{"id":"36fd17e4-a8a4-469e-9ba6-77deb656ba3a","arxiv_id":"2506.09962","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":7,"one_line_summary":"A 3D diagram combining [N II]/Hα, [O III]/Hβ, and velocity dispersion separates star formation, AGN, and shocks, and its application to NGC5728 reveals a star-forming ring, an AGN bicone, and a central fast-shock disk.","lead":"This paper proposes a theoretical three-dimensional diagram that uses two optical emission-line ratios plus gas velocity dispersion to separate star formation, active galactic nuclei, and shocks in galaxy spectra. The authors test it on the nearby galaxy NGC5728 and identify a star-forming ring, an AGN ionization bicone, and a shock-dominated region at the base of the AGN outflow.","discovery_kind":"new_method","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The shock separation rests on treating observed single-Gaussian σ_OIII as shock velocity; the NGC5728 fast-shock detection is vulnerable to beam smearing and multi-component kinematics, and the paper itself acknowledges projection effects without correcting them.","rationale":"Reader's verdict conditional is appropriate. I agree with the weakest assumption: the σ_OIII axis is the load-bearing discriminator. The paper is honest about the projection limitation but does not propagate it into the classification, and the main new result is fully exposed to it. Two independent IFU datasets seeing similar structures is genuine supporting evidence, but both are subject to the same single/multi-component ambiguity at the nucleus; seeing the fast-shock region in S7 at lower resolution can be explained either by a real compact shock or by greater PSF smoothing of the same AGN broad-line/outflow emission. The strongest independent anchors are the known AGN bicone and star-forming ring, which were already known from BPT and multiwavelength work; the shock maps are the novel part and are least anchored. I would not reject: the proposed diagram is a reasonable and useful diagnostic concept, the MAPPINGS grids are self-consistent, and the method may work once the σ axis is validated. The requested two-component refit would directly test the fast-shock identification without requiring new observations. Therefore the reader's CONDITIONAL verdict should stand unchanged; the condition should be an explicit validation of the σ axis (multi-component fits or simulated PSF/beam-smearing tests) before the shock maps are interpreted as physical detections.","tokens_in":28461,"tokens_out":8445,"duration_ms":107245,"concrete_test":"Re-fit the MUSE cube for all spaxels assigned ≥80% fast-shock contribution (Fig. 3, upper right) with a two-component Gaussian model for [O III] λ5007 and Hβ, after subtracting the MUSE line-spread function. If the narrow component alone has log([O III]/Hβ) below the adopted fast-shock threshold and σ<200 km/s, while the broad component is centered near the AGN systemic velocity and carries most of the σ excess, the fast-shock map is produced by single-Gaussian fitting of a multi-component AGN outflow. If instead the best-fit spectrum requires a broad component whose [O III]/Hβ, [N II]/Hα, and velocity width are jointly consistent with the fast-shock model grid, the detection survives. A useful cross-check is to repeat the same test on the S7/LZIFU multi-component fits, where the components are already separated.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central novelty is the σ_OIII axis. For HII and AGN, its value is fixed empirically from BPT-classified spaxels in the same galaxy (Sec. 4: σ_HII=100, σ_AGN=125 km/s); for shocks, σ is set equal to the model shock velocity under an upright viewing angle. The paper concedes projection can lower the observed σ and that the diagram is most effective for shocks moving toward the observer, yet no projection correction or inclination estimate is applied to the NGC5728 maps. The MUSE data were fitted with a single Gaussian per line (Sec. 2), so a nuclear spaxel with an unresolved AGN NLR plus outflow, or with beam-smeared [O III] from the bright bicone, can acquire σ>300 km/s without any fast shock being present. Step (v) then converts any spaxel with z>z3 and log([O III]/Hβ)>0.7 into a 100% fast-shock classification. The main new detection, the fast-shock region at the base of the bicone, is exactly where this degeneracy is strongest. In addition, the fast- and pure-shock grids are calibrated to the same galaxy: the precursor-to-shock Hβ ratios (0.7:0.3 and 0.1:0.9) and the 100% shock basis spaxels are chosen by visual inspection of the NGC5728 data (Secs. 3.3, 5). The Monte Carlo uncertainties in Step (vi) propagate only line-flux and σ measurement errors, not these calibration choices, so the quoted <5% errors do not cover the main systematic risk.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper proposes a three-dimensional diagnostic diagram for classifying ionizing sources in optical IFU data, with axes log([N II]/Hα), log([O III]/Hβ), and emission-line velocity dispersion. The diagram is populated with MAPPINGS-based theoretical grids for H II regions, AGN narrow-line regions, pure shocks, and fast shocks. Applied to VLT/MUSE and WiFeS/S7 observations of NGC 5728, the method produces a star-forming ring at ~1 kpc, an AGN bicone extending ~2-5 kpc, and a fast-shock-dominated region at the base of the bicone, along with maps of fractional contributions from each mechanism. The authors argue that the diagram simultaneously separates star formation, AGN, and shocks using only optical data.","tokens_in":28838,"tokens_out":3988,"duration_ms":50503,"significance":"If the method holds up, it would be a valuable addition to the IFU toolkit, potentially applicable to large surveys such as SAMI and MaNGA and to high-redshift JWST observations. The paper has real strengths: the model grids are built with a common, self-consistent set of MAPPINGS calculations; the application to two independent datasets (MUSE and S7) yields broadly consistent spatial structures; and the Monte Carlo uncertainty propagation is clearly described. However, the central claim that the z-axis separates fast shocks from AGN and H II regions rests on an assumption that the authors themselves qualify, and several calibration choices are made from the same galaxy to which the method is then applied. These issues are load-bearing for the quantitative classification, so the present version requires substantial revision.","major_comments":[{"comment":"","section":"Section 4, velocity dispersion axis"},{"comment":"","section":"Section 5, Steps (iv)-(vi)"},{"comment":"","section":"Sections 3.3 and 5, calibration of shock and basis grids"},{"comment":"","section":"Section 5, Step (v), fast/pure shock divider"}],"minor_comments":[{"comment":"","section":"Section 5, Step (ii)"},{"comment":"","section":"Section 5, Step (v)"},{"comment":"","section":"Figures 4 and 7"},{"comment":"","section":"Introduction"},{"comment":"","section":"Section 6 and Table 1"}],"recommendation":"major_revision","confidential_remarks":"The manuscript is within the scope of the journal and presents a potentially useful diagnostic tool. My main concern is that the central new detection—fast shocks at the base of the AGN bicone in NGC 5728—depends on an assumption about the velocity dispersion axis that the authors themselves acknowledge is not corrected for projection or beam-smearing effects, and on calibration choices made from the same galaxy. These issues are fixable with additional analysis, so I recommend major revision rather than rejection. I would also encourage the editor to request that the model grids and analysis code be made available as part of the revision, since the paper states only that the models \"will be shared upon request.\""},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Read this one. It is a serious attempt to solve a real problem—separating star formation, AGN, and shocks with optical IFU data alone—and it is not just a reheated version of D'Agostino or Johnston. The three axes are the BPT ratios plus [O III] velocity dispersion, and for the first time the HII, AGN, and shock loci are populated with self-consistent MAPPINGS v5.2 grids using the same atomic data and abundance scale. Applied to NGC5728 in two independent IFU datasets, the method recovers the known star-forming ring and AGN bicone, and it turns up a spatially resolved fast-shock-dominated region at the base of the bicone. That is a real, novel result, and the consistency between MUSE and WiFeS/S7 is genuinely encouraging. The paper is also transparent about most of its choices, which I appreciate.\n\nNow the soft spots. The third axis is not theoretical in the way the title promises. For HII and AGN, the z position is the average observed sigma of BPT-selected spaxels in the same galaxy (100 and 125 km/s here); for shocks, z is equated to the model shock velocity under an upright viewing angle. Projection can lower the observed sigma, and the paper says so, but no inclination correction is applied. The MUSE data are single-Gaussian fits, so beam smearing and unresolved multi-component [O III] in the nuclear region can inflate sigma and push spaxels into the shock classification region. The main new detection—fast shocks at the base of the bicone—is exactly where that degeneracy is strongest.\n\nSeveral steps are hand-set: the precursor-to-shock Hβ ratios (0.7:0.3 and 0.1:0.9) are chosen by visual inspection, the 100% basis spaxels are selected from the same data, and the y = 0.7 divider is ad hoc. The Monte Carlo uncertainties propagate only line-flux and sigma measurement errors, not these calibration choices, so the quoted <5% errors understate the real uncertainty. For a method paper, the lack of released code or fixed parameter tables is also a problem; the models are available only on request.\n\nI would push back on the idea that the whole method is circular. The HII and AGN grids on the BPT plane are genuinely theoretical and independent; what is empirical is the z-axis anchoring and the basis-spaxel selection. That distinction matters. But the quantitative classification is not yet robust enough to be used as a standard tool without validation.\n\nBottom line: the diagnostic concept is defensible and deserves referee time. I would send it to a serious referee and ask for a simulated or independently calibrated validation, a projection/beam-smearing treatment or at least an explicit quantitative caveat, and release of code and fixed parameters. With those, this could become a useful community tool.","headline":"A genuinely new and promising diagnostic idea, but the third axis is empirically anchored and the headline fast-shock detection sits exactly on the method's least-tested assumption.","tokens_in":29432,"tokens_out":3733,"would_cite":true,"duration_ms":42130,"reading_group":"yes","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"A theoretical three-dimensional diagram built from two optical line ratios and the [O III] velocity dispersion separates star formation, AGN, and shocks simultaneously in galaxy IFU data.","keywords":["3D diagnostic diagram","excitation mechanisms","star formation","active galactic nuclei","shocks","integral field spectroscopy","velocity dispersion","NGC 5728"],"falsifier":"Simulate or observe a fast shock whose propagation direction lies nearly in the plane of the sky, so its line-of-sight velocity dispersion is near zero; the 3D diagram would place those spaxels at low [O III] dispersion and classify them as HII or AGN, while an independent tracer such as a broad kinematic component or an X-ray temperature measurement would identify them as shocks, falsifying the projection assumption.","tokens_in":28204,"feed_emoji":"🔭","tokens_out":10726,"duration_ms":109170,"temperature":0.7,"pith_summary":"The paper proposes a three-dimensional diagnostic for classifying what excites the gas in a galaxy's optical spectra. Its axes are the line ratios [N II]/Hα and [O III]/Hβ together with the velocity dispersion of the [O III] line, and it places theoretical grids for HII regions, AGN photoionization, and radiative shocks in that space. The claim is that the three families occupy separated locations, so each spatial pixel (spaxel) of an integral-field spectrum can be assigned fractional contributions from star formation, AGN, fast shocks, and pure shocks, while also reading off gas metallicity, ionization parameter, pressure, and shock velocity. Applied to NGC 5728, the diagram recovers a star-forming ring, an AGN ionization bicone, and a fast-shock region at the bicone's base. This matters because most galaxies lack the X-ray and radio data usually needed to tell AGN from shocks, and previous optical methods either required special BPT sequences or left many galaxies ambiguous.","feed_headline":"Three-axis diagram separates star formation, AGN, and shocks","feed_subtitle":"Two optical IFU datasets of NGC 5728 reveal a star-forming ring, an AGN bicone, and a fast-shock base.","key_machinery":"The load-bearing object is the theoretical 3D diagram itself, with axes log([N II]/Hα), log([O III]/Hβ), and the [O III] velocity dispersion. The HII and AGN model grids come from photoionization calculations, while the shock grids come from self-consistent radiative shock and precursor models; because photoionization models do not predict kinematics, the velocity-dispersion coordinates of HII and AGN spaxels are set empirically from the observed average dispersion of spaxels selected by BPT classification, whereas for shocks the shock velocity is used as a proxy for dispersion assuming an upright viewing angle. The separation mechanism is geometric: each spaxel's fractional contribution is computed as the normalized inverse distance to four basis spaxels representing HII, AGN, fast shock, and pure shock, with special rules assigning 100 percent shock contribution to spaxels lying above the shock basis points. The same geometry lets the models' underlying physical parameters be read off from the nearest model grid lines.","core_discovery":"On its own terms, the paper claims that the space defined by [N II]/Hα, [O III]/Hβ, and the [O III] velocity dispersion, populated by theoretical model grids for HII regions, AGN narrow-line regions, and radiative shocks, separates the three excitation families well enough to classify every spaxel. All models are computed with the same atomic data, so differences in predicted line ratios come from the excitation mechanism rather than from inconsistent model assumptions. Applying the diagram to NGC 5728, the paper assigns each spaxel fractional contributions from HII, AGN, fast shocks, and pure shocks by inverse distance to four basis spaxels; the result is a roughly 1 kpc star-forming ring, an AGN bicone extending about 2 kpc from the nucleus, and a fast-shock-dominated structure at the bicone base that is a new detection for this galaxy, with pure shocks forming a surrounding shell. Because each spaxel lands near a model grid line, the same diagram yields gas metallicity, ionization parameter, and pressure for the separated regions.","pith_inferences":["If projection effects were corrected using the galaxy inclination, the same diagram could classify shocks moving across the line of sight rather than only shocks propagating toward the observer, which is the orientation where the current version is most effective.","Because the three model families share atomic data and abundance sets, the same construction could be transplanted to other line-ratio pairs or to rest-optical lines at high redshift, making the diagram a template for classifying excitation in regimes beyond the local universe.","The basis-point and inverse-distance recipe is a heuristic, not a fit; its accuracy could be tested directly by comparing the resulting shock fractions against independent tracers such as X-ray plasma temperatures, radio jet structures, or mid-infrared H2 line ratios in the same galaxies.","Applying the method to large IFU samples could produce a statistical census of how often fast shocks accompany AGN outflows, but beam smearing must be controlled at coarse resolution or the shock fractions will be systematically biased."],"forward_implications":["Galaxies observed only in optical light can be mapped into star-formation-dominated, AGN-dominated, and shock-dominated regions without X-ray or radio data, enlarging the sample for AGN and feedback studies.","Separating mechanisms before measuring emission-line fluxes changes derived quantities; for NGC 5728 the Hα-based star formation rate of the ring is estimated at 1.17 ± 0.05 solar masses per year from MUSE data and 0.57 ± 0.04 from the lower-resolution data, both lower than estimates that did not remove shock and AGN contributions.","IFU data at roughly 1 arcsec resolution reproduce the same excitation structures as higher-resolution data, so the method can be applied to large optical IFU surveys and, at lower physical resolution, to high-redshift space-based IFU observations.","For NGC 5728 the diagram identifies a fast-shock-dominated region at the base of the AGN bicone and a surrounding shell of pure shock emission, structures that were not visible in 2D BPT classification."],"supporting_citations":[{"why":"Built the first empirical 3D diagram for star formation, AGN, and shock separation, which this paper replaces with a theoretical grid.","marker":"D'Agostino et al. (2019a)"},{"why":"Proposed a multidimensional diagnostic diagram that still leaves many galaxies ambiguous between AGN, shocks, and beam smearing; this paper aims to overcome that limitation.","marker":"Johnston et al. (2023)"},{"why":"Supplies the HII region photoionization model grid used on the diagram and documents that shocks can mimic AGN on 2D optical diagrams.","marker":"Kewley et al. (2019b)"},{"why":"Supplies the AGN narrow-line region photoionization model grid used on the diagram.","marker":"Zhu et al. (2023)"},{"why":"Provides the self-consistent radiative shock and precursor models that define the shock grids.","marker":"Sutherland & Dopita (2017)"},{"why":"Provides the time-dependent shock and precursor model calculations used for the fast-shock grids.","marker":"Dopita & Sutherland (2017)"},{"why":"Gives the statistical [O III] velocity dispersions used to anchor HII and AGN spaxels on the vertical axis.","marker":"Joh et al. (2021)"},{"why":"Previous BPT-based separation of NGC 5728 from MUSE data provides the ring and bicone structures this paper compares with.","marker":"Durré & Mould (2018)"},{"why":"CO and X-ray observations confirm the star-forming ring and AGN bicone structures used to validate the separation.","marker":"Shin et al. (2019b)"},{"why":"Deep X-ray observations show shocked-gas emission in the bicone region, supporting the shock component identified by the diagram.","marker":"Falcao et al. (2023)"}],"fun_headline_variants":["3D diagram separates star formation, AGN, and shocks in galaxies","Theoretical 3D plot untangles galaxy excitation sources","3D excitation diagram maps NGC 5728's ring, bicone, and shock base","Separating starburst, AGN, and shock emission with a 3D diagram","3D diagram plus IFU data reveals galaxy excitation anatomy"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The load-bearing assumption is that a single measured [O III] velocity dispersion faithfully represents each excitation mechanism: the HII and AGN values are set empirically from selected spaxels, and for shocks the shock velocity is equated to the observed dispersion for shocks moving toward the observer, so any projection, beam smearing, or kinematic blending that changes the dispersion shifts spaxels vertically in the diagram and can turn a shock into an AGN or star-formation classification.","fun_headline_variants_meta":{"raw":{"variants":["3D diagram separates star formation, AGN, and shocks in galaxies","Theoretical 3D plot untangles galaxy excitation sources","3D excitation diagram maps NGC 5728's ring, bicone, and shock base","Separating starburst, AGN, and shock emission with a 3D diagram","3D diagram plus IFU data reveals galaxy excitation anatomy"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000941,"raw_usage":{"total_tokens":4102,"prompt_tokens":1103,"completion_tokens":2999,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":719,"completion_tokens_details":{"reasoning_tokens":2911}},"tokens_in":719,"tokens_out":2999,"duration_ms":22808,"temperature":1.0,"reasoning_tokens":2911,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-07T04:37:36.012107+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Simulate or observe a fast shock whose propagation direction lies nearly in the plane of the sky, so its line-of-sight velocity dispersion is near zero; the 3D diagram would place those spaxels at low [O III] dispersion and classify them as HII or AGN, while an independent tracer such as a broad kinematic component or an X-ray temperature measurement would identify them as shocks, falsifying the projection assumption.","supporting_citations":[{"cited_title":"2021, Publications of the Astronomical Society of Japan, 73, 1152, 10.1093/pasj/psab065","cited_arxiv_id":null,"evidence_quote":"Gives the statistical [O III] velocity dispersions used to anchor HII and AGN spaxels on the vertical axis."},{"cited_title":"Deep Chandra Observations of NGC 5728: Morphology and Spectral Properties of the Extended X-ray Emission","cited_arxiv_id":"2303.00789","evidence_quote":"Deep X-ray observations show shocked-gas emission in the bicone region, supporting the shock component identified by the diagram."}],"review_version":1}