{"id":"7f83c0d8-54f4-4012-9a5a-442d32b26763","arxiv_id":"2411.16611","paper_version":2,"verdict":"ACCEPT","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":5,"one_line_summary":"MaNGA LINERs show suprasolar nitrogen-to-oxygen ratios that are insensitive to the assumed ionizing source, while oxygen abundances shift with the assumed AGN, old-star, or ADAF scenario.","lead":"This paper measures oxygen and nitrogen in the centers of 105 LINER galaxies using photoionization models for three possible ionizing sources. It finds nitrogen-to-oxygen ratios are robust to the assumed source and slightly above solar, while oxygen abundances depend on the source.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The suprasolar N/O result rests on pure-photoionization grids; the explicit omission of fast radiative shocks (Sec. 5.3) leaves an untested mechanism that can mimic LINER line ratios and bias N/O high.","rationale":"The paper is careful, and much of the comparative result is internally robust: Appendix A.2 shows one-to-one agreement among N/O estimates from different SED families, and Fig. 9 shows that nuclear N/O matches the extrapolation from spatially resolved HII-region gradients, supporting the claim that N/O is less SED-sensitive than O/H. These checks, however, do not address the absolute N/O scale because all grids share the same assumption of pure photoionization (density 500 cm^-3, standard dust, no shocks). The manuscript explicitly acknowledges only in Sec. 5.3 that shocks are omitted; shocks are nonetheless a canonical LINER mechanism and can boost [N ii]/Hβ at fixed N/O, so a pure-photoionization fit could overestimate log(N/O). The suprasolar conclusion hinges on the difference between the derived median (−0.69/−0.79) and the adopted solar value (−0.86); the systematic offset from an omitted shock contribution could plausibly be comparable to or larger than this 0.1–0.2 dex gap. The proposed shock-grid test would settle this directly. If the test were to leave the median unchanged, the central claim would stand; if not, the conclusion should be softened to 'suprasolar only under pure-photoionization excitation.' I therefore recommend a conditional acceptance pending this test or a correspondingly qualified statement.","tokens_in":40932,"tokens_out":15974,"duration_ms":149315,"concrete_test":"Re-fit the 105 central spaxels with a composite grid adding a fast radiative shock+precursor component (e.g., Allen et al. 2008 MAPPINGS III), letting shock velocity (100–500 km/s) and shock fraction vary. Compare the new median log(N/O) and best-fit shock fractions with Table 3. If the median drops by more than ~0.1 dex toward or below the solar value (−0.86), or shocks dominate the objects driving the suprasolar tail, the suprasolar conclusion is an artifact of omitting shocks; if the shock fraction is negligible and the median stays ≈ −0.7, the concern is settled.","verdict_should_be":"CONDITIONAL","load_bearing_attack":"Shocks are one of the three canonical LINER excitation mechanisms listed in the Introduction (Dopita & Sutherland 1995; Allen et al. 2008), yet all grids in Sec. 3.2 are pure Cloudy photoionization models, and Sec. 5.3 explicitly omits shocks 'due to the large number of free parameters required to model this emission.' For fast shocks in LINER-like conditions, the hot post-shock gas and the photoionized precursor produce [N ii]/Hβ and [O i]/Hα ratios that mimic photoionization but with little sensitivity to the actual gas-phase N/O; a pure-photoionization fit will therefore tend to absorb the extra [N ii] into an overestimated log(N/O). The central claim has two parts: N/O is nearly independent of the assumed SED family (supported by internal consistency, Fig. A.2) and is suprasolar (log(N/O) ≈ −0.7). The first part only compares three pure-photoionization SED families, so it does not establish robustness against the shock mechanism; the second part depends on the absolute N/O scale, which the shock omission directly threatens. The internal consistency of Fig. 9 (nuclear N/O matches the HII-region gradient extrapolation) argues against a large bias, but it is not a substitute for a model test because both nuclear grids share the same pure-photoionization assumption.","agreement_with_reader":"partial"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript presents a systematic analysis of nuclear oxygen and nitrogen abundances in 105 LINERs from the MaNGA survey. The authors construct photoionization model grids for three ionizing-source scenarios (AGN with varying α_OX, pAGB stars with three effective temperatures, and ADAF) and use the HII-CHI-Mistry code to derive 12+log(O/H), log(N/O), and log(U) from optical line ratios. They find that oxygen abundances depend on the assumed source (median 12+log(O/H) = 8.69 for AGN, 8.53 for pAGB), whereas the derived log(N/O) is nearly independent of the assumed source, with median values around -0.69 to -0.79, i.e., suprasolar. They further examine the N/O-O/H relation, the mass-metallicity and mass-N/O relations, and compare nuclear estimates with extrapolations from HII-region gradients. The paper concludes that the pAGB scenario requires a complex inflow/outflow picture, whereas the AGN scenario is consistent with standard enrichment, and it acknowledges that shocks are not modeled.","tokens_in":10,"tokens_out":6627,"duration_ms":117719,"significance":"If the central result holds, the paper makes a valuable contribution: it provides the first large-sample, spatially resolved abundance analysis of LINER nuclei across multiple ionizing-source scenarios, and it identifies log(N/O) as a comparatively robust diagnostic. The analysis is reproducible in design: it uses the public HII-CHI-Mistry code, clearly specifies the Cloudy grids, and provides full tables at CDS. The internal consistency check against HII-region gradients (Fig. 9) is a useful falsifiable cross-check. The claim that N/O is largely independent of the assumed SED family is well supported among the pure-photoionization grids (Fig. A.2). However, the absolute N/O scale and the inference of suprasolar values rest on the assumption that fast radiative shocks, a canonical LINER mechanism, do not contribute significantly; this assumption is stated but not tested.","major_comments":[{"comment":"The omission of fast radiative shocks is load-bearing for the central claim. The Introduction identifies shocks (Dopita & Sutherland 1995; Allen et al. 2008) as one of the three canonical LINER ionization mechanisms, but all grids in Sec. 3.2 are pure Cloudy photoionization models, and Sec. 5.3 states that shocks were omitted \"due to the large number of free parameters required.\" In fast shocks, the hot post-shock gas and the photoionized precursor produce [N ii]/Hβ and [O i]/Hα ratios that mimic photoionization with little sensitivity to the gas-phase N/O; a pure-photoionization fit may therefore absorb the excess [N ii] into an overestimated log(N/O). The comparison in Fig. 9 with HII-region gradients does not resolve this concern, because the HII-region abundances are derived from the same pure-photoionization methodology. I request a quantitative test: for a representative subset, fit grids that include shock+precursor components (e.g., MAPPINGS) or use literature shock models to estimate the potential bias on the median log(N/O). Without such a test, the statement \"the nitrogen-to-oxygen ratio we derived is much less affected by the assumptions on the ionizing source\" (Abstract) is established only within the restricted space of pure-photoionization SEDs, not across the full range of proposed LINER mechanisms.","section":"Sec. 5.3, Sec. 3.2"},{"comment":"The description of the ionization parameter range is internally inconsistent, and the imposed prior may affect the results. The text states that log(U) \"varied in the range -4.0 < log(U) < -0.5 in steps of 0.25 dex,\" but then says the parameter was \"preliminarily constrained in the range -4.0 < log(U) < -2.5 based on previous findings.\" If the grid actually covers only -4.0 to -2.5, the sentence should be corrected; if the full -4.0 to -0.5 grid is computed but subsequent inference is restricted to -2.5, the paper should justify that the truncation does not bias the derived log(N/O), since log(U) and log(N/O) can be degenerate in the fitted line ratios. Please clarify which range was used and report the sensitivity of the median abundances to this choice.","section":"Sec. 3.2"}],"minor_comments":[{"comment":"The text says the preliminary sample contains 429 galaxies, but the \"Total\" row of Table 1 sums to 428 (329 + 48 + 51). Please reconcile this discrepancy.","section":"Table 1, Sec. 2.2"},{"comment":"The caption says \"The solid back line represents the fit\"; this should be \"solid black line.\"","section":"Fig. 5 caption"},{"comment":"The phrase \"all these stars ionize the gas nowadays today\" is awkward; suggest \"these stars ionize the gas at the present day.\"","section":"Sec. 5.3, after Eq. (2)"},{"comment":"The line \"Tables B1–B5B.5 are available\" contains a typo; it should read \"Tables B.1–B.5.\"","section":"Data availability"},{"comment":"The total number of models is given as 54,208, which equals 4,928 × 11; it would be helpful to state explicitly that this corresponds to 7 AGN SEDs + 3 pAGB temperatures + 1 ADAF.","section":"Sec. 3.2"},{"comment":"The exclusion of pAGB models with Teff = 5×10^4 K from the discussion (Sec. 5) should be stated earlier, since Table 3 shows these models yield the highest median log(N/O) (-0.58), which would otherwise appear to contradict the robustness claim.","section":"Sec. 4.1 and Sec. 5"}],"recommendation":"major_revision","confidential_remarks":"The paper is within scope for A&A. The main risk is not circularity but unmodeled physics; the authors' openness about the shock omission is commendable, but it makes the absolute N/O scale less secure than the abstract suggests. If the authors can add a shock test or at least a quantitative bound on the shock contribution, the central claim would be substantially strengthened."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"The thing to know: this paper gives observers a practical result—log(N/O) in LINER nuclei is nearly invariant across AGN, pAGB, and ADAF photoionization scenarios, while 12+log(O/H) shifts with the assumed SED. The medians are suprasolar N/O (-0.69 to -0.79) and solar-ish O/H (8.69 for AGN, 8.53 for pAGB). That claim is the load-bearing result, and it is mostly supported by the internal comparisons in Fig. A.2 and the agreement with HII-region gradient extrapolations in Fig. 9.\n\nWhat the paper does well: the sample is decent (105 MaNGA LINERs), the grids are comparable across three SED families with consistent assumptions, and the authors are transparent about their priors, the restricted ionization parameter range, and the post-hoc exclusion of low-T pAGB models. They also make a sensible physical argument against pAGB as the dominant ionizer by counting the required stars. The public code and full data tables are a plus.\n\nSoft spots, in proportion: the biggest one is also the one the authors openly flag. Shocks are a canonical LINER mechanism, but all grids are pure Cloudy photoionization, and fast radiative shocks can produce LINER-like [NII] and [OI] ratios while carrying little sensitivity to true gas-phase N/O. So the \"robustness\" result is really robustness among three pure-photoionization SED families, not against the shock mechanism. The suprasolar N/O scale could in principle absorb shock-enhanced [NII]. The Fig. 9 consistency check is reassuring, but as the authors use the same photoionization machinery for both nuclear and HII region estimates, it is not an independent test. This is a limitation of scope, not a fatal flaw, because the paper says so explicitly and frames its conclusions accordingly.\n\nMinor: the fixed density of 500 cm^-3, standard dust, and single-phase gas are assumptions typical for this kind of work, but they do affect the absolute scale. The AGN alpha_OX=-1.2 model behaving differently in the N/O-O/H correlation is a curiosity that could use a brief physical explanation.\n\nBottom line: this is a solid subfield paper. It is worth a serious referee, and I would cite it if I worked on LINER abundances. My own verdict is close to the reader's ACCEPT, though I would push the authors in revision to add a shock-grid test or at least an explicit paragraph on how shocks might alter the N/O scale. That would turn a good paper into a definitive one.","headline":"A careful, useful paper on LINER abundances whose central N/O robustness claim is solid among photoionization models but is not yet tested against the shock mechanism.","tokens_in":41783,"tokens_out":1503,"would_cite":true,"duration_ms":17725,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"The nitrogen-to-oxygen ratio of LINER nuclear gas is nearly independent of the assumed ionizing source and is suprasolar, while oxygen abundances shift with that assumption.","keywords":["LINERs","chemical abundances","nitrogen-to-oxygen ratio","oxygen abundance","photoionization models","MaNGA survey","active galactic nuclei","post-AGB stars"],"falsifier":"Measure auroral lines such as $[\\mathrm{O\\,III}]\\,\\lambda4363$ or $[\\mathrm{N\\,II}]\\,\\lambda5755$ in a sample of LINER nuclei; a direct-method $\\log(\\mathrm{N/O})$ that comes out solar or subsolar would contradict the suprasolar model-based values, while agreement would confirm them. Alternatively, split the sample by X-ray detection: if X-ray-bright and X-ray-quiet LINERs give the same $\\log(\\mathrm{N/O})$, the ratio is genuinely source-independent, and if they differ, the apparent invariance is a grid artifact.","tokens_in":40753,"feed_emoji":"🌌","tokens_out":7641,"duration_ms":69520,"temperature":0.7,"pith_summary":"The paper aims to establish that the chemical enrichment of LINER nuclei can be traced reliably by the nitrogen-to-oxygen ratio even though the ionizing source of LINERs is debated. For 105 MaNGA galaxies with LINER-like central spaxels, the authors fit optical emission lines with photoionization model grids built for three competing ionizing sources: standard AGN spectra, hot post-AGB stars, and advection-dominated accretion flows. The derived oxygen abundance depends strongly on the assumed source, with median $12+\\log(\\mathrm{O/H}) = 8.69$ (solar) for AGN models and $8.53$ for pAGB models. The derived $\\log(\\mathrm{N/O})$ barely moves, clustering around $-0.69$ to $-0.79$, slightly above the solar ratio of $-0.86$. If correct, this makes N/O a robust abundance tracer for the most common type of active galaxy in the local Universe.","feed_headline":"Nitrogen-rich LINER nuclei keep their ratio under every ionizing model","feed_subtitle":"Oxygen estimates swing with the assumed light source, but N/O stays slightly above solar—a stable enrichment tracer.","key_machinery":"The load-bearing machinery is the Bayesian-like code HII-CHI-Mistry, which estimates $12+\\log(\\mathrm{O/H})$, $\\log(\\mathrm{N/O})$, and the ionization parameter $\\log(U)$ by comparing observed line ratios $[\\mathrm{O\\,II}]/\\mathrm{H}\\beta$, $[\\mathrm{Ne\\,III}]/\\mathrm{H}\\beta$, $[\\mathrm{O\\,III}]/\\mathrm{H}\\beta$, $[\\mathrm{N\\,II}]/\\mathrm{H}\\beta$, and $[\\mathrm{S\\,II}]/\\mathrm{H}\\beta$ against precomputed grids of photoionization models. For each ionizing source the authors build 4,928 Cloudy models spanning $6.9<12+\\log(\\mathrm{O/H})<9.1$, $-2.0<\\log(\\mathrm{N/O})<0.0$, and $-4.0<\\log(U)<-0.5$, at fixed density $500\\,\\mathrm{cm^{-3}}$, standard dust, and two stopping criteria. The grids differ only in the assumed spectral energy distribution: AGN power laws with $\\alpha_{\\mathrm{OX}}$ from $-0.8$ to $-2.0$, post-AGB atmospheres at $T_{\\mathrm{eff}}=5\\times10^4$, $1\\times10^5$, and $1.5\\times10^5$ K, and an ADAF average SED. Because N/O is largely set by the $[\\mathrm{N\\,II}]/[\\mathrm{O\\,II}]$ ratio while O/H is tied to the harder ionization stages, the N/O output survives the grid choice and the O/H output does not.","core_discovery":"The central discovery is that the nitrogen-to-oxygen abundance ratio in LINER nuclear gas is insensitive to the assumed ionizing source and is suprasolar, whereas the oxygen abundance is not. Across AGN grids with seven spectral slopes, post-AGB grids at three effective temperatures, and an ADAF grid, the median $\\log(\\mathrm{N/O})$ stays in the range roughly $-0.79$ to $-0.69$, while median $12+\\log(\\mathrm{O/H})$ moves from about $8.69$ under AGN models to $8.53$ under pAGB models. The paper further reports that under pAGB ionization the N/O versus O/H relation becomes a strong anticorrelation, with Pearson coefficients around $-0.62$ to $-0.71$, which would require inflows or outflows to explain, while the AGN assumption keeps LINERs inside the scatter of the standard enrichment relation. A pAGB-only scenario also requires about $3.2\\times10^4$ ionizing post-AGB stars within 1 kpc, a number the authors argue is hard to reconcile with observed globular cluster counts. The authors conclude that AGN activity is the chemically conservative interpretation, with old stellar populations possibly contributing but not uniquely powering the emission.","pith_inferences":["An implication the authors leave implicit is that the gradient comparison is testable: if nuclear N/O matches the radial extrapolation while O/H does not, the central gas may have been nitrogen-enriched without corresponding oxygen enrichment, for example by secondary nitrogen production in intermediate-mass stars or by selective removal of oxygen-rich gas.","The N/O invariance across SED families could be used to calibrate cheap empirical nitrogen-based metallicity indicators for LINERs, since existing N-based calibrations inherit the O/H ambiguity this paper documents.","Adding shock-dominated photoionization models to the same Bayesian machinery would test whether the pAGB anticorrelation is an artifact of the shock contribution that all grids explicitly omit.","A natural next observational check is to compare X-ray-detected and X-ray-quiet LINERs: if their derived $\\log(\\mathrm{N/O})$ values agree, the source-independence is genuine, and if not, the apparent invariance is a grid artifact."],"forward_implications":["If the paper is right, $\\log(\\mathrm{N/O})$ can be used as an ionizing-source-independent chemical enrichment indicator for LINERs, removing one major systematic from studies of the most common local active galaxies.","Oxygen abundances of LINER nuclei should be reported with the assumed ionizing source made explicit, since the same line ratios yield solar medians under AGN SEDs and subsolar medians under pAGB SEDs.","Under the AGN interpretation, LINER nuclei follow the standard N/O versus O/H enrichment relation, so no exotic inflow or outflow is required to explain their chemistry.","Under the pAGB interpretation, a strong N/O versus O/H anticorrelation appears, implying gas flows that scale with chemical abundance or some other non-standard enrichment channel.","The nuclear $\\log(\\mathrm{N/O})$ estimates agree with values extrapolated from each galaxy's radial gradient, while nuclear $12+\\log(\\mathrm{O/H})$ does not, reinforcing N/O as the more trustworthy nuclear tracer."],"supporting_citations":[{"why":"Supplies the HII-CHI-Mistry Bayesian fitting code that maps line ratios to abundances.","marker":"Pérez-Montero (2014)"},{"why":"Extends HII-CHI-Mistry to AGN ionizing sources and provides the AGN-grid methodology reused here.","marker":"Pérez-Montero et al. (2019)"},{"why":"Previous LINER abundance analysis with AGN models that this paper extends to multiple SED families.","marker":"Pérez-Díaz et al. (2021)"},{"why":"Independent pAGB-model study of LINER N/O that this paper's suprasolar N/O result agrees with.","marker":"Oliveira et al. (2024b)"},{"why":"Cloudy v17 is the photoionization code used to compute all 4,928-model grids.","marker":"Ferland et al. (2017)"},{"why":"Provides the NLTE post-AGB stellar atmosphere SEDs used for the pAGB grids.","marker":"Rauch (2003)"},{"why":"Supplies the average ADAF spectral energy distribution used for the inefficient-accretion grid.","marker":"Nemmen et al. (2014)"},{"why":"Sets the solar reference values defining solar versus suprasolar O/H and N/O.","marker":"Asplund et al. (2009)"}],"fun_headline_variants":["LINER nitrogen ratios hold steady across all ionizing models","N/O in LINERs stays solar+ no matter the light source","Oxygen varies, nitrogen solid: LINER abundance key","Suprasolar N/O survives every LINER ionizing scenario","Chemical enrichment in LINERs: N/O is the reliable tracer"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The load-bearing premise is that each family of photoionization models maps the observed five line ratios to one unique set of gas-phase abundances, with fixed density, standard dust, no shocks, and a single ionizing SED per grid; if the real gas is multiphase or shock-excited, the medians shift.","fun_headline_variants_meta":{"raw":{"variants":["LINER nitrogen ratios hold steady across all ionizing models","N/O in LINERs stays solar+ no matter the light source","Oxygen varies, nitrogen solid: LINER abundance key","Suprasolar N/O survives every LINER ionizing scenario","Chemical enrichment in LINERs: N/O is the reliable tracer"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000191,"raw_usage":{"total_tokens":1461,"prompt_tokens":1178,"completion_tokens":283,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":794,"completion_tokens_details":{"reasoning_tokens":195}},"tokens_in":794,"tokens_out":283,"duration_ms":3438,"temperature":1.0,"reasoning_tokens":195,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-12T12:55:56.281830+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Measure auroral lines such as $[\\mathrm{O\\,III}]\\,\\lambda4363$ or $[\\mathrm{N\\,II}]\\,\\lambda5755$ in a sample of LINER nuclei; a direct-method $\\log(\\mathrm{N/O})$ that comes out solar or subsolar would contradict the suprasolar model-based values, while agreement would confirm them. Alternatively, split the sample by X-ray detection: if X-ray-bright and X-ray-quiet LINERs give the same $\\log(\\mathrm{N/O})$, the ratio is genuinely source-independent, and if they differ, the apparent invariance is a grid artifact.","supporting_citations":[{"cited_title":"J., Chatzikos, M., Guzmán, F., et al","cited_arxiv_id":null,"evidence_quote":"Cloudy v17 is the photoionization code used to compute all 4,928-model grids."},{"cited_title":"S., Storchi-Bergmann, T., & Eracleous, M","cited_arxiv_id":null,"evidence_quote":"Supplies the average ADAF spectral energy distribution used for the inefficient-accretion grid."}],"review_version":1}