{"id":"a0c9be91-7939-4789-9e7a-4331aba45205","arxiv_id":"2507.01549","paper_version":1,"verdict":"ACCEPT","confidence":"MODERATE","novelty_score":5.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":0,"one_line_summary":"Three of five type-1 AGN observed with ALMA follow the dense gas to star formation scaling relation, and optical and sub-millimetre excitation classifications agree where all lines are detected.","lead":"Using new ALMA observations, this paper detects dense molecular gas in three of five nearby bright AGN and asks whether the gas still follows the normal star formation scaling relation. The three detections do follow it, and in the one galaxy with all needed line detections, the sub-millimetre and optical AGN diagnostics agree.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Beam dilution of compact AGN-excited HCN undermines the sub-mm/optical agreement as evidence for AGN-excited dense gas; the paper's central interpretation is unsupported but explicitly speculative.","rationale":"The reader's weakest-assumption analysis correctly identifies beam dilution as the key limitation. Our stress test confirms that this is the most load-bearing concern for the paper's central interpretive claim: the sub-mm/optical 'agreement' in HE1108-2813 and the Gao–Solomon consistency in three sources are both consistent with the alternative interpretation that the sub-mm diagnostics only trace large-scale star-forming dense gas, with any AGN-excited component diluted below significance. The paper is transparent about this: Section 5 cites Viti et al. (2014), Audibert et al. (2021), and explicitly states that the ALMA beams 'may be too large to separate out the AGN HCN excitation with the AGN impact being diluted.' The conclusions also label the interpretation as 'speculative.' Thus the concern lands, but it is already disclosed and does not contradict the paper's stated claims, which are appropriately hedged. The data, stacking analysis, and the one complete sub-mm diagram are still useful observational contributions, and the raw data are public. We therefore see no reason to change the reader's ACCEPT verdict. A concrete high-resolution follow-up or re-imaging test would determine whether the beam-dilution concern actually suppresses an AGN-excited component in these specific sources.","tokens_in":18096,"tokens_out":9303,"duration_ms":105577,"concrete_test":"Use the ALMA visibility data for HE0433-1028 and HE1108-2813 to re-image the HCN(4-3) and HCO+(4-3) lines with a smaller synthesized beam, for example via uniform weighting or a shorter uv-taper, recovering the highest resolution allowed by the uv coverage. If the HCN/HCO+ (and HCN/CS, where detectable) ratio moves toward the AGN region of the sub-mm HCN diagram as the beam shrinks, the low-resolution starburst classification is an artifact of beam dilution, and the claimed sub-mm/optical agreement does not constrain AGN excitation of dense gas. If the ratios remain starburst-like at the smaller beam, the dilution concern is alleviated for these targets. Alternatively, archival or new high-resolution (~0.1 arcsec) ALMA observations of the same lines in the central r<200 pc would settle the question directly.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central interpretive claim—that AGN can contribute to the excitation of both the warm ionised gas and the dense molecular gas—rests on the sub-mm HCN diagram classifications. However, the ALMA beams (373–633 pc) are much larger than the r<200 pc scale on which AGN-enhanced HCN excitation has been found (Viti et al. 2014; Audibert et al. 2021). Since the HCN(4-3), HCO+(4-3) and CS(7-6) fluxes are measured within these large beams, any compact AGN-excited component is mixed with and diluted by circumnuclear star-forming dense gas. Consequently, the observed starburst-like HCN/HCO+ and HCN/CS ratios in all three sources (including the only completely measured source, HE1108-2813) do not test whether the AGN contributes to dense-gas excitation. The same dilution applies to the L_IR–L_HCN comparison: agreement with the Gao–Solomon relation only shows that total dense-gas luminosity tracks star formation on large scales. The paper itself notes this limitation in Section 5, so the data presentation is sound, but the conclusion that AGN can simultaneously excite warm ionised and dense molecular gas is not actually supported by the sub-mm data; it is a speculation consistent with the upper limits. Because this is the central claim of the abstract, the concern is load-bearing even though the authors hedge it.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper presents new ALMA Band 7 observations of HCN(4-3), HCO+(4-3), and CS(7-6) for five Type-1 AGN from the CARS survey, together with MUSE-based BPT classifications. The authors detect dense gas in three of five targets, find that these three follow the Tan et al. (2018) L_IR-L_HCN relation (with p-values 0.32 and 0.51 from a one-sample t-test), and place the three sources on the Izumi et al. (2016a) sub-mm HCN diagram, where all fall in the starburst region. For the one source with all three lines detected (HE1108-2813), the sub-mm classification agrees with the composite/star-forming BPT classification. The paper concludes that AGN may contribute to the excitation of both warm ionised and dense molecular gas, but explicitly labels this interpretation as speculative and notes the small sample size.","tokens_in":18337,"tokens_out":6719,"duration_ms":73600,"significance":"The paper provides new, useful ALMA observations of dense gas tracers in nearby AGN and a transparent comparison of optical and sub-mm diagnostics. Its strengths include a careful stacking analysis, explicit treatment of upper limits, and an honest discussion of limitations, including beam-dilution effects. The sample size is very small (three detections, one with a complete sub-mm measurement), but the paper is appropriately cautious in its language and does not overstate the statistical support. If the results hold, they suggest that on large scales AGN do not produce a measurable enhancement of dense-gas excitation, and that the sub-mm HCN diagram can be consistent with optical BPT classifications, providing a useful pilot result for future surveys.","major_comments":[{"comment":"The abstract's concluding claim that 'AGN can contribute to the excitation of both the low density gas in the warm ionised medium and the high density gas in molecular clouds simultaneously' is not directly tested by the sub-mm data because the ALMA beams (373-633 pc; Section 2.1.2) are much larger than the r<200 pc region where AGN-enhanced HCN excitation has been found (Viti et al. 2014). Although Section 5 explicitly acknowledges this beam-dilution issue, the abstract presents the interpretation without this caveat. Please add a sentence to the abstract stating that the sub-mm/optical agreement is based on one source with complete detections and that the current resolution cannot rule out a compact AGN-excited component.","section":"Abstract and Section 5"},{"comment":"The statement 'Where it was possible to complete the analysis we find general agreement between optical and sub-mm classified gas excitation mechanisms' is based on a single galaxy, HE1108-2813, which has all three sub-mm lines detected. For HE0433-1028 and HE1029-1831, only lower limits on HCN/CS are available and the sub-mm diagram cannot discriminate between AGN and starburst excitation. The paper should explicitly say that the agreement is established for one source only, rather than 'general agreement' that could be read as applying to the sample as a whole.","section":"Abstract and Section 4.2.2"}],"minor_comments":[{"comment":"There is a typo: 'calibrated using the the Common Astronomy Software Applications' should read 'calibrated using the Common Astronomy Software Applications'.","section":"Section 2.1"},{"comment":"In the table notes, the galaxy names 'HE0433-1831' and 'HE1108-1813' are misspelled and should be 'HE0433-1028' and 'HE1108-2813'.","section":"Table 1"},{"comment":"The caption is confusing: it states the CARS targets are 'plotted in green and red, as labelled by the colour bar', but the colour bar represents BPT classification, not a green/red dichotomy. Please clarify how points are colour-coded.","section":"Figure 8 caption"},{"comment":"The improvement in signal-to-noise from ∼3.7 to ∼4.1 after stacking is modest; please specify the S/N of each detection before and after stacking to help the reader assess the benefit of the stacking procedure.","section":"Section 3.2"},{"comment":"The paper uses 'sub-mm HCN diagram' and 'submm-HCN diagram' inconsistently; please use a single spelling.","section":"Throughout"}],"recommendation":"minor_revision","confidential_remarks":"This is a carefully written small-sample study with transparent limitations. The statistics are weak, but the authors are appropriately cautious and the data are novel. The main issue is the abstract's phrasing, which slightly overreaches relative to the single complete sub-mm measurement and the acknowledged beam dilution. A minor revision can fix this. The paper fits MNRAS well."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Worth a look if you work on dense gas tracers in AGN. The genuinely new thing here is the ALMA data: HCN(4-3), HCO+(4-3) and CS(7-6) in five CARS type-1 AGN, with detections in three of five and a stacking pipeline that pulls out a faint CS detection in one source. The Gao–Solomon comparison is reasonable, the residual test against Tan et al. is appropriate, and the line ratios are placed on the Izumi sub-mm diagram in a way that makes the small-sample limitations visible rather than burying them. Credit where due: the paper ships real public-archive observations and is unusually candid about what it cannot say.\n\nThe soft spots are exactly the ones the authors disclose. The ALMA beams (373–633 pc) are much larger than the sub-kiloparsec scales where AGN-enhanced HCN excitation has been resolved (Viti et al.; Audibert et al.), so the sub-mm HCN diagram is probing mostly large-scale, star-formation-dominated dense gas. That means the sub-mm/optical 'agreement' in the one complete case, HE1108-2813, is consistent with AGN exciting both the warm ionised and dense molecular gas, but it is not a strong test of that idea. The paper says this in Section 5, so the hedging is adequate. The abstract's final sentence, though, slightly overstates the support: the data suggest, they do not demonstrate, that AGN can simultaneously excite both phases.\n\nStatistically it is a small sample: three HCN/HCO+ detections, one CS detection, and two sources placeable only as lower limits. The infrared luminosities are derived from SFRs via Kennicutt, which carries its own systematic uncertainty. None of this is hidden, and none of it breaks the stated claims once you read the hedging carefully. There is no circular reasoning: everything is compared to fixed external calibrations.\n\nWho is this for? Someone assembling reference samples for sub-mm HCN diagnostics, or checking the dense-gas–star-formation relation around luminous AGN. It is a modest data point, not a breakthrough. I would send it to peer review: a competent referee will check the flux calibration and the stacking, and the discussion will likely be tightened, but the core data and the honest analysis deserve publication.\n\nRecommendation: accept with minor revision, keeping the speculative interpretation clearly labelled.","headline":"Honest small-sample ALMA study with new dense-gas detections in five CARS AGN; the central interpretation is speculative but the paper explicitly says so.","tokens_in":18996,"tokens_out":1851,"would_cite":false,"duration_ms":25443,"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 paper claims that three nearby AGN follow the dense-gas/star-formation scaling relation despite the AGN, and that the sub-mm and optical classification diagrams agree in the one case where both could be completed.","keywords":["active galactic nuclei","dense molecular gas","HCN","HCO+","sub-mm HCN diagram","BPT diagram","Gao-Solomon relation","AGN diagnostics"],"falsifier":"Detect CS(7-6) in HE0433-1028, the source the BPT classifies as AGN-dominated, and measure HCN(4-3)/CS(7-6); if the ratio exceeds about 8 the source moves into the AGN region and the claimed agreement for the fully measurable set fails, or observe these galaxies at <200 pc resolution and check whether the sub-mm ratios climb to AGN levels while the optical classification stays AGN-dominated, which would show the reported agreement is a beam-dilution artefact.","tokens_in":17873,"feed_emoji":"🔭","tokens_out":8971,"duration_ms":85575,"temperature":0.7,"pith_summary":"This paper tests whether a luminous AGN leaves a measurable imprint on the dense molecular gas that traces star formation, using ALMA observations of HCN(4-3), HCO+(4-3) and CS(7-6) in five nearby Type-1 AGN from the CARS survey. It finds that in the three galaxies where the dense-gas lines are detected, the infrared and HCN luminosities follow the Gao-Solomon relation, so whatever effect the AGN has on these lines is not enough to break the global scaling. It then compares the sub-mm HCN diagnostic diagram against the classic optical BPT classification built from MUSE data: in the one source where all three sub-mm lines are detected, both diagrams agree, and in the other two the available lower limits do not rule out agreement. The paper reads this as evidence that AGN can excite both the warm ionised gas and the dense molecular gas at the same time, and that strong star formation can still dominate both phases even close to an AGN.","feed_headline":"Dense gas in AGN hosts still follows star-formation scaling","feed_subtitle":"Optical and sub-mm diagnostics agree in the one fully measured case; big ALMA beams may hide AGN effects.","key_machinery":"The argument runs on three tools. The Gao-Solomon relation, in the $L_{\\rm IR}$-$L_{\\rm HCN(4-3)}$ and $L_{\\rm IR}$-$L_{\\rm HCO^+(4-3)}$ forms calibrated by Tan et al. (2018), is the scaling relation the AGN are tested against. The sub-mm HCN diagram of Izumi et al. (2016a) uses the HCN(4-3)/HCO+(4-3) and HCN(4-3)/CS(7-6) line-intensity ratios to separate AGN- from starburst-dominated dense gas. The optical BPT diagram, with the Kauffmann, Kewley and Schawinski demarcation lines, classifies the warm ionised gas in the same nuclear regions, isolated within one ALMA beam. The comparison between the two classification schemes, and the t-test residuals against the scaling relation, carry the central claim.","core_discovery":"The paper reports ALMA band-7 detections of HCN(4-3) and HCO+(4-3) in three of five CARS Type-1 AGN and a stacked CS(7-6) detection in one. It claims that, despite the potential impact of the AGN on line fluxes, these sources follow the Gao-Solomon $L_{\\rm IR}$-$L_{\\rm HCN}$ relation within the scatter of the Tan et al. (2018) calibration, with one-sample t-tests giving p=0.32 for HCN and p=0.51 for HCO+. It further claims that in the only source with all three lines measured (HE1108-2813), the sub-mm HCN diagram and the optical BPT diagram agree, both classifying the excitation as star-formation-driven, while in the two remaining sources the lower limits are consistent with either conclusion. The authors interpret this as evidence that AGN can excite both the warm ionised medium and dense molecular clouds simultaneously, possibly through X-ray, cosmic-ray or shock heating, while strong star formation can still dominate both phases.","pith_inferences":["If the beam-dilution reading is right, the apparent sub-mm/optical agreement is a resolution effect, not a physical statement about AGN: at <200 pc resolution, HCN(4-3)/CS(7-6) and HCN(4-3)/HCO+(4-3) should rise toward the AGN region while the BPT stays AGN-dominated, and a targeted high-resolution ALMA follow-up of these three sources would test this directly.","HE0433-1028, BPT-classified as AGN-dominated, sits on a HCN/CS lower limit; if CS(7-6) is detected and the ratio exceeds about 8, the two diagrams would agree in the AGN sense, which would strengthen the paper's conclusion that optical and sub-mm classifications agree.","The implied simultaneous excitation of warm ionised and dense molecular gas favours volume-filling mechanisms such as X-ray or cosmic-ray heating and turbulence over pure photoelectric heating in photodissociation regions; mapping shock- or cosmic-ray-sensitive tracers like [Fe II] or H2 at the same nuclear scales would provide a cross-check."],"forward_implications":["The $L_{\\rm IR}$-$L_{\\rm HCN}$ and $L_{\\rm IR}$-$L_{\\rm HCO^+}$ scaling relations survive in these AGN hosts at the current resolution, so they remain usable as dense-gas-to-star-formation calibrators even when a luminous AGN is present.","In the one source with all three lines measured (HE1108-2813), the sub-mm HCN and optical BPT diagrams agree on star-formation-driven excitation, showing the two diagnostics can be cross-checked on identical nuclear scales.","Non-detections of CS(7-6) are the main limitation: in the other two sources only HCN/CS lower limits are available, so their sub-mm classification is not settled.","If AGN enrich HCN as suggested for other systems, the enrichment is not large enough in these three sources to shift them off the global scaling relations in log-log space.","The results are consistent with AGN exciting both warm ionised and dense molecular gas through X-ray, cosmic-ray or shock heating, while strong star formation still dominates both phases when present."],"supporting_citations":[{"why":"Defines the $L_{\\rm IR}$-$L_{\\rm HCN}$ relation for star-forming galaxies that the paper tests against AGN hosts.","marker":"Gao & Solomon (2004a,b)"},{"why":"Provides the HCN(4-3) and HCO+(4-3) versions of the Gao-Solomon relation used for the residuals and t-tests.","marker":"Tan et al. (2018)"},{"why":"Origin of the sub-mm HCN diagram and its AGN/starburst separation, whose classification the paper compares with the BPT.","marker":"Izumi et al. (2016a)"},{"why":"Introduced the BPT optical diagnostic diagram used for the ionised-gas classification.","marker":"Baldwin et al. (1981)"},{"why":"Supplies the theoretical upper boundary separating composite from AGN/LINER ionisation in the BPT.","marker":"Kewley et al. (2001)"},{"why":"Supplies the empirical boundary separating star-forming from composite regions in the BPT.","marker":"Kauffmann et al. (2003)"},{"why":"Supplies the line dividing AGN from LINER regions in the BPT.","marker":"Schawinski et al. (2007)"},{"why":"Reports enhanced HCN excitation only within r<200 pc, the premise behind the beam-dilution caveat.","marker":"Viti et al. (2014)"}],"fun_headline_variants":["AGN dense gas still tracks star-formation scaling","Sub-mm and optical AGN diagnostics agree in one case","Dense gas in AGN obeys L_IR-HCN relation","CARS: ALMA limits but optical and sub-mm concur","AGN excitation: optical and sub-mm find common ground"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The comparison assumes that the ALMA beam (373-633 pc) covers the region where AGN influence on dense gas would appear, even though higher-resolution work places enhanced HCN excitation within r<200 pc, so the large beams may dilute any AGN-excited component and push the sources into the starburst part of the sub-mm diagram.","fun_headline_variants_meta":{"raw":{"variants":["AGN dense gas still tracks star-formation scaling","Sub-mm and optical AGN diagnostics agree in one case","Dense gas in AGN obeys L_IR-HCN relation","CARS: ALMA limits but optical and sub-mm concur","AGN excitation: optical and sub-mm find common ground"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.00019,"raw_usage":{"total_tokens":1421,"prompt_tokens":1105,"completion_tokens":316,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":721,"completion_tokens_details":{"reasoning_tokens":232}},"tokens_in":721,"tokens_out":316,"duration_ms":4305,"temperature":1.0,"reasoning_tokens":232,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-06T20:48:38.380263+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Detect CS(7-6) in HE0433-1028, the source the BPT classifies as AGN-dominated, and measure HCN(4-3)/CS(7-6); if the ratio exceeds about 8 the source moves into the AGN region and the claimed agreement for the fully measurable set fails, or observe these galaxies at <200 pc resolution and check whether the sub-mm ratios climb to AGN levels while the optical classification stays AGN-dominated, which would show the reported agreement is a beam-dilution artefact.","supporting_citations":[{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Provides the HCN(4-3) and HCO+(4-3) versions of the Gao-Solomon relation used for the residuals and t-tests."}],"review_version":1}