{"id":"7ecd355e-1f7f-497c-8d15-819be84b86a2","arxiv_id":"2505.09728","paper_version":2,"verdict":"CONDITIONAL","confidence":"HIGH","novelty_score":7.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":2,"one_line_summary":"In a z=1.46 grand-design spiral starburst, the ratio of CO-inferred infrared luminosity to 8-micron PAH emission is about twice as high in the nucleus as in the spiral arms, revealing the first spatially resolved PAH deficit at z>1.","lead":"JWST and ALMA images of a distant starburst galaxy at redshift 1.46 reveal a spiral galaxy whose bright core emits much less 8-micron PAH light than its dense gas emission would suggest. This is the first resolved map at z>1 of the ratio between total infrared and 8-micron luminosity, pointing to destruction of PAH molecules in the intense radiation of the starburst core.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The PAH-deficit claim rests on applying the Daddi+15 CO(5-4)-to-L_TIR conversion uniformly across PACS-830; if core CO excitation is enhanced, L_IR is overestimated and IR8 is inflated.","rationale":"I read the paper in good faith and find the strongest claim—a spatially resolved PAH deficit in the starburst core at z>1—genuinely interesting and potentially correct. The morphology is convincing, and the paper is transparent about the key systematics in Sec. 4.2 and Sec. 5.1. However, the single most load-bearing premise is the uniform application of Eq. (1) to convert CO(5-4) to L_IR across regions with very different physical conditions. If CO excitation is higher in the core, the conversion inflates L_IR there and creates a spurious IR8 enhancement. The paper's rebuttal rests on the different radial concentration of F1800W and CO(5-4), but F1800W is a broadband proxy whose continuum behavior is unmeasured, and the VLA 3 GHz data in Appendix C independently suggest that the CO map may not trace L_IR in the same way across the galaxy. This is a testable concern: the ALMA dust continuum and the VLA radio map are both already available, so the check I propose does not require new observations. If the IR8 contrast persists when L_IR is derived from an independent tracer, the PAH-deficit interpretation is strongly supported; if it flattens, the central claim does not survive. I therefore keep the CONDITIONAL verdict, with the condition being the outcome of that cross-check.","tokens_in":22402,"tokens_out":5685,"duration_ms":61221,"concrete_test":"Recompute the IR8 map using an independent L_IR tracer already in hand: the ALMA 1.33 mm dust continuum (Fig. 1k), calibrated to the global SED-based L_IR (Liu et al. 2021) with a fixed dust SED, at the same 0.59'' resolution and mask as Fig. 3. If the core/arm IR8 contrast drops below ~1.5 (or the Fig. 4 core point moves below the critical-density locus), the PAH-deficit claim is not robust. As a second check, measure the CO(5-4)/1.33 mm continuum ratio in 0.3'' apertures at the core and eastern arm; an elevated ratio in the core would directly flag enhanced CO excitation and invalidate the uniform Eq. (1) assumption.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim of a resolved PAH deficit in the core of PACS-830 hinges on the IR8 map of Fig. 3, whose numerator is L_IR obtained by applying Eq. (1) (Daddi et al. 2015) pixel-by-pixel with a single constant. That relation was calibrated on galaxy-integrated CO(5-4) luminosities, and the CO(5-4) upper level sits ~55 K above ground; in the warm, dense starburst core the line-to-total-CO ratio can be elevated, so the fixed conversion would overestimate L_IR there. The paper acknowledges this in Sec. 4.2 but argues that the dissimilar morphologies of F1800W and CO(5-4) make systematics unlikely. That rebuttal is not quantitative: F1800W is a broad filter including continuum and stellar light, and its lower central concentration is exactly what one expects if PAH/continuum emission is suppressed by the same intense radiation field that excites CO(5-4). Independent tension is visible in the paper's own Appendix C: VLA 3 GHz shows two peaks of comparable strength in the core and eastern arm, whereas CO(5-4) is far more centrally concentrated. If radio emission traces L_IR, the CO-based L_IR map underweights the arm relative to the core, and the IR8 enhancement is at least partly a CO-excitation artifact. This concern is load-bearing because both Figs. 4 and 5 and the title's 'PAH deficit' inherit the uniform conversion.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript presents resolved JWST (NIRCam and MIRI F1800W) and ALMA CO(J=5-4) observations of PACS-830, a z=1.46 starburst galaxy, and shows that it has a grand-design spiral morphology with a central bulge, two arms, and clumps. The authors construct a spatially resolved IR8 (= L_IR / L_8) map by converting the CO(5-4) map to L_IR via Eq. (1) (Daddi et al. 2015) and using F1800W as an L_8 proxy, finding that IR8 peaks near 18 in the core, about twice the arm values. They interpret this as a PAH deficit in the starburst core, likely due to PAH destruction by intense radiation or reduced PDR emission, and argue that this is the first resolved detection of such a deficit at z>1. The paper also discusses the morphological evidence for a non-major-merger origin of the starburst and the possible role of a mini-merger companion.","tokens_in":22700,"tokens_out":3829,"duration_ms":39958,"significance":"If the central claim holds, the paper provides the first spatially resolved measurement of IR8 variations at z>1, with direct implications for using PAH emission as an SFR tracer in high-redshift galaxies and for the physical conditions in high-z starburst cores. The analysis is technically careful: it uses spatially matched JWST and ALMA data, performs foreground subtraction, pixel-by-pixel SED fitting, and uv-plane modeling, and it explicitly relies on external calibrations without fitting new free parameters. The morphological evidence for a spiral starburst that is not a major merger is a valuable addition to the growing JWST-based picture of disk-like high-z starbursts. However, the PAH-deficit conclusion rests on the assumption that a single CO(5-4)-to-L_IR conversion applies uniformly across the galaxy, an assumption that the paper itself identifies as a systematic uncertainty and that is not fully resolved by the current analysis.","major_comments":[{"comment":"The uniform application of the Daddi et al. (2015) CO(5-4)-to-L_TIR conversion is the load-bearing step for the IR8 map, but the relation is calibrated on galaxy-integrated luminosities and there is no evidence that the same constant holds in the nucleus and the arms of PACS-830. In the warm, dense starburst core, enhanced CO(5-4) excitation relative to the total CO reservoir would cause Eq. (1) to overestimate L_IR, inflating IR8 and producing a spurious PAH deficit. The rebuttal in Section 4.2, based on the morphological difference between F1800W and CO(5-4), is not quantitative, and the manuscript's own Appendix C shows VLA 3 GHz emission with two comparable peaks in the core and the eastern arm, whereas CO(5-4) is strongly centrally concentrated; if radio emission traces L_IR, this indicates that the CO-based conversion underweights the arm relative to the core, which is exactly the direction needed to create the claimed IR8 enhancement. The authors should test the robustness of the IR8 map with an alternative L_IR tracer (e.g., radio continuum or available dust continuum) or provide an explicit excitation-correction model, rather than relying on the qualitative morphological argument.","section":"Section 3.2, Eq. (1)"},{"comment":"The F1800W flux is converted to L_8 by applying a single M82-template k-correction (factor 1.3), but the 8 micron band is a broad filter that includes PAH features and underlying continuum, and the spectral shape may vary spatially across the galaxy. The authors acknowledge in Section 5.1 that the continuum contribution is uncertain, yet they do not quantify how this uncertainty propagates to the IR8 map. If the continuum fraction is higher in the core (e.g., due to hot dust), the L_8 map would be overestimated there, partially masking a PAH deficit; if lower, it would enhance the deficit. The manuscript should provide an estimate of the spatially varying continuum contribution to F1800W, or at least show that the claimed factor-of-two IR8 variation is robust to plausible continuum variations.","section":"Section 3.2 (F1800W k-correction)"},{"comment":"The claim that the PAH distribution is more disk-like than the L_IR distribution is supported by comparing the Sérsic index from the F1800W image (n ~ 1, fitted in the image plane with Galight) to the Sérsic index from the CO map (n ~ 1.7, derived from a Spergel model fitted in the uv-plane with GILDAS). These indices are obtained with different fitting methods, different data representations (image vs. visibilities), and different noise properties, and the reported uncertainties of ±0.2 do not include systematic differences between the two approaches. Before using this index difference as supporting evidence for a PAH deficit, the authors should demonstrate that the difference is not an artifact of the fitting methodology, for example by fitting both maps in the same plane or at matched S/N ratio.","section":"Sections 3.3-3.4"}],"minor_comments":[{"comment":"The color-bar label 'IR8 = L_CO5 4 IR /L_F1800W' is difficult to parse; please use the same notation as in the text and Eq. (1) (e.g., IR8 = L_IR / L_8) and define all symbols in the caption.","section":"Figure 3"},{"comment":"The northern spiral arm is not robustly detected in CO(5-4) and is treated as an upper limit, but the text does not specify how the mask (CO S/N > 4) and the upper-limit treatment are applied consistently in Figure 4; please clarify this procedure and state whether the shown northern-arm points are the actual measurements or the upper limits.","section":"Section 3.2 and Figure 4"},{"comment":"The statement 'SFE is proportional to IR8 (SFE = SFR/Mgas ∝ IR8)' assumes a constant ratio between PAH luminosity and molecular gas mass; this proportionality should be stated as an assumption rather than an equality, since spatial variations in PAH abundance or excitation are the very subject of the paper.","section":"Section 5.1"},{"comment":"The claim of 'the first time at z>1' is a strong novelty statement; consider clarifying it as 'the first spatially resolved IR8 map at z>1' or adding a brief caveat about prior global measurements, to avoid overstatement.","section":"Abstract and Section 6"}],"recommendation":"major_revision","confidential_remarks":"The paper is a strong observational study with high-quality data and a novel resolved IR8 map, and it is well suited for MNRAS if the central systematic concern is addressed. The main risk is that the CO(5-4)-to-L_IR conversion may be biased in the dense core, and the VLA 3 GHz data in Appendix C appear to contradict the CO-based L_IR distribution. I would like the editor to encourage the authors to either provide a quantitative test using an alternative L_IR tracer or substantially reframe the claim as a CO-based IR8 variation with explicit caveats on the excitation issue."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"X,\n\nShort version: this is the first resolved map of L_IR/L_8 (IR8) at z>1, and it shows a clear spatial gradient in a grand-design spiral starburst at z=1.46, with the core having a higher IR8 than the arms. That is a genuinely new observation and worth taking seriously. But the quantitative claim of a 'PAH deficit' in the core rests on a uniform CO(5-4)-to-L_TIR conversion that is not calibrated per pixel, and the supporting morphology argument is weaker than the authors suggest.\n\nThe data themselves are good. MIRI F1800W and ALMA CO(5-4) are well matched in resolution and depth; the spiral structure is clear in both. The Sersic vs Spergel fits give a useful independent hint that the PAH distribution is more extended than the CO. The paper is also transparent about known systematics, which is more than many papers do.\n\nThe soft spot is the conversion. Eq. 1 from Daddi et al. (2015) was derived from integrated galaxy luminosities. Applying it pixel-by-pixel assumes the CO(5-4)-to-total-IR ratio is constant across the galaxy. CO(5-4) has an upper level ~55 K, so in a warm, dense starburst core you'd expect higher excitation and thus a higher line-to-total-CO ratio. That alone would inflate L_IR (and IR8) in the core. The authors say the different morphologies of F1800W and CO rule out systematics, but that doesn't follow: if PAH is genuinely suppressed in the core, F1800W would be less centrally peaked than CO. The morphology is consistent with a PAH deficit, but it can't distinguish that from an excitation effect.\n\nThe Appendix C radio data are the most concrete worry. VLA 3 GHz shows comparable flux in the core and eastern arm, while CO is much more centrally concentrated. If the radio continuum is tracing recent star formation, then the CO-based L_IR underweights the arm relative to the core, and at least part of the IR8 enhancement is a conversion artifact. The authors acknowledge this but don't close the loop.\n\nSo: a solid descriptive paper with an important new measurement, but the headline interpretation should be softened until additional data (e.g., CO(1-0)/CO(2-1) mapping, or MIRI spectroscopy) can calibrate the conversion spatially. A good referee would likely ask for that, or for a revision that frames the result as a spatial variation in the F1800W/CO ratio rather than a definite PAH deficit.\n\nMy recommendation: send to peer review. It deserves a serious referee and will be useful even if the final version is more hedged.","headline":"First resolved IR8 map at z>1 shows a core PAH deficit in a spiral starburst, but the claim rests on a uniform CO conversion that needs per-pixel validation.","tokens_in":23545,"tokens_out":3370,"would_cite":true,"duration_ms":32468,"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":"In PACS-830, a starburst spiral at $z=1.46$, the core's $\\mathrm{IR}_8$ reaches about 18, roughly twice the spiral-arm value, indicating a deficit of 8-micron PAH emission.","keywords":["PAH deficit","IR8","starburst galaxy","high-redshift spiral","JWST MIRI","ALMA CO(5-4)","bulge formation","interstellar medium"],"falsifier":"Rebuild the $\\mathrm{IR}_8$ map using an $L_{\\rm IR}$ tracer that does not depend on CO(5-4) excitation, such as matched-resolution ALMA dust continuum or CO(1-0); if the core-to-arm ratio drops to near unity, the reported PAH deficit is a conversion artifact rather than a physical deficit.","tokens_in":22172,"feed_emoji":"🌌","tokens_out":12335,"duration_ms":105171,"temperature":0.7,"pith_summary":"This paper reports that PACS-830, a starburst galaxy at $z=1.46$ that JWST shows to be a grand-design spiral (two well-defined arms) rather than a merger remnant, has a core where the ratio $\\mathrm{IR}_8 = L_{\\rm IR}/L_8$ reaches about 18, roughly twice the value in its two spiral arms; here $L_8$ tracks the 8-micron emission of polycyclic aromatic hydrocarbons (PAHs), the mid-infrared-emitting dust grains. The authors construct the first spatially resolved $\\mathrm{IR}_8$ map at $z>1$ by converting ALMA CO(5-4) emission into total infrared luminosity and using the JWST MIRI F1800W image for the rest-frame 8-micron luminosity. They interpret the elevated core ratio as a deficit of 8-micron PAH emission, likely from destruction of PAH molecules by the intense radiation field or suppressed photodissociation-region emission in a forming bulge. If correct, the result cautions against using PAH emission as a spatially resolved star-formation tracer and shows that an intense starburst at cosmic noon can be sustained by disk processes, not only by a major merger.","feed_headline":"Distant starburst core doubles IR8, a sign of destroyed PAHs","feed_subtitle":"First resolved z>1 map ties the 8-micron PAH deficit to a growing bulge, so PAH tracers can mislead.","key_machinery":"The central object is the spatially resolved $\\mathrm{IR}_8$ map, $\\mathrm{IR}_8 \\equiv L_{\\rm IR}/L_8$, which acts as a resolved version of the galaxy-wide infrared-to-PAH ratio. The map is built from two matched tracers: the ALMA CO($J=5-4$) map, converted pixel-by-pixel to $L_{\\rm IR}$ with the empirical relation $\\log L_{\\rm TIR}/L_\\odot = \\log L'_{\\rm CO[5-4]}/({\\rm K\\,km\\,s^{-1}\\,pc^2}) + 2.52$, and the JWST MIRI F1800W image, which captures rest-frame 8-micron emission where the 7.7-micron PAH feature dominates, k-corrected with an M82 SED template. The CO map is convolved to the 0.59-arcsecond MIRI PSF and masked at low signal-to-noise before division. Companion Sérsic and uv-plane Spergel fits quantify how much more compact the CO-based infrared emission is than the PAH-tracing F1800W emission, providing the geometric evidence for the deficit.","core_discovery":"On its own terms, the paper establishes that in PACS-830 the spatial distribution of rest-frame 8-micron emission does not track the distribution of star formation traced by CO(5-4). With $L_{\\rm IR}$ inferred from the ALMA CO(5-4) map through the adopted empirical conversion and $L_8$ from the MIRI F1800W image, the $\\mathrm{IR}_8$ map peaks near 18 in the 0.5-arcsec core and drops to roughly half that in the spiral arms. Two-dimensional modeling supports the difference: the CO emission is more centrally concentrated than the F1800W emission, with Sérsic indices around 1.7 and 1.0 respectively. The authors conclude that the core has a genuine PAH deficit, unlikely to be caused by an AGN (there is no AGN evidence, and the F1800W image is fainter, not brighter, in the core), and they attribute it to PAH destruction by the hard ionizing radiation or reduced PDR emission. They further conclude that PACS-830 is a face-on grand-design spiral with a growing bulge, with one arm carrying about 21% of the CO flux, so a disruptive major-merger trigger is not required.","pith_inferences":["An untested consequence of the radiative-destruction explanation is that the same core should show suppressed 6.2- and 11.3-micron PAH features with elevated continuum temperature; MIRI MRS spectroscopy of PACS-830 would test this directly.","An independent $L_{\\rm IR}$ tracer at matched resolution, such as ALMA dust continuum or CO(1-0), could separate a true PAH deficit from an overestimate of $L_{\\rm IR}$ in the core caused by enhanced CO(5-4) excitation.","Applied to larger samples, resolved $\\mathrm{IR}_8$ maps could reveal how many apparent main-sequence galaxies hide growing bulges, linking the deficit to inside-out quenching rather than only to starburst outliers."],"forward_implications":["Resolved PAH-based star-formation maps at high redshift will undercount star formation in dense starburst cores unless the $\\mathrm{IR}_8$ enhancement is taken into account.","Starburst activity well above the main sequence at $z\\sim1.5$ can be powered by a spiral disk with clumps and a forming bulge, weakening the assumption that such systems are major mergers.","An elevated $\\mathrm{IR}_8$ in a compact core can be used as a spatial signpost for a bulge in formation or an extreme star-forming region, extending the global $\\mathrm{IR}_8$-starburstiness relation to resolved scales.","The core's infrared surface density sits near the critical value where $\\mathrm{IR}_8$ begins rising, so the observed deficit is consistent with the dust-bounded, high-radiation-field picture rather than requiring an AGN."],"supporting_citations":[{"why":"It supplies the empirical CO(5-4)-to-total-infrared conversion (Equation 1) used to turn the ALMA map into $L_{\\rm IR}$.","marker":"Daddi et al. (2015)"},{"why":"It defines the $\\mathrm{IR}_8$ ratio and the galaxy-wide $L_{\\rm IR}/L_8$ relation that the paper extends to resolved kpc scales.","marker":"Elbaz et al. (2011)"},{"why":"It supplies the $\\mathrm{IR}_8$ versus $\\Sigma_{\\rm IR}$ relation and the critical-density trend used to interpret the core enhancement.","marker":"Elbaz et al. (2018)"},{"why":"Global SED fits set PACS-830's starburst properties, stellar mass, and negligible AGN contribution, and anchor the PAH SED shown alongside F1800W photometry.","marker":"Liu et al. (2021)"},{"why":"BPT analysis and X-ray non-detections establish that PACS-830 is a star-forming galaxy without significant AGN.","marker":"Silverman et al. (2015)"},{"why":"It introduces the dust-bounded critical surface density idea used to explain why $\\mathrm{IR}_8$ rises above a threshold.","marker":"Díaz-Santos et al. (2017)"},{"why":"It supplies the theoretical basis that hard radiation destroys PAH molecules or suppresses their photodissociation-region emission.","marker":"Draine & Li (2007)"},{"why":"It establishes the pixel-by-pixel SED fitting pipeline and a companion starburst case that supports reading PACS-830 as a rotation-supported disk.","marker":"Liu et al. (2024b)"}],"fun_headline_variants":["First resolved z>1 PAH deficit in a starburst spiral core","Distant spiral's core lacks PAHs, first resolved z>1 map","JWST & ALMA see PAH deficit in starburst core of spiral galaxy","Core of distant starburst spiral shows PAH deficit","PAH destruction in spiral core: first z>1 resolved map"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The argument assumes that the same empirical CO(5-4)-to-total-infrared conversion holds in the starburst core and in the spiral arms; if the warm, dense core excites CO(5-4) more strongly, $L_{\\rm IR}$ would be overestimated there and the $\\mathrm{IR}_8$ peak could be an artifact.","fun_headline_variants_meta":{"raw":{"variants":["First resolved z>1 PAH deficit in a starburst spiral core","Distant spiral's core lacks PAHs, first resolved z>1 map","JWST & ALMA see PAH deficit in starburst core of spiral galaxy","Core of distant starburst spiral shows PAH deficit","PAH destruction in spiral core: first z>1 resolved map"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.001014,"raw_usage":{"total_tokens":4396,"prompt_tokens":1176,"completion_tokens":3220,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":792,"completion_tokens_details":{"reasoning_tokens":3123}},"tokens_in":792,"tokens_out":3220,"duration_ms":24348,"temperature":1.0,"reasoning_tokens":3123,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-15T21:26:41.124292+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Rebuild the $\\mathrm{IR}_8$ map using an $L_{\\rm IR}$ tracer that does not depend on CO(5-4) excitation, such as matched-resolution ALMA dust continuum or CO(1-0); if the core-to-arm ratio drops to near unity, the reported PAH deficit is a conversion artifact rather than a physical deficit.","supporting_citations":[],"review_version":1}