survey. Our search identified five objects at 0.2 < z < 0.4 0.2<z<0.4 that share the defining characteristics of the LRD. Our findings are summarized below.

S4: 4 Summary / block 1 · html

Unlike high-redshift LRDs, our low-redshift analogues are unambiguously classified as AGN by BPT/VO diagnostics. This suggests they reside in more chemically mature, relatively metal-rich host galaxies.

S0.SSx3: Signatures of opacity-driven pulsation / block 1 · html

Projecting the illustrative f-mode light curve model into the T eff T_{\rm eff} - L BB L_{\rm BB} space with blackbody model visualizes the system’s potential temporal sequence in the pulsation scenario (Materials and Methods). The resulting trajectory in Figure 3 (B) shows a counterclockwise loop, resembling that of classical pulsating stars like Cepheids. In this pattern, the envelope is hotter (and smaller) during the brightening phase than the dimming phase at a given luminosity, a consequence of the hysteresis between temperature and radius variations [ 21 , 6 ] (see also Figure S9 ). Such a lag arises from finite thermal inertia: when compressed, the gas heats faster than it can radiate, and when expanding, it cools more slowly, mimicking a heat engine in which heating and cooling are out of phase with mechanical motion. Consistent with this picture, the expansion velocity inferred ( 132 ​ km ​ s − 1 132\,{\rm km\,s^{-1}} from R2211-RX1.3 to R2211-RX1.1) is larger than the contraction velocity ( 61 ​ km ​ s − 1 61\,{\rm km\,s^{-1}} from R2211-RX1.1 to R2211-RX1.2), as expected for opacity-driven pulsations. These velocities are obtained by dividing the change in R ph R_{\rm ph} between the epochs by the corresponding rest-frame time intervals. While we caution that this trajectory is a model-dependent inference, the data’s natural accommodation of such a phase lag lends qualitative support to the stellar-like envelope hypothesis.

Sx2.SSx1.SSSx3: Construction of Empirical LRD Templates / block 1 · html

These twelve empirical templates, together with the global median composite are shown in Figure S5 (B). They provide a flexible basis set for EAZY z phot z_{\mathrm{phot}} estimation, allowing the continuum slope and Balmer break strength to vary smoothly. The resulting library effectively captures the observed diversity of LRDs across UV-optical colors while maintaining realistic spectral shapes at all wavelengths. The library is available at https://github.com/Zijian-astro/LRD-template-Zhang25 .

Sx1: Figures and Tables / block 1 · html

Figure 2 : JWST NIRCam F444W slitless spectra of the Forges . We also show the best-fit results and associated residuals. Prominent emission features (e.g., He i , O i λ ​ 11287 \lambda 11287 , Pa γ \gamma , Pa δ \delta ) are labeled. Short vertical lines at the top, labeled with line names, mark the theoretical positions of the emission lines. For Forge II , A zoom-in on the Pa δ \delta absorption feature is shown in the subpanel, and additional spectroscopic coverage from DESI (covering the wavelength ranges of Ly α \alpha , N v , O i λ ​ 1304 \lambda 1304 , C iv , He ii , and C iii] ) and HST G141 (Mg ii ) is also displayed. The gray-shaded regions indicate the wavelength ranges masked during the fitting process due to contamination from neighboring sources in the NIRCam spectra.

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We estimate the central BH masses M BH M_{\mathrm{BH}} and bolometric luminosities L bol L_{\rm bol} of the Forges using the spectral and photometric data above. We first model their continuum and line emission and absorption features in the JWST slitless spectra (Methods), and the best-fit results are listed in Extended Data Table 1 . The full widths at half maximum (FWHM) of the emission lines are about several thousand km s -1 , confirming their AGN nature. In addition, the strong O i λ ​ 11287 \lambda 11287 emission agrees with the predicted signature of high-accretion SMBHs 29 . Assuming that these lines originate from the AGN broad-line regions, we calculate BH mass using the broad Paschen emission lines and the 1 μ \mu m continuum luminosity 30 . The resultant masses are ( 3.72 ± 1.86 ) × 10 8 ​ M ⊙ (3.72\pm 1.86)\times 10^{8}\,M_{\odot} and ( 9.78 ± 4.89 ) × 10 8 ​ M ⊙ (9.78\pm 4.89)\times 10^{8}\,M_{\odot} ( M ⊙ M_{\odot} is the Solar mass), respectively. Forge II was also observed by the DESI spectroscopic survey and HST G141 slitless spectroscopy. Only narrow Ly α \alpha emission line (FWHM = 556 ​ km ​ s − 1 =556\rm\penalty\ km\penalty\ \,s^{-1} ) is detected by DESI, shown in the lower panel of Figure 2 . Canonical AGN broad lines, including Ly α \alpha , N v λ ​ 1240 \lambda 1240 , C iv λ ​ 1549 \lambda 1549 , He ii λ ​ 1640 \lambda 1640 , and C iii] λ ​ 1909 \lambda 1909 , are not detected, which is consistent with previous results that high-redshift LRDs have weak or no broad UV emission lines 7 , 28 . We assume that the central AGNs are obscured in the rest-frame UV, but dominate in the near-IR, radio, and X-ray bands, then we obtain AGN bolometric luminosities L bol L_{\rm bol} by integrating their reprocessed SEDs (Extended Data Figure 3 ). The inferred Eddington ratios are L bol / L Edd = 0.17 − 0.08 + 0.25 L_{\rm bol}/L_{\rm Edd}=0.17^{+0.25}_{-0.08} and 0.24 − 0.12 + 0.42 0.24^{+0.42}_{-0.12} , where the Eddington luminosity is L Edd = 1.3 × 10 38 ​ ( M BH / M ⊙ ) ​ erg ​ s − 1 L_{\rm{Edd}}=1.3\times 10^{38}\ (M_{\rm{BH}}/M_{\odot})\ \rm{erg\ s^{-1}} .

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One of the most mysterious results from observations of the James Webb Space Telescope (JWST) is the detection of numerous, high-redshift, very red, extremely compact, broad-line sources termed “little red dots” (LRDs). It is unclear whether the LRDs belong to an active galactic nucleus (AGN) or simply a collection of very compact star clusters. We build spectral energy distributions (SEDs) for 29 LRDs at z ≈ 3 − 8.5 z\approx 3-8.5 based on JWST photometric and spectroscopic observations. We find that the V-shaped SEDs of these LRDs exhibit a roughly similar break frequency at ν b = 10 14.96 ± 0.06 \nu_{\rm b}=10^{14.96\pm 0.06} Hz, which corresponds to λ b = 3287 − 424 + 487 ​ Å \lambda_{\rm b}=3287_{-424}^{+487}\mathrm{\AA} in the rest frame. We propose that this unique SED can be explained by the combination of an inner standard disk and an outer gravitationally unstable accretion disk with Toomre parameter Q ∼ 1 Q\sim 1 . The outer disk has a temperature of ∼ 2000 − 4000 \sim 2000-4000 K for typical AGN parameters, which can well reproduce the near-infrared to optical bump as observed in LRDs. This model can naturally explain the strong infrared to optical emission and the V-shaped SED with a similar break frequency ≃ 10 15 \simeq 10^{15} Hz for LRDs without invoking strong dust extinction or unusual stellar contribution from a host galaxy. Most LRDs stay in sub-Eddington state based on the SED modeling, which are intrinsically weak in X-rays.

S1: 1 Introduction / block 1 · html

James Webb Space Telescope (JWST) is opening up a new era in the study of the early Universe, thanks to its remarkable sensitivity and angular resolution at infrared wavelength [ 1 ] . One of the most surprising results from JWST is the discovery of numerous faint, optically red and extremely compact sources ( R e ≈ 100 R_{\mathrm{e}}\approx 100 pc) [ 2 , 3 , 4 ] . These sources are referred to as “Little Red Dots” [ 5 , 6 , 7 , 3 ] , which are widely considered as candidates for high-redshift active galactic nuclei (AGNs) based on their broad emission lines, and can play an important role in understanding the evolution of galaxies and supermassive black holes at cosmic dawn [ 8 , 6 , 7 , 9 , 10 , 11 ] . The LRDs exhibit spectral energy distributions (SEDs) characterized by a “V-shaped” continuum with a luminous red component and a faint blue component in the rest-frame spectrum, which is notably different from that of local AGNs [ 5 , 7 , 3 ] . Furthermore, the LRDs are less variable and are X-ray weaker compared to classical AGNs [ 12 , 13 , 14 ] . Interpreting the LRD SEDs remains difficult, which prevents us from further understanding the possible SMBH growth or galaxy evolution in high redshift [ 15 , 16 ] . The common AGN-only or galaxy-only models cannot adequately describe the two distinct spectral components in V-shaped SEDs. For the AGN-only scenario, the red/optical component is dust-reddened AGNs due to the dust in the torus and/or interstellar medium, ( 17 , 8 , e.g.,) . It has also been proposed that the LRDs may be powered by the pure dusty starburst galaxies [ 18 , 19 , 20 , 21 ] . Alternatively, some studies also consider the co-existence of AGNs and galaxies: either a galaxy dominates the rest-frame UV SED while a reddened AGN contributes in the rest-frame infrared to optical range; or the AGN contributes significantly to the rest-frame UV while a dusty stellar population contributes in the rest-frame UV SED [ 17 , 18 , 21 ] . A fraction of LRDs exhibit a clear Balmer break in their spectra, which indicates either an old stellar population [ 22 , 23 ] or extremely dense gas surrounding an AGN [ 24 , 25 , 26 ] . Most of these models predict sign

S10: 10 Summary and conclusions / block 1 · html

After cross-matching with existing AGN catalogues and considering the SED-fitting results with CIGALE , we find that (at least) 9.4% of the V-shaped-SED selected sources at z > 4 z>4 could host an AGN. However, interestingly, most of the classically selected AGN via X-ray surveys and bright 24 ​ µ ​ m 24\,\rm$\mathrm{\SIUnitSymbolMicro m}$ sources at z > 4 z>4 do not comply with the double power-law SED criteria, but rather constitute a disjoint population. This is consistent with the fact that LRDs are X-ray weak, and that in general most AGN do not have colours or β \beta values consistent with LRDs.

S10: 10 Summary and conclusions / block 1 · html

We have studied the properties of a population of Euclid /IRAC-selected, V-shaped-SED galaxies at z > 4 z>4 in ∼ 0.75 ​ deg 2 \sim 0.75\,\rm deg^{2} of the COSMOS field. Our sample contains a total of 16 and 233 sources, for two different cuts in the rest-frame optical SED power-law slopes, namely β red > 0 \beta_{\rm red}>0 and β red > − 1 \beta_{\rm red}>-1 , respectively. Out of these sources, a total of 16 satisfy also having a compactness 1 ​ σ 1\sigma above the median compactness of all z > 4 z>4 galaxies, as defined from their Euclid H E H_{\scriptscriptstyle\rm E} -band images. We consider these 16 sources to be the most robust Euclid LRD/LBD candidates. We note that imposing compactness on the H E H_{\scriptscriptstyle\rm E} band is stricter than imposing it at longer wavelengths, as compactness typically increases with wavelength, so our selection is very conservative.

abstract · abstract

The multi-cycle JWST Treasury program NEXUS will obtain cadenced imaging and spectroscopic observations around the North Ecliptic Pole during 2024-2028. Here we report a systematic search for nuclear variability among $\sim 25\,$k sources covered by NIRCam (F200W+F444W) imaging using the first two NEXUS epochs separated by 9 months in the observed frame. Difference imaging techniques reach $1σ$ variability sensitivity of 0.18~mag (F200W) and 0.15~mag (F444W) at 28th magnitude (within 0".2 diameter aperture), improved to $0.01$~mag and $0.02$~mag at $<25$th magnitude, demonstrating the superb performance of NIRCam photometry. The difference imaging results represent significant improvement over aperture photometry on individual epochs (by $>30\%$). We identify 465 high-confidence variable sources among the parent sample, with 2-epoch flux difference at $>3σ$ from the fiducial variability sensitivity. Essentially all these variable sources are of extragalactic origin based on preliminary photometric classifications, and follow a similar photometric redshift distribution as the parent sample up to $z_{\rm phot}>10$. While the majority of these variability candidates are likely normal unobscured AGNs, some of them may be rare nuclear stellar transients and tidal disruption events that await confirmation with spectroscopy and continued photometric monitoring. We also constrain the photometric variability of ten spectroscopically confirmed broad-line Little Red Dots (LRDs) at $3\lesssim z \lesssim 7$, and find none of them show detectable variability in either band. We derive stringent $3σ$ upper limits on the F444W variability of $\sim 3-10\%$ for these LRDs, with a median value of $\sim 5\%$. These constraints imply weak variability in the rest-frame optical continuum of LRDs.

S6: 6 Summary and conclusions / block 1 · html

Mock NIRCam observations reveal extended rest-frame UV structures, including host galaxy emission, merging companions, and stellar clumps, surrounding a compact red core. This morphology arises because the host remains visible in the UV while the central AGN dominates at longer wavelengths. These features closely resemble those observed in a substantial fraction of LRDs, with extended rest-UV emission reported in ∼ \sim 40% of the sample ( Rinaldi et al., 2025 ; Baggen et al., 2026a ) .

S6: 6 Summary and conclusions / block 1 · html

The simulated LRDs reproduce the UV-bright portion of the observed population ( − 22.5 < M UV < − 19.5 -22.5<M_{\rm UV}<-19.5 ) and exhibit moderate V-band attenuation ( A V < 3.5 A_{\rm V}<3.5 ). Their stellar populations produce moderate Balmer breaks, F 4000 / F 3600 ∼ 1 F_{4000}/F_{3600}\sim 1 – 2 2 , consistent with approximately 40 40 – 50 % 50\% of observed LRDs. The strongest observed discontinuities require additional ingredients, such as more evolved stellar populations or absorption and reprocessing by dense excited neutral hydrogen.