{"id":"43201558-f1f3-4497-80d3-888611bd8b2f","arxiv_id":"2606.12851","paper_version":1,"verdict":"UNVERDICTED","confidence":"LOW","novelty_score":5.0,"correctness_risk":"unknown","formal_verification":"none","parameter_count":2,"one_line_summary":"Lenient heavy-seed models in BRAHMA simulations produce black hole merger rates above 100 per year and near-unity occupation fractions down to low-mass galaxies, while strict models yield only about 1 merger per year and occupation fractions below 10 percent for galaxies under 10^8 solar masses.","lead":"The paper runs new cosmological simulations called BRAHMA to compare lenient and strict models for forming heavy black hole seeds of 10,000 to 100,000 solar masses, including dynamical friction effects. Smart readers might care because the differing predictions for black hole merger rates and how many galaxies host black holes today offer testable ways to interpret JWST data and future LISA observations.","discovery_kind":"new_application","skeptic_critique":{"model":"grok-4.3","headline":"Subgrid dynamical friction implementation is the least-secured step linking seed prescriptions to predicted merger rates","rationale":"The reader's weakest_assumption directly identifies the subgrid modeling step that controls the quantitative outputs. Because the paper's claim is that future GW and occupation-fraction measurements will discriminate the models, the robustness of that discrimination rests on the DF treatment; the concrete_test isolates exactly that variable.","tokens_in":2021,"tokens_out":293,"duration_ms":9920,"concrete_test":"Recompute the z=0 merger rate and occupation fraction statistics after replacing the subgrid DF implementation with the analytic Chandrasekhar formula evaluated at the same resolution (or with the DF model from Tremmel et al. 2015); if the lenient-to-strict rate ratio drops below ∼10 the central observational discriminator weakens.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The headline distinction (≳100 yr^{-1} vs ∼1 yr^{-1} merger rates; near-unity vs ≲10% occupation fractions) is produced by the combination of lenient vs strict seed formation plus the subgrid DF model. The abstract states that growth is “dominated by seeding and mergers,” so any systematic bias in the DF drag force or merger criterion at unresolved scales directly scales the reported rates. No independent check against resolved high-z merger calculations or against an alternative DF formulation is described in the provided text.","agreement_with_reader":"agree"},"referee_report":{"model":"grok-4.3","summary":"The manuscript presents results from the BRAHMA cosmological simulations investigating the assembly of supermassive black holes from heavy seeds under different seed formation prescriptions coupled with a subgrid dynamical friction model. It reports that lenient seed models, allowing seed formation in halos with sufficient dense and metal-poor gas for ~10^4 and ~10^5 M_sun seeds, lead to high BH merger rates of ≳100 yr^{-1} and near-unity local occupation fractions even in low-mass galaxies (M_* ≲ 10^7 M_⊙). In contrast, a strict seed model with additional direct-collapse constraints yields merger rates of ~1 yr^{-1} and occupation fractions ≲10% for M_* ≲ 10^8 M_⊙. The models produce M_*-M_BH relations consistent with local observations for massive galaxies, with lenient models also producing overmassive BHs.","tokens_in":2138,"tokens_out":409,"duration_ms":14724,"significance":"If the results hold, they demonstrate that variations in seed prescriptions can lead to dramatically different predictions for observable quantities like merger rates and BH occupation fractions, providing potential constraints from JWST, LISA, and local universe observations. The work highlights the sensitivity of high-redshift BH assembly to seeding criteria.","major_comments":[{"comment":"The central claims regarding the distinction in merger rates (≳100 vs ~1 yr^{-1}) and occupation fractions (near-unity vs ≲10%) between lenient and strict models rely on the subgrid dynamical friction implementation and the specific seed formation criteria. The provided text reports simulation outputs but provides no details on validation, convergence tests, or error quantification for the subgrid DF model or the 'dense & metal-poor gas' thresholds, making the quantitative predictions model-dependent without independent checks against resolved high-z calculations.","section":"Abstract (subgrid dynamical friction model and seed criteria)"}],"minor_comments":[],"recommendation":"major_revision","confidential_remarks":null},"author_rebuttal":{"model":"grok-4.3","summary":"We thank the referee for their constructive comments on our manuscript. We address the major comment below.","responses":[{"response":"We agree that the manuscript would benefit from expanded details on the subgrid DF model and seed criteria. Section 2 describes the DF implementation (following the formulation validated in our prior BRAHMA papers) and the seed thresholds (motivated by dense, metal-poor gas conditions for direct-collapse scenarios). In the revision we will add an explicit paragraph summarizing the referenced validation and convergence tests from those works, plus a short discussion of threshold sensitivity. We will also note the model dependence of the quantitative rates more prominently. Direct new comparisons to fully resolved high-z calculations remain outside the scope of this large-volume study.","revision_made":"partial","referee_comment":"[Abstract (subgrid dynamical friction model and seed criteria)] The central claims regarding the distinction in merger rates (≳100 vs ~1 yr^{-1}) and occupation fractions (near-unity vs ≲10%) between lenient and strict models rely on the subgrid dynamical friction implementation and the specific seed formation criteria. The provided text reports simulation outputs but provides no details on validation, convergence tests, or error quantification for the subgrid DF model or the 'dense & metal-poor gas' thresholds, making the quantitative predictions model-dependent without independent checks against resolved high-z calculations."}],"tokens_in":1690,"tokens_out":291,"duration_ms":22498,"standing_objections":["Independent checks against resolved high-z calculations for the subgrid DF model and seed thresholds"]},"desk_editor":{"model":"grok-4.3","letter":"The main things to know are that lenient heavy-seed models in these BRAHMA runs produce BH merger rates of over 100 per year and near-unity occupation fractions down to low-mass galaxies, while the strict model gives rates around 1 per year and fractions below 10 percent for galaxies under 10^8 solar masses. These are presented as discriminants for LISA and local surveys.\n\nThe paper brings new simulation volumes and tracks the populations from high redshift to z=0 under the different prescriptions, including dynamical friction. It shows that the models can reproduce the local M star to M BH relation for galaxies above 10^9 solar masses, and that lenient seeding can create overmassive systems consistent with some JWST claims.\n\nThis is useful work in laying out concrete numbers from the simulations.\n\nThe soft spot is the reliance on the subgrid dynamical friction model. The abstract notes that growth is dominated by seeding and mergers, so the DF implementation directly influences the reported rates and fractions. The stress-test concern holds up here because there is no description of checks against resolved merger calculations or different DF formulations. The seed formation criteria are also specific choices that drive the differences.\n\nThis paper is aimed at the community working on black hole seeding mechanisms and their observational signatures. Readers interested in simulation predictions for high-redshift BHs and future gravitational wave constraints will find the quantitative outputs helpful.\n\nI would send it to peer review. The predictions are sharp enough to warrant referee input on the modeling assumptions.","headline":"Lenient heavy-seed models in the new BRAHMA runs produce merger rates above 100 per year and near-unity occupation fractions, versus roughly 1 per year and low fractions for the strict model, but the subgrid dynamical friction implementation is the main uncertainty.","tokens_in":2710,"tokens_out":408,"would_cite":true,"duration_ms":21807,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"grok-4.3","headline":"Lenient heavy black hole seed models in BRAHMA simulations produce merger rates above 100 per year and near-unity occupation fractions in galaxies down to 10 million solar masses, while strict models yield rates around 1 per year and fracti","keywords":["supermassive black holes","heavy seeds","dynamical friction","cosmological simulations","black hole mergers","occupation fraction","JWST","LISA"],"falsifier":"A measurement of the present-day black hole occupation fraction in galaxies with stellar mass near 10 million solar masses that is either near 100 percent or below 10 percent would distinguish the lenient from the strict seed models.","tokens_in":2932,"feed_emoji":"🌌","tokens_out":929,"duration_ms":21459,"temperature":0.7,"pith_summary":"The paper compares two lenient heavy-seed prescriptions, in which black hole seeds of 10,000 or 100,000 solar masses form in every halo containing sufficient dense and metal-poor gas, against a strict prescription that adds further direct-collapse constraints. These prescriptions are implemented inside the BRAHMA cosmological simulations together with a subgrid dynamical friction model that governs how seeds sink and merge. By redshift 5 the lenient cases produce overmassive black holes whose luminosities overlap with those inferred for JWST sources, while by the present day they also generate high merger rates and high occupation fractions even in low-mass galaxies. The differences imply that measurements of merger rates and local occupation fractions can distinguish the dominant seeding pathways.","feed_headline":"Black hole seed models predict merger rates of 1 versus 100 per year","feed_subtitle":"Lenient seeding in BRAHMA simulations produces near-unity occupation fractions in dwarf galaxies while strict models do not.","key_machinery":"The subgrid dynamical friction model combined with lenient versus strict criteria for forming 10,000 and 100,000 solar-mass seeds inside halos that contain sufficient dense and metal-poor gas, run inside the BRAHMA cosmological simulations.","core_discovery":"In the BRAHMA simulations, lenient seed models in which all halos with sufficient dense and metal-poor gas form 10,000 and 100,000 solar-mass seeds generate by z approximately 5 multiple systems with black-hole-to-stellar-mass ratios at or above 0.01 that reach luminosities of 10^43 to 10^45 erg per second, while at z equals 0 they produce merger rates of at least 100 per year and occupation fractions near unity even for galaxies with stellar masses below 10 million solar masses; the strict seed model, which forms 100,000 solar-mass seeds only under additional direct-collapse conditions, instead produces merger rates of only about 1 per year and occupation fractions below 10 percent for gala","pith_inferences":["If observations favor the lenient models, then mergers contribute a larger fraction of black hole growth at high redshift than they do under strict seeding.","The overmassive black holes produced at redshift 5 in lenient models suggest that seeding alone, without requiring sustained super-Eddington accretion, can account for some JWST detections.","High occupation fractions in present-day dwarf galaxies would favor lenient seeding even if the strict model matches other observables.","The efficiency of the dynamical friction model in driving seed mergers sets the scale of the predicted rate differences between the two classes of models."],"forward_implications":["Lenient production of 100,000 solar-mass seeds produces multiple overmassive systems with black-hole-to-stellar-mass ratios at or above 0.01 inside galaxies that host 100 million to 1 billion solar-mass black holes.","All three seed models produce black-hole-to-stellar-mass relations broadly consistent with the local universe for galaxies above 1 billion solar masses by redshift 5.","The lenient scenarios also generate systems near the upper envelope of the observed local scatter in the black-hole-to-stellar-mass relation.","Future gravitational-wave event rates and measurements of local black hole occupation fractions will constrain the dominant pathways for high-redshift black hole assembly."],"fun_headline_variants":["Lenient seeds drive 100 black hole mergers per year","Strict models yield only 1 merger yearly","Near unity BH occupation in dwarfs via lenient seeds","Early overmassive BH systems in lenient BRAHMA runs"],"cache_read_input_tokens":2112,"weakest_assumption_plain":"The subgrid dynamical friction model and the specific criteria defining sufficient dense and metal-poor gas for lenient seeding versus additional direct-collapse constraints for strict seeding accurately represent unresolved high-redshift astrophysical processes.","fun_headline_variants_meta":{"raw":{"variants":["Lenient seeds drive 100 black hole mergers per year","Strict models yield only 1 merger yearly","Near unity BH occupation in dwarfs via lenient seeds","Early overmassive BH systems in lenient BRAHMA runs"]},"model":"grok-4.3","cost_usd":0.006375,"raw_usage":{"total_tokens":3154,"prompt_tokens":993,"num_sources_used":0,"completion_tokens":61,"cost_in_usd_ticks":63749500,"prompt_tokens_details":{"text_tokens":993,"audio_tokens":0,"image_tokens":0,"cached_tokens":256},"completion_tokens_details":{"audio_tokens":0,"reasoning_tokens":2100,"accepted_prediction_tokens":0,"rejected_prediction_tokens":0}},"tokens_in":993,"tokens_out":61,"duration_ms":12764,"temperature":1.0,"reasoning_tokens":2100,"cache_read_input_tokens":256,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-06-27T06:53:19.054182+00:00","model_set":{"reader":"grok-4.3"},"falsifier":"A measurement of the present-day black hole occupation fraction in galaxies with stellar mass near 10 million solar masses that is either near 100 percent or below 10 percent would distinguish the lenient from the strict seed models.","supporting_citations":[],"review_version":1}