{"id":"656d6bae-c235-4dca-b31e-9f89e32125fe","arxiv_id":"2501.01289","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":7.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":8,"one_line_summary":"The HI disks of 35 nearby galaxies, measured down to 0.01 M_sun/pc2, share a universal radial profile when scaled by their deep outer radius R001, and R001 follows a very tight relation with HI mass.","lead":"Astronomers mapped the faint outer gas of 35 nearby galaxies with the FAST telescope, reaching surface densities 100 times deeper than standard HI surveys. The results show that the faintest gas still arranges itself into a common disk shape, giving a new way to test how galaxies accrete gas.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The outer-HI deprojection under a thin, fixed-inclination disk assumption is the load-bearing step for the claimed universal profile; a warp or flare beyond R1 would bias R001 and the shallow-slope profiles.","rationale":"The reader's weakest_assumption identifies the deprojection geometry as the vulnerable step, and I agree that this is the single most load-bearing concern. The claim of a universal profile down to 0.01 M_sun/pc^2 is an empirical statement about the face-on surface-density distribution in the outermost HI. If the thin-disk approximation fails there, both R001 and the profile shapes are systematically biased, which would undermine the universality and the derived R001-MHI relation. The paper acknowledges the caveat only for dwarfs and defers quantification, so the central result rests on an unverified assumption. Other concerns, such as the partial tautology of normalizing by the measured radius, are real but secondary: the authors' comparison of normalizations (kpc, rvir, R1, R001) shows R001 gives the tightest alignment, and the increased scatter beyond R001 in Table 2 indicates the result is not purely forced by the crossing point. The beam-smoothing simulations in Appendix B provide independent support for the R1/R001 measurements at the claimed precision, so the measurement pipeline is not the weakest link. Given that the deprojection concern can be tested with existing or upcoming deep HI observations, a CONDITIONAL verdict remains appropriate, with the condition being a quantitative check of the outer-disk geometry.","tokens_in":18573,"tokens_out":9839,"duration_ms":97325,"concrete_test":"Select a representative subsample of about six non-dwarf galaxies spanning the observed range of R001/R1 and M_HI/M_* that already have or can obtain deep, high-resolution HI observations (e.g., MeerKAT/MHONGOOSE or VLA). Fit a tilted-ring model with radius-dependent inclination, position angle, and (if resolved) vertical scale height, and recompute the face-on Sigma_HI(r) using the 3D geometry rather than a single fixed cos(i). Then remeasure R001 and the R001-normalized profiles. If the scatter around the median profile increases from ~0.2 dex to >0.3 dex, or if R001 changes by >0.1 dex for the non-dwarf galaxies, the thin-disk assumption is load-bearing and the universal-profile claim needs to be qualified.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim that non-tidally-disturbed galaxies have self-similar HI profiles down to 0.01 M_sun/pc^2 depends on the accuracy of the face-on surface densities at large radii. Section 2 sets the ellipse geometry from the 10^20 cm^-2 isophote and multiplies all annuli by a single cos(i), i.e., a thin, unwarped, circular disk. R001 is measured at roughly 2R1, far outside the geometry-defining contour. In real galaxies, warps and flares commonly begin around R1, and the low-column-density HI may be vertically thick or extraplanar, especially in dwarf irregulars (the paper explicitly flags this caveat). If the outer HI layer has a different inclination or position angle than the inner disk, the deprojected Sigma_HI at large radii is systematically wrong. This would bias the measured R001 and the shape of the outer profile, potentially creating or destroying the apparent universal profile and shifting points along or off the R001-MHI relation. The paper's supporting evidence from W24 and D. Yang in prep is qualitative and does not quantify the vertical structure at the 0.01 M_sun/pc^2 level across the sample. Without a quantitative test of the thin-disk assumption in the low-column-density regime, the main physical interpretation is not fully secured.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"This paper presents deep HI surface density radial profiles for 35 nearby galaxies observed with FAST in the FEASTS program, reaching a column density sensitivity of about 10^17.7 cm^-2 (0.004 M_sun/pc^2). The authors define R001 as the radius at which Sigma_HI = 0.01 M_sun/pc^2, and show that, compared to normalization by kiloparsecs, R1, or rvir, the profiles align most tightly when the radius is normalized by R001. From the aligned profiles they derive a 'universal' median profile with a scatter of about 0.2 dex and an outer exponential scale length of 0.11 R001. They also derive a new R001-MHI size-mass relation with a scatter of 0.02 dex and slope 0.49, and report that R001/R1 anti-correlates with HI-to-stellar mass ratio and specific star formation rate for non-interacting galaxies. The paper interprets these results as evidence for self-similar HI disk structure down to 0.01 M_sun/pc^2 and uses them to discuss gas inflow and CGM interaction scenarios.","tokens_in":18939,"tokens_out":6350,"duration_ms":63289,"significance":"If the central claims hold, this is an important observational contribution: it is among the first statistical samples of HI radial profiles extending to surface densities two orders of magnitude below the conventional 1 M_sun/pc^2 level. The proposed R001-MHI relation could become a useful distance-independent size indicator for HI surveys, and the apparent universality of the outer HI profile would provide a strong constraint on models of gas accretion and disk formation. The paper is careful in several respects: it uses total-power FAST data that are not affected by short-spacing problems, it calibrates the beam-smoothing bias using high-resolution combined images, and it provides detailed tables of the measured radii and the median profile. These strengths make the paper valuable for future deep HI surveys with MeerKAT, ngVLA, and SKA. However, the physical interpretation rests on two assumptions that need scrutiny: the thin-disk deprojection at radii well beyond the geometry-defining isophote, and the use of R001 as a normalization, which forces all profiles through one common point by construction.","major_comments":[{"comment":"The interpretation in terms of gas inflow rates (Figure 6) is speculative and goes beyond the data. The paper does not directly measure gas flows; it infers them from the correlation between R001/R1 and MHI/M*. The discussion of 'faster inflow from R001 to R1' is a plausible scenario but is not uniquely constrained by the observations. I recommend softening the language in Section 4 (e.g., 'may indicate', rather than 'implies') and explicitly stating that the kinematic evidence is not yet available. This is a presentation issue, but it affects how the results will be used by the community.","section":"Section 4 (Discussion)"}],"minor_comments":[{"comment":"The abstract states that the R001-MHI relation has a scatter of 0.02 dex, but the text in Section 3.2 notes that the standard deviation around the relation is 0.02 dex, while the intrinsic scatter is <0.001. Please be consistent in terminology ('scatter' vs. 'intrinsic scatter') throughout the paper.","section":"Abstract"}],"recommendation":"major_revision","confidential_remarks":"The paper presents valuable new data and a potentially important relation, but the two main claims (universal profile and tight R001-MHI relation) depend on the thin-disk deprojection assumption at radii where warps and flares are common. The authors should be encouraged to provide a quantitative sensitivity test, even if it is approximate. The circularity concern about R001 normalization should be addressed explicitly, as it will be raised by many readers. The paper is likely to be highly cited and is a good fit for the journal, but it is not acceptable in its present form."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Best quick take: this is the first statistical sample of HI radial profiles at 0.01 M_sun pc^-2, and the new R001-MHI relation is a solid, useful result. The paper deserves peer review.\n\nWhat's new: FEASTS total-power images reach three times lower column density than typical interferometry, and the authors use them to measure HI radii for 35 galaxies. The R001-MHI relation has a scatter of 0.02 dex and a slope near 0.5, consistent with the older R1-MHI relation. The R001/R1 anti-correlation with MHI/M* survives a partial-correlation test (R=-0.55, p=0.01). These are real advances over Wang et al. (2016).\n\nWhat it does well: the beam-smoothing simulations in Appendix B are the right way to check size measurements at FAST resolution. The paper flags the thin-disk deprojection caveat itself, and it clearly separates strongly interacting galaxies from the main analysis.\n\nSoft spots: the outer deprojection assumption is the load-bearing one. R001 sits around 2R1, well beyond the 10^20 contour that sets the ellipse geometry. Warps and flares are common past R1, and a few degrees of inclination error can shift R001 and flatten or steepen the outer profile. The paper mentions this for dwarfs but doesn't quantify it across the sample; the cited W24 and D. Yang results are qualitative. That means the claimed 'universal profile' scatter of 0.2 dex is probably a lower bound on real structural diversity.\n\nAlso, the R001-normalized alignment is partly built in: define R001 as where every profile crosses the same surface density, and the profiles are guaranteed to meet at r/R001=1. The tighter scatter in that normalization is expected, not a discovery. The claim that the scatter is ~0.2 dex at all radii is mostly a claim about outer-slope similarity, and that is exactly where the deprojection uncertainty bites hardest.\n\nThe abstract says sSFR correlates strongly with R001/R1, but the paper's own partial-correlation test reduces that to R=-0.23, p=0.30 after controlling for distance. I would soften the abstract.\n\nWould I cite it? Yes, the R001-MHI relation is a handy benchmark. Bring to reading group? Yes, for a discussion of what 'universal profiles' really mean. Referee? Definitely, the measurements are worth scrutiny, and the concerns are addressable with more careful geometry tests.","headline":"A genuinely deeper HI survey and a tight new R001-MHI relation, with an outer-disk geometry assumption that needs scrutiny before the 'universal profile' claim is trusted.","tokens_in":19532,"tokens_out":2783,"would_cite":true,"duration_ms":28820,"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":"Deep 21-cm maps show galaxy HI disks are self-similar down to 0.01 M_sun/pc2, with a deep edge predictable from total HI mass to 0.02 dex.","keywords":["neutral hydrogen","HI 21-cm line","galaxy disks","HI surface density","size-mass relation","self-similarity","gas accretion","deep HI imaging"],"falsifier":"Take a few of the 35 galaxies, including two dwarfs, and observe their outer HI with high resolution and full kinematic information to measure the vertical scale height and warp of the 0.01 $M_\\odot$ pc$^{-2}$ emission. If that emission is distributed in a vertically thick or warped layer rather than a flat disk, the thin-disk deprojection underpinning $R_{001}$ fails and the claimed 0.02-dex $R_{001}$-$M_{\\rm HI}$ relation and universal profile would need revision. A simpler check is to recompute $R_{001}$ from full 3D kinematic models of the same galaxies and see whether the 0.02-dex scatter persists.","tokens_in":18406,"feed_emoji":"🌌","tokens_out":10001,"duration_ms":87816,"temperature":0.7,"pith_summary":"Using deep 21-cm images that reach surface densities a hundred times below the usual 1 $M_\\odot\\,\\mathrm{pc}^{-2}$ level, this paper maps the neutral-hydrogen disks of 35 nearby, not-too-inclined galaxies. It finds that, once the radius is scaled by the radius $R_{001}$ where the HI surface density falls to 0.01 $M_\\odot\\,\\mathrm{pc}^{-2}$, every galaxy's outer disk follows nearly the same profile, with a scatter of about 0.2 dex and an exponential scale-length of $0.11R_{001}$. It also establishes a tight relation between $R_{001}$ and total HI mass, with slope about 0.5 and scatter 0.02 dex. If correct, the deep outer HI disk is a self-similar structure set by the galaxy's own HI reservoir, not by stellar mass, halo mass, or the group environment; only strong tidal interactions break the pattern. These results sharpen constraints on how gas is accreted and recycled in galaxy disks.","feed_headline":"One radius predicts every galaxy's deep HI disk to 0.02 dex","feed_subtitle":"New deep 21-cm maps show HI disks are self-similar 100x deeper than before, sharpening gas-inflow tests.","key_machinery":"The central object is the iso-density radius $R_{001}$, the radius at which the HI surface density reaches 0.01 $M_\\odot\\,\\mathrm{pc}^{-2}$, measured by interpolating deprojected elliptical-annulus profiles from total-power 21-cm images. Normalizing radii by $R_{001}$ collapses all non-tidally-disturbed profiles onto one universal curve, and its tight relation with $M_{\\rm HI}$ (slope about 0.5) becomes the new size-mass law. The analytical fit to the median profile uses the double-exponential form $y=\\log\\left(\\frac{I e^{-x/r_{s1}}}{1+(I/J-1)e^{-x/r_{s2}}}\\right)$, with $r_{s1}=0.114R_{001}$, along with a cored power-law alternative. The deprojection assumes a thin circular disk whose geometry is set by the $10^{20}$ cm$^{-2}$ contour.","core_discovery":"The paper claims that at surface densities 100 times lower than previously probed, the neutral hydrogen disks of galaxies reorganize into a universal shape. Its central discovery is a new characteristic radius $R_{001}$, the radius at which $\\Sigma_{\\rm HI}=0.01\\,M_\\odot\\,\\mathrm{pc}^{-2}$ (about $10^{18.1}$ cm$^{-2}$). When radial profiles are plotted against $r/R_{001}$, the 35 galaxies, excluding the most strongly interacting, collapse onto one median profile with approximately 0.2 dex scatter; the outer part is exponential with scale-length $0.11R_{001}$. The paper further derives an $R_{001}$-$M_{\\rm HI}$ relation, $\\log R_{001}=0.49\\log M_{\\rm HI}-3.11$, with 0.02 dex scatter and essentially the same slope as the known $R_1$-$M_{\\rm HI}$ relation, but shifted outward by a factor of about two. The ratio $R_{001}/R_1$ anti-correlates with the HI-to-stellar mass ratio (Pearson $R=-0.66$, $p=0.00$) and with specific star formation rate, but not with stellar mass, HI mass, dark-matter mass, or star formation rate; partial-correlation tests indicate the $M_{\\rm HI}/M_*$ relation is the robust one. The authors interpret this as evidence that physical processes cooperate to keep the outer HI disk self-similar, and that gas-rich galaxies transport HI inward from $R_{001}$ to $R_1$ more efficiently, so their disks grow faster near $R_1$.","pith_inferences":["A testable extension: push the same analysis toward the $10^{17}$ cm$^{-2}$ regime, the Lyman-limit boundary. If self-similarity continues, the $R_{001}$-$M_{\\rm HI}$ slope should stay near 0.5 and the outer exponential should persist; a break would locate the physical edge set by the ionizing background or CGM pressure.","The tight $R_{001}$-$M_{\\rm HI}$ relation implies that the deep HI edge of a galaxy could be predicted from $M_{\\rm HI}$ alone, which could serve as a prior for classifying Lyman-limit absorption systems or planning absorption-line observations toward background quasars.","For strongly interacting galaxies, the roughly 1.4 times excess in $R_{001}$ at fixed $M_{\\rm HI}$ and the flattened profiles offer a potential single-parameter diagnostic of tidal disturbance, applicable to larger samples to identify recent mergers from HI morphology alone."],"forward_implications":["A galaxy's deep HI edge can be estimated from its total HI mass alone, to 0.02 dex, without needing deep imaging; this gives observers a direct target for where to look for accretion and stripping.","Any galaxy-formation model must reproduce a universal, $R_{001}$-normalized outer HI profile with about 0.2 dex scatter and outer scale-length $0.11R_{001}$, independent of stellar mass, halo mass, or star formation rate.","The $R_{001}/R_1$ anti-correlation with HI-to-stellar mass ratio means gas-richer galaxies concentrate their outer HI more steeply, implying inward transport from $R_{001}$ to $R_1$ is faster in those systems, a quantitative test for gas-regulator or bathtub models.","Most HI mass lies within $R_1$ (less than 20% outside), so the deep HI distribution is primarily a diagnostic of the disk-CGM interface and gas flows rather than a significant hidden mass reservoir.","Strong tidal interactions flatten the outer HI profile and inflate $R_{001}$ by about 1.4 times at fixed $M_{\\rm HI}$, so the universal relation holds only for galaxies without major interactions, and deviations flag interaction state."],"supporting_citations":[{"why":"Supplies the earlier $R_1$-$M_{\\rm HI}$ relation and $R_1$-normalized median profile that this paper extends to 100 times lower surface density, plus the radius-measurement procedure used here.","marker":"Wang et al. (2016)"},{"why":"Defines the FEASTS survey, its data reduction, and the deep total-power HI images that enable the 0.01 $M_\\odot$ pc$^{-2}$ measurements.","marker":"W23"},{"why":"Establishes the data depth (3-sigma about $10^{17.7}$ cm$^{-2}$) and identifies strongly interacting galaxies, while also providing the 3D deblending approach.","marker":"W24"},{"why":"Provides the z0MGS catalog of stellar masses, star formation rates, and distances used for the correlations and sample characterisation.","marker":"Leroy et al. (2019)"},{"why":"Supplies group membership, group masses, and virial radii used for the $r_{\\rm vir}$ normalization and halo-mass checks.","marker":"Kourkchi & Tully (2017)"},{"why":"Provides the double-exponential analytic form that the paper fits to the median $R_{001}$-normalized profile.","marker":"Wang et al. (2014)"},{"why":"Provides the 3D watershed deblending pipeline used to separate HI emission in interacting pairs so that isolated galaxy profiles can be measured.","marker":"Huang et al. (2024)"},{"why":"Supplies the emcee sampling method used to estimate slopes, intercepts, and intrinsic scatter of the $R_{001}$-$M_{\\rm HI}$ relation.","marker":"Foreman-Mackey et al. (2013)"}],"fun_headline_variants":["Deep HI maps reveal universal disk shape 100x deeper","New radius R_001 unifies HI disks of nearby galaxies","HI disks self-similar down to 0.01 Msun/pc^2","One deep radius predicts galaxy HI disk to 0.02 dex","FAST survey: HI disks collapse onto universal profile"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The load-bearing premise is that the low-column-density HI seen down to 0.01 $M_\\odot$ pc$^{-2}$ lies in a thin, circular, coplanar disk whose inclination is set by the $10^{20}$ cm$^{-2}$ contour; if the outer HI is instead a thick, warped, or extraplanar layer, most plausibly in dwarf irregulars, the deprojected surface densities, $R_{001}$, and the claimed universal profile would be systematically biased.","fun_headline_variants_meta":{"raw":{"variants":["Deep HI maps reveal universal disk shape 100x deeper","New radius R_001 unifies HI disks of nearby galaxies","HI disks self-similar down to 0.01 Msun/pc^2","One deep radius predicts galaxy HI disk to 0.02 dex","FAST survey: HI disks collapse onto universal profile"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000686,"raw_usage":{"total_tokens":3306,"prompt_tokens":1333,"completion_tokens":1973,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":949,"completion_tokens_details":{"reasoning_tokens":1884}},"tokens_in":949,"tokens_out":1973,"duration_ms":14132,"temperature":1.0,"reasoning_tokens":1884,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-10T22:30:52.142579+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Take a few of the 35 galaxies, including two dwarfs, and observe their outer HI with high resolution and full kinematic information to measure the vertical scale height and warp of the 0.01 $M_\\odot$ pc$^{-2}$ emission. If that emission is distributed in a vertically thick or warped layer rather than a flat disk, the thin-disk deprojection underpinning $R_{001}$ fails and the claimed 0.02-dex $R_{001}$-$M_{\\rm HI}$ relation and universal profile would need revision. A simpler check is to recompute $R_{001}$ from full 3D kinematic models of the same galaxies and see whether the 0.02-dex scatter persists.","supporting_citations":[{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Supplies group membership, group masses, and virial radii used for the $r_{\\rm vir}$ normalization and halo-mass checks."},{"cited_title":"2014, , 441, 2159","cited_arxiv_id":null,"evidence_quote":"Provides the double-exponential analytic form that the paper fits to the median $R_{001}$-normalized profile."},{"cited_title":"WALLABY Pilot Survey: Star Formation Enhancement and Suppression in Gas-rich Galaxy Pairs","cited_arxiv_id":"2410.22406","evidence_quote":"Provides the 3D watershed deblending pipeline used to separate HI emission in interacting pairs so that isolated galaxy profiles can be measured."}],"review_version":1}