{"id":"8154f271-a95f-4473-be2d-a7d973ccc856","arxiv_id":"2411.11446","paper_version":3,"verdict":"REJECT","confidence":"HIGH","novelty_score":4.0,"correctness_risk":"high","formal_verification":"none","parameter_count":2,"one_line_summary":"Using a semi-analytic spin estimator, the authors claim halo spin strongly correlates with HI-to-stellar mass ratio in ALFALFA galaxies, but the estimator shares a definitional dependence on HI mass with the ratio.","lead":"The paper estimates dark matter halo spins for thousands of HI-rich galaxies from the ALFALFA survey and reports a correlation between halo spin and the HI-to-stellar mass ratio. The catch is that the spin estimate is built from the same HI mass that defines the ratio, so part of the correlation may be an artifact of the method.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The spin estimator is a deterministic function of M_HI: Eq. (1) uses R_HI,d derived from M_HI via Eqs. (2)-(4), giving λ_h ∝ M_HI^0.51/V_rot^1.5, so the reported η–λ_h correlations are largely built into the construction.","rationale":"The reader's weak point is exactly the load-bearing issue. The paper's central claim is a strong positive correlation between halo spin and HI fraction, but the halo spin estimator is not an independent observable: Eq. (1) depends on R_HI,d, which is solved from M_HI through Eqs. (2)-(4). The algebra shows R_HI,d ∝ M_HI^0.51, so λ_h is nearly a power law in M_HI. Because η = log M_HI - log M_*, any object with an unusually high M_HI at fixed M_* will simultaneously have a high η and a high λ_h. This creates a positive correlation by construction, regardless of whether halo spin physically regulates gas retention. The paper's reported coefficients, slopes, and binned trends are therefore not evidence for the proposed formation scenario. The physical mechanism invoked may be correct in nature, but the analysis as presented cannot support it. The paper does not provide a baseline test, a partial correlation controlling for M_HI, or an independent spin proxy, all of which would be straightforward. The error analysis on inclination and the environmental isolation do not address the constructional coupling. Therefore the reader's REJECT verdict is correct and should remain unchanged. I see no other concern that is more load-bearing: the sample selection and inclination corrections are secondary if the primary correlation is an artifact. The conclusion is unsupported, not necessarily false, and a revised analysis with an independent spin estimate could rescue the result.","tokens_in":7728,"tokens_out":5075,"duration_ms":52661,"concrete_test":"Using the same ALFALFA/SDSS sample, compute a null spin variable λ_null = 21.8 (R_null/kpc)/(V_rot/km s^-1)^1.5, where R_null is obtained by solving Eqs. (2)-(4) with the M_HI values randomly shuffled among galaxies (preserving the marginal R distribution but breaking the M_HI-λ_h link). Re-measure the Spearman correlation and linear slope between η and log λ_null in the same low-mass and massive subsamples. If the null gives ρ ≈ 0.40 and 0.50, or slopes comparable to the reported 0.73 and 1.39, the observed correlation is an artifact of the estimator construction. An additional check would use a subset with resolved HI kinematics to measure R_HI,d independently of M_HI and see whether the correlation survives.","verdict_should_be":"UNCHANGED","load_bearing_attack":"Section 2.3 constructs λ_h from exactly the quantity whose ratio with M_* is the dependent variable. Combining Eqs. (2)-(4), the HI scale radius is the solution of M_HI = 2π Σ_HI,0 R_HI,d^2 and 1 M_sun/pc^2 = Σ_HI,0 exp(-r_HI/R_HI,d), with log r_HI = 0.51 log M_HI - 3.59. Writing x = r_HI/R_HI,d gives log M_HI = const + 1.02 log M_HI + x log_10 e - 2 log x, so x varies only weakly over the sample; hence R_HI,d ∝ M_HI^0.51. Equation (1) then yields log λ_h ≈ 0.51 log M_HI - 1.5 log V_rot + const. Within a fixed stellar-mass bin, variation in M_HI is exactly variation in η, so every galaxy with higher η also receives a larger λ_h through R_HI,d before any halo physics is invoked. The reported Spearman coefficients 0.40/0.50 and the positive slopes are therefore expected from the estimator alone. The paper's argument that inclination uncertainties do not systematically bias the result does not address this constructional coupling, because the coupling enters through M_HI, not through inclination. A valid test requires breaking the M_HI dependence of λ_h or comparing against a permutation/null model.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper estimates dark matter halo spin parameters λ_h for a large sample of HI-rich galaxies from the ALFALFA survey using the semi-analytic approach of Hernandez et al. (2007), in which the HI disk scale length is derived from the empirical r_HI-M_HI relation. It then reports a positive correlation between the HI-to-stellar mass ratio η and λ_h in both low-mass (M* < 10^9 Msun) and massive (M* > 10^9 Msun) subsamples, with Spearman coefficients of 0.40 and 0.50, respectively. The authors interpret this as evidence for a universal formation scenario in which higher halo spin reduces angular momentum loss, suppresses star formation and feedback, and thus helps retain HI gas.","tokens_in":8086,"tokens_out":6432,"duration_ms":59012,"significance":"If the reported correlation were physically real, it would provide an interesting observational constraint on the role of halo spin in regulating the HI content of galaxies across a large sample. The paper has some positive features: it uses a large homogeneous sample from ALFALFA, selects isolated galaxies to minimize environmental effects, and uses the kurtosis of HI line profiles to exclude dispersion-dominated systems. However, the significance of the result is critically undermined by the likelihood that the correlation is built into the spin estimator itself, as detailed in the major comments. The central claim cannot be accepted on the present evidence.","major_comments":[{"comment":"In Eqs. (2)-(4), R_HI,d is solved from M_HI using the empirical relation log r_HI = 0.51 log M_HI - 3.59. Combining these equations gives R_HI,d ∝ M_HI^0.51 to a very good approximation, since the exponent in the M_HI dependence of the central surface density is only 2α−1 = 0.02. Substituting into Eq. (1), log λ_h ≈ 0.51 log M_HI - 1.5 log V_rot + const. Within a fixed stellar-mass bin, variation in M_HI is exactly variation in η, so a positive η-λ_h trend is partly generated by the estimator itself. The paper does not address this constructional coupling.","section":"Section 2.3, Eqs. (1)-(4)"},{"comment":"The reported Spearman coefficients (0.40 and 0.50) and the linear fits in Section 3 are presented as evidence for a physical correlation. Because λ_h is an increasing function of M_HI by construction, the authors must show that the correlation is not an artifact. A null test (e.g., permuting M_HI values among galaxies with the same V_rot and M_*, or correlating residuals of λ_h after subtracting the best-fit M_HI dependence) is required. No such control is provided, so the central claim is not supported.","section":"Section 3, Figure 1"},{"comment":"The discussion of inclination misalignment in Section 3 only addresses scatter in V_rot and does not affect the M_HI-based coupling via R_HI,d. Even if inclinations are perfectly known, galaxies with larger M_HI would still have systematically larger λ_h through the r_HI-M_HI relation. Thus the robustness argument is insufficient to address the main concern.","section":"Section 3, inclination discussion"}],"minor_comments":[{"comment":"The phrase 'Total HI masMHI' is a typo; it should read 'Total HI mass M_HI'.","section":"Section 2.3"},{"comment":"The parameter q0 is introduced without a clear definition; please state that it is the intrinsic axial ratio of the HI disk and provide the reference for the adopted values.","section":"Section 2.3"},{"comment":"The figure caption should report the number of galaxies in each bin and the method used to compute the 1σ error bars.","section":"Section 3, Figure 1"},{"comment":"The word 'universal' overstates the conclusion because the sample is limited to isolated, HI-rich, double-horned galaxies; this caveat should be stated in the abstract or conclusions.","section":"Section 4"},{"comment":"The reference list contains formatting errors (e.g., 'Guo, Q. et al. 2020, NewA, 4, 246' appears to have an incorrect volume/journal format); please check the bibliography.","section":"References"}],"recommendation":"reject","confidential_remarks":"The central result is very likely an artifact of the spin estimator, which is a deterministic function of M_HI through the r_HI-M_HI relation. A null test or an independent spin proxy would be required to rescue the claim. I recommend rejection."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Colleague,\n\nThe punchline is straightforward: the central correlation in this paper is largely built into the estimators. The authors estimate halo spin λ_h from Eq. (1) using R_HI,d, which they solve from M_HI through the empirical r_HI–M_HI relation in Eqs. (2)–(4). Since the dependent variable is η = log M_HI – log M_*, within a fixed stellar-mass bin any galaxy with higher η automatically gets a larger λ_h before any halo physics is invoked. The Spearman coefficients 0.40 and 0.50 are about what you'd expect from that constructional coupling alone.\n\nCredit where it's due: the paper extends the UDG-specific result from Rong et al. (2024a) to the full ALFALFA sample, split at 10^9 M_sun, with careful isolation criteria and environmental controls. The sample selection, inclination corrections, and profile-shape filtering are handled thoughtfully. The writing is clear.\n\nThe soft spot is load-bearing. The authors' response to inclination uncertainty misses the point—the coupling enters through M_HI, not through inclination. They never present a null test, a partial correlation controlling for M_HI, or a permutation analysis. Without breaking the M_HI dependence of λ_h, the observed trend is not evidence that halo spin regulates HI fraction. Their physical scenario may be correct, but this analysis does not demonstrate it.\n\nIf I'm being fair, this is a useful cautionary example of semi-analytic estimators sharing variables with the response. I'd encourage the authors to redo the analysis with a spin estimate that does not rely on M_HI (e.g., from resolved kinematics or a stellar-mass-based scaling) or at least to show that the correlation survives partial correlation tests. As it stands, I would not publish it.\n\nWould I send it to peer review? Yes—the question is significant and the flaw is subtle enough that a good referee can help the authors see it. But I'd expect the referee to catch this, and the authors will need to do more than cite inclination uncertainty. For my own work, I wouldn't cite this result until the coupling is addressed.","headline":"The reported η–λ_h correlation is largely an artifact of the spin estimator's dependence on M_HI; the paper needs a null test or an independent spin proxy to be convincing.","tokens_in":8622,"tokens_out":3414,"would_cite":false,"duration_ms":34417,"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 the HI-to-stellar mass ratio rises with dark-matter halo spin in both low-mass and massive isolated galaxies, making halo spin a likely universal regulator of gas retention.","keywords":["galaxy formation","halo spin","HI-to-stellar mass ratio","neutral hydrogen","dark matter halos","galaxy evolution","star formation feedback","21-cm survey"],"falsifier":"Recompute halo spins for the same galaxies using an estimator that does not use $M_{\\rm HI}$ as input--for example, one based on stellar disk scale lengths and rotation velocities only--and check whether the $\\eta$--$\\lambda_h$ correlation coefficients stay near 0.4 and 0.5. If the correlation largely disappears or drops below significance, the claimed universal spin--gas relation is not supported by the current method.","tokens_in":7536,"feed_emoji":"🌀","tokens_out":9465,"duration_ms":80569,"temperature":0.7,"pith_summary":"This paper tries to establish that the amount of neutral hydrogen a galaxy holds, relative to its stars, is set to a significant degree by how fast its dark-matter halo spins. Using a semi-analytic estimator of halo spin applied to thousands of isolated HI-rich galaxies, it reports a positive correlation between the HI-to-stellar mass ratio and the spin parameter in both low-mass and massive systems, with correlation coefficients of about 0.4 and 0.5. If the correlation is real, halo spin would be a universal regulator of gas retention: high-spin halos keep their gas from collapsing quickly, which slows star formation and weakens feedback, leaving more HI in place. A sympathetic reader would care because this would connect a readily observable gas fraction to a fundamental dark-matter property, giving a new handle on galaxy formation models.","feed_headline":"Halo spin drives gas retention in dwarf and massive galaxies","feed_subtitle":"Two galaxy subsamples show HI-to-stellar mass ratios climbing with halo spin, pointing to a universal regulator.","key_machinery":"The object doing the work is the semi-analytic spin estimator $\\lambda_h \\simeq 21.8\\,(R_{\\rm HI,d}/{\\rm kpc})\\,(V_{\\rm rot}/{\\rm km\\,s^{-1}})^{-3/2}$ from Hernandez et al. (2007), which assumes an isothermal dark-matter halo with negligible baryonic gravity and a thin, rotationally supported exponential HI disk. The disk scale length $R_{\\rm HI,d}$ is not measured directly; it is computed from the total HI mass $M_{\\rm HI}$ through the empirical $r_{\\rm HI}$--$M_{\\rm HI}$ relation, while the rotation velocity $V_{\\rm rot}$ comes from the measured HI line width corrected by an optical inclination. This estimator converts large HI-survey data into halo spins, and the analysis then compares $\\eta$ against $\\lambda_h$ separately for the two mass regimes, restricting to isolated systems with double-horned HI profiles to remove environmental gas loss and dispersion-dominated kinematics.","core_discovery":"The central claim is that the HI-to-stellar mass ratio, defined as $\\eta = \\log M_{\\rm HI} - \\log M_{\\star}$, increases with the halo spin parameter $\\lambda_h$ for both low-mass ($M_\\star < 10^9\\,M_\\odot$) and massive ($M_\\star > 10^9\\,M_\\odot$) isolated galaxies in the sample. The paper reports correlation coefficients of $0.40$ and $0.50$ for the two subsamples and linear fits with positive slopes (steeper for massive galaxies), and interprets this as evidence for a universal formation scenario: a higher-spin halo receives high-angular-momentum gas that resists infall and condensation, so star formation proceeds gently and supernova feedback is too weak to expel the gas, leaving a high HI fraction. The same mechanism previously proposed for ultra-diffuse galaxies is thereby extended across a broad stellar mass range.","pith_inferences":["The reported correlation may be inflated by construction: $\\lambda_h$ depends on $M_{\\rm HI}$ through $R_{\\rm HI,d}$, whereas $\\eta$ is defined from $M_{\\rm HI}$, so part of the correlation could reflect the same quantity appearing on both sides; testing with a spin estimator that uses only stellar sizes or rotation velocities would separate the physical relation from this arithmetic coupling.","If confirmed, the spin--gas connection could sharpen quenching models: at fixed halo mass, high-spin halos should host galaxies with lower current star formation rates per stellar mass and more gas-rich disks, a prediction testable with resolved HI and star-formation maps.","The sample selection (isolated, double-horned, high-SNR galaxies) may bias toward regular rotators; extending the same analysis to interacting or dispersion-dominated systems could reveal whether the spin--HI link persists in populations whose gas is most easily disturbed."],"forward_implications":["If the correlation is genuine, halo spin becomes a primary observable predictor of a galaxy's HI-to-stellar mass ratio, comparable in importance to halo mass and environment.","The positive $\\eta$--$\\lambda_h$ relation should hold across the full stellar mass range, implying that spin-driven gas retention is a universal rather than dwarf-specific process.","Simulations and semi-analytic models of galaxy formation would need to reproduce a stronger HI fraction in high-spin halos; a model that predicts the opposite or a flat relation would conflict with these data.","Large HI surveys can be used statistically to estimate halo spins for thousands of galaxies without resolved 21-cm mapping, opening a cheap way to study angular momentum in the field."],"supporting_citations":[{"why":"Supplies the semi-analytic formula that converts HI disk scale length and rotation velocity into the halo spin parameter $\\lambda_h$.","marker":"Hernandez et al. 2007"},{"why":"Provides the thin exponential disk model used to relate total HI mass to the disk scale length $R_{\\rm HI,d}$.","marker":"Mo et al. 1998"},{"why":"Gives the empirical $r_{\\rm HI}$--$M_{\\rm HI}$ relations that connect observed HI mass to the disk radius used in the spin estimator.","marker":"Wang et al. 2016; Gault et al. 2021"},{"why":"Provides the 21-cm catalog from which HI masses, line widths, and signal-to-noise ratios are taken.","marker":"Haynes et al. 2018"},{"why":"Supplies stellar masses and optical axis ratios for the galaxies, used both for defining $\\eta$ and for inclination corrections.","marker":"Durbala et al. 2020"},{"why":"Proposes that excessive halo spin impedes gas condensation and raises gas fractions in ultra-diffuse galaxies, the mechanism this paper extends to the full mass range.","marker":"Rong et al. 2024a"},{"why":"Identifies the $10^9\\,M_\\odot$ stellar-mass transition used to split the sample into low-mass and massive regimes.","marker":"Di Cintio et al. 2019"},{"why":"Provides the group catalog used to identify and remove galaxies in dense environments.","marker":"Saulder et al. 2016"},{"why":"Establishes the kurtosis criterion used to select double-horned, rotation-dominated HI spectra.","marker":"Hua et al. 2024"}],"fun_headline_variants":["Halo spin boosts HI fraction in galaxies of all masses","Spinning halos keep gas, suppress star formation in galaxies","Galaxy gas retention tied to halo spin, even for dwarfs","High spin halos hoard gas, low star formation","HI-rich galaxies reveal spin-gas link across masses"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The load-bearing assumption is that the semi-analytic halo spin estimate measures the dark-matter halo's true angular momentum and is not merely a re-expression of the galaxy's neutral-hydrogen mass, because the spin estimate is derived from that same gas mass, which also defines the ratio being tested.","fun_headline_variants_meta":{"raw":{"variants":["Halo spin boosts HI fraction in galaxies of all masses","Spinning halos keep gas, suppress star formation in galaxies","Galaxy gas retention tied to halo spin, even for dwarfs","High spin halos hoard gas, low star formation","HI-rich galaxies reveal spin-gas link across masses"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000401,"raw_usage":{"total_tokens":2031,"prompt_tokens":820,"completion_tokens":1211,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":436,"completion_tokens_details":{"reasoning_tokens":1143}},"tokens_in":436,"tokens_out":1211,"duration_ms":8832,"temperature":1.0,"reasoning_tokens":1143,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-12T18:31:35.784169+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Recompute halo spins for the same galaxies using an estimator that does not use $M_{\\rm HI}$ as input--for example, one based on stellar disk scale lengths and rotation velocities only--and check whether the $\\eta$--$\\lambda_h$ correlation coefficients stay near 0.4 and 0.5. If the correlation largely disappears or drops below significance, the claimed universal spin--gas relation is not supported by the current method.","supporting_citations":[{"cited_title":"2007, , 375, 163","cited_arxiv_id":null,"evidence_quote":"Supplies the semi-analytic formula that converts HI disk scale length and rotation velocity into the halo spin parameter $\\lambda_h$."},{"cited_title":"S., Serra , P., van der Hulst , T., Roychowdhury , S., Kamphuis , P., Chengalur , J","cited_arxiv_id":null,"evidence_quote":"Gives the empirical $r_{\\rm HI}$--$M_{\\rm HI}$ relations that connect observed HI mass to the disk radius used in the spin estimator."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Provides the 21-cm catalog from which HI masses, line widths, and signal-to-noise ratios are taken."},{"cited_title":"V., Mikske , S., Zeilinger , W","cited_arxiv_id":null,"evidence_quote":"Provides the group catalog used to identify and remove galaxies in dense environments."}],"review_version":1}