{"id":"ea1d16db-cd72-46ae-b76f-75a61abd4013","arxiv_id":"1908.02945","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":7.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":2,"one_line_summary":"A new adaptive phase-space definition of diffuse stellar material in simulations shows the intra-halo stellar mass fraction rises with system mass and is most strongly anti-correlated with the central galaxy's V/sigma at Milky Way scales.","lead":"Astronomers used a new phase-space galaxy-finder on the Horizon-AGN simulation to identify the diffuse intra-halo stars that belong neither to a galaxy nor its satellites, from Milky Way size systems to galaxy clusters. They find the diffuse stellar fraction grows with total system mass, and the central galaxy's rotation versus random motion is the strongest predictor of this fraction in Milky Way-like systems.","discovery_kind":"new_method","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Claimed 2 dex scatter at M*~1e11 may be dominated by particle shot noise; the paper never subtracts the N_IHSC < 100 floor before quoting scatter.","rationale":"The reader's verdict (CONDITIONAL) is appropriate, but the single most load-bearing concern is not the flx,6D calibration, because the paper itself demonstrates in Fig. 3 that the broad mass trend and slopes are robust to the phase-space threshold. The more serious internal weakness is the unquantified particle-noise floor at low IHSC masses. The paper's central quantitative claim is the 2 dex scatter at M*_tot ~ 1e11 Msun and its decrease to 0.3 dex; with a stellar particle mass of ~3e6 Msun, the low-f end of the quoted scatter corresponds to a handful of particles, where Poisson fluctuations alone produce order-dex scatter. This affects not only the scatter measurement but also the V/sigma anti-correlation, since the high-V/sigma branch sits at f < 0.1%, i.e., below ~30 particles at M*_tot ~ 1e11. The proposed test -- a simple N_IHSC >= 100 cut and a comparison to the Poisson floor -- would settle whether the headline scatter and the V/sigma trend survive. The paper deserves credit for a new adaptive phase-space definition, explicit discussion of resolution limits at low masses, and transparent presentation of the parameter dependence; these strengths mean the concern warrants a conditional revision rather than rejection.","tokens_in":28074,"tokens_out":4172,"duration_ms":49577,"concrete_test":"Restrict the z=0 sample to systems with N_IHSC >= 100 in each M*_tot bin, recompute the 16th-84th percentile scatter, and compare it to the expected Poisson scatter sigma_poisson ~ sqrt(N_IHSC)/N_IHSC in log10 f. If the low-mass scatter drops from ~2 dex to <1 dex, the headline scatter is largely a resolution artifact; also rerun the V/sigma sub-sample medians under the same N_IHSC cut to test whether the anti-correlation persists.","verdict_should_be":"UNCHANGED","load_bearing_attack":"Horizon-AGN stellar particles have mass ~3e6 Msun. At M*_tot ~ 1e11 Msun, the quoted scatter spans f_M*,IHSC ~ 1e-4 to ~1e-2, corresponding to roughly 3 to 300 IHSC particles per system; the 2 dex spread is therefore close to the range set by counting statistics. The blue dashed/dot-dashed lines in Fig. 2 mark N_IHSC = 100 and 10, but the paper does not quantify the Poisson noise floor or subtract it before attributing the scatter to growth mechanisms in Section 3. At f < 0.001, individual systems have only tens of particles, so the low-f tail and the high-V/sigma branch of Fig. 9 (with f < 0.1%) rest on measurements where shot noise and discreteness are order unity. The paper acknowledges resolution-induced diagonal patterns only below M*_tot ~ 1e10 Msun (Section 3), but the same discreteness contaminates low-f measurements at higher masses. Without a noise-floor correction, the headline quantitative claims -- 'scatter decreasing from 2 dex at M*_tot ~ 1e11 to 0.3 dex at group masses' and the V/sigma anti-correlation at low f -- are not securely established. The flx,6D calibration issue flagged by the reader is real but less load-bearing, because Fig. 3 shows the qualitative mass dependence and normalization-independent slopes survive changes in flx,6D; the shot-noise floor directly undermines the paper's central quantitative result.","agreement_with_reader":"partial"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper introduces an adaptive phase-space definition of the diffuse stellar component, which the authors call the Intra-Halo Stellar Component (IHSC), and applies it to the Horizon-AGN cosmological hydrodynamical simulation. The IHSC is defined as stellar particles not linked to galaxies by a 6D friends-of-friends search, so it represents the kinematically hot, diffuse stellar background around galaxies, groups, and clusters. Using this definition, the paper reports the z = 0 f_M*,IHSC-M*_tot relation, characterizing its slope, normalization, and the mass dependence of its scatter; it then studies correlations of the scatter with the number of satellites, the central-to-satellite mass ratio, the central galaxy's V/sigma, and sSFR. The paper also follows individual systems through cosmic time with merger trees, distinguishing quiescent and merger-rich evolutionary paths and discussing the tentative behavior of galaxy clusters. The central quantitative claims are that f_M*,IHSC increases with total stellar mass on average, that the scatter decreases from roughly 2 dex at M*_tot ~ 1e11 Msun to about 0.3 dex at group masses, and that V/sigma of the central galaxy is the strongest predictor of f_M*,IHSC at fixed mass for M*_tot below about 1e12 Msun.","tokens_in":28388,"tokens_out":5901,"duration_ms":62337,"significance":"If the results are robust, the paper provides a useful unifying framework: the same adaptive, shape-independent definition applies to stellar halos, intra-group light, and intracluster light, which is a genuine improvement over fixed or variable spherical apertures. The stability of the M_IHSC-M_sats power-law slopes across flx,6D = 0.1-0.8 is a credible and falsifiable prediction, and the V/sigma anti-correlation at Milky-Way and group masses gives observers a concrete kinematic proxy for the diffuse stellar mass fraction. The authors are also transparent about the parameter choice that sets the normalization and about the limited cluster sample. The main significance risk is quantitative: part of the reported scatter and of the low-f_M*,IHSC population rests on systems with very few IHSC particles, and the paper does not quantify this counting-noise floor before presenting its headline numbers.","major_comments":[{"comment":"The paper quotes a scatter of about 2 dex at M*_tot around 1e11 Msun and uses the low-f_M*,IHSC tail (f < 0.1%) to support the V/sigma anti-correlation, but at these masses a large part of that range contains only tens of IHSC particles. With the stated stellar particle mass of ~3e6 Msun, f_M*,IHSC = 1e-4 at M*_tot = 1e11 Msun corresponds to about 3 particles, and f_M*,IHSC = 1e-3 at M*_tot = 3e10 Msun corresponds to about 10 particles. The blue dashed and dot-dashed lines in Fig. 2 mark the N_IHSC = 100 and N_IHSC = 10 boundaries, but these boundaries are not applied as cuts and no Poisson or jackknife uncertainty in f_M*,IHSC is propagated into the scatter or into the sub-sample medians in Figs. 7-10. The text identifies resolution effects only at M*_tot < 1e10 Msun, yet the same discreteness affects low-f systems at higher masses. Please add a quantitative noise-floor analysis: for example, compute Monte Carlo uncertainties from the IHSC particle counts, show how the scatter-versus-mass trend and the Fig. 9 medians change when N_IHSC >= 100 or N_IHSC >= 1000 cuts are imposed, and verify explicitly that the 2-dex-to-0.3-dex claim and the V/sigma anti-correlation survive such cuts.","section":"Section 3, Fig. 2; Section 4.3, Fig. 9"},{"comment":"The parameter flx,6D = 0.4 is adopted in Section 3.1 specifically because it makes the simulated f_M*,IHSC-M* relation agree with the observed stellar-halo and ICL fractions of Merritt et al. (2016), Harmsen et al. (2017), and Morishita et al. (2017). Given that, the statement in Section 3.2 that the method 'predicts' IHSC mass fractions in agreement with observations is circular for the normalization: those anchor points are matched by construction. The shape of the relation, the slope of M_IHSC-M_sats in Fig. 4, and the V/sigma-dependent ordering in Fig. 9 are genuine predictions, but the absolute normalization is not. Please state this distinction explicitly and avoid phrasing that presents the agreement at the calibration points as an independent success.","section":"Section 3.1 and Section 3.2"}],"minor_comments":[{"comment":"The caption contains two typographical errors: 'as labellled' should read 'as labelled', and 'the medians of each sub-sample do nor overlap' should read 'do not overlap'.","section":"Fig. 9 caption"},{"comment":"The word 'fracton' in the caption text should be 'fraction'.","section":"Section 4.2, Fig. 8 caption"},{"comment":"The sentence 'At z = 2.12 these systems have total stellar masses between M*_tot~1e10 and 1e10.5 and a f_M*,IHSC ~0.03' is slightly ambiguous; clarify that the ~0.03 value refers to the same redshift.","section":"Section 5.1.1"},{"comment":"The discussion of the 2D versus 3D V/sigma difference is noted in the text, but it would help the observational reader to state whether the reported V/sigma values are the 3D values and to indicate the expected size of the projection-induced scatter.","section":"Section 4.3"}],"recommendation":"major_revision","confidential_remarks":"The authors are honest about the flx,6D calibration and about the limited cluster sample; the referee's main concern is the unquantified particle-counting floor in the low-f regime. If the authors can show that the scatter trend and the V/sigma correlation survive N_IHSC cuts, and if they relabel the normalized comparisons as calibrated rather than predicted, the paper should be publishable."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"The useful thing here is the method. Defining the Intra-Halo Stellar Component as the kinematically hot material that 6DFOF phase-space linking leaves unassigned to galaxies is genuinely adaptive and shape-independent, and the paper applies it across roughly three decades in halo mass in one simulation. Two results earn credit: the M_IHSC-M_sats power-law slope is nearly independent of the linking parameter flx,6D, and V/sigma of the central galaxy is the strongest scatter driver of the IHSC fraction at fixed stellar mass below 10^12 Msun. The paper is also careful in several respects—it flags resolution with particle-number lines, tests flx,6D from 0.1 to 0.8, compares against spherical apertures, and follows individual merger histories. The cluster results are explicitly tentative, which is honest.\n\nThe soft spots are two. First, the reader's circularity flag is real: flx,6D = 0.4 is adopted because it makes the predicted f-M* relation match the observed stellar halo and ICL fractions. The text says so plainly in Section 3.1, but the abstract and conclusions still present the resulting normalization as a prediction. It is a calibration. That said, the qualitative mass dependence and the power-law slopes survive changes in flx,6D, so this is a moderate concern, not a fatal one.\n\nSecond, the stress-test concern lands harder. The quoted 2 dex scatter at M* ~ 10^11 Msun is probably inflated by particle shot noise. Horizon-AGN stellar particles are ~3e6 Msun, so at f ~ 1e-4 you are working with a handful of particles. The blue lines in Fig. 2 mark 100 and 10 particles, but the paper never subtracts a Poisson floor before attributing the scatter to growth mechanisms in Section 3. The low-f tail and the high-V/sigma branch of Fig. 9 sit exactly where discreteness is order unity. The qualitative V/sigma anti-correlation may survive subtraction, but the headline claim—scatter decreasing from 2 dex to 0.3 dex—is not secure as presented.\n\nThis is not a fatal flaw in the method. The parameter-sensitivity tests, the comparison to spherical apertures, and the stability of the mass relations make a convincing case that the phase-space definition is better behaved across mass. The paper deserves peer review, but revision should quantify and subtract the shot-noise floor, present the flx,6D calibration as calibration, and add error bars to fits and sub-sample medians. I'd bring this to reading group, and I'd cite it if I worked on stellar halos or ICL.","headline":"Adaptive phase-space definition of diffuse stellar material is a real new tool; the paper's quantitative scatter claims need a shot-noise subtraction before they can stand.","tokens_in":28990,"tokens_out":1935,"would_cite":true,"duration_ms":20183,"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":"Using an adaptive phase-space definition, the diffuse stellar component can be measured consistently from Milky Way-mass halos to galaxy clusters; its mass fraction grows with total stellar mass and, below group masses, tracks the central…","keywords":["intra-halo stellar component","stellar halo","intracluster light","cosmological hydrodynamical simulations","phase-space galaxy finder","galaxy kinematics","galaxy mergers","galaxy formation"],"falsifier":"Measure $V/\\sigma$ and the diffuse stellar fraction for a sample of $M_{*,\\mathrm{tot}}\\sim10^{11}\\,M_\\odot$ galaxies: the paper predicts that rotation-supported galaxies stay below $f_{M_*,\\mathrm{IHSC}}\\sim0.1\\%$, so finding many high-$V/\\sigma$ systems with diffuse fractions above 1% would falsify the claimed anti-correlation.","tokens_in":27788,"feed_emoji":"🌌","tokens_out":10966,"duration_ms":110501,"temperature":0.7,"pith_summary":"This paper introduces and applies an adaptive, shape-independent way to identify the diffuse stellar component in cosmological hydrodynamical simulations: the collection of stars too kinematically hot and phase-space distant to belong to any galaxy. Using this Intra-Halo Stellar Component (IHSC) definition in the Horizon-AGN simulation, it claims that the IHSC mass fraction grows with total stellar mass on average, while its scatter shrinks from roughly two orders of magnitude at Milky Way masses to about 0.3 dex at group masses. The main physical driver of that scatter below $\\sim10^{12}\\,M_\\odot$ is the central galaxy's kinematic morphology: rotation-supported galaxies have IHSC fractions below 0.1%, whereas dispersion-supported systems reach about 5%. If correct, this gives observers a measurable proxy, $V/\\sigma$, for the assembly history encoded in diffuse starlight, and unifies the study of stellar halos and intracluster light under one definition.","feed_headline":"Use galaxy spin to predict its diffuse halo mass","feed_subtitle":"Rotation-supported galaxies keep under 0.1% of their stars diffuse; dispersion-supported systems reach about 5%.","key_machinery":"The central object is an adaptive phase-space Friends-of-Friends search that identifies galaxies as six-dimensional phase-space overdensities. The IHSC is defined as all stellar particles not linked to any galaxy in that search, meaning kinematically hot, diffuse background stars and tidal debris. The user-set parameter $f_{l_x,6D}=0.4$ sets the phase-space density threshold separating galaxy from IHSC, and the paper shows that the shapes and slopes of the resulting mass relations are robust to this choice, with only the normalization shifting. The observational proxy that carries the physical argument is the central galaxy's $V/\\sigma$, the ratio of rotational velocity to velocity dispersion, because it encodes merger history in a single measurable number.","core_discovery":"The central discovery is that the same adaptive phase-space criterion identifies the diffuse stellar component across the full mass range, and that the resulting $f_{M_*,\\mathrm{IHSC}}$ versus $M_{*,\\mathrm{tot}}$ relation has a mass-dependent scatter with two distinct regimes. For $M_{*,\\mathrm{tot}}<10^{12}\\,M_\\odot$, the diffuse fraction is strongly anti-correlated with the central galaxy's stellar rotation-to-dispersion ratio $V/\\sigma$; high-$V/\\sigma$ galaxies sit at $f_{M_*,\\mathrm{IHSC}}<0.1\\%$, while low-$V/\\sigma$ galaxies reach about 5%, and this correlation is the strongest among all halo and galaxy properties examined. At $M_{*,\\mathrm{tot}}>10^{12}\\,M_\\odot$, all centrals are dispersion-supported, and the diffuse fraction instead tracks the dynamical state of the system, quantified by the mass ratio of the largest satellite to the central galaxy; cluster-scale systems reach 10 to 20 percent by $z\\sim1$ and then evolve weakly, though the paper cautions that the cluster sample is small. The paper interprets the low-mass regime as reflecting the diversity of accretion histories: quiescent, rotation-supported galaxies simply do not have much stripped stellar material, while dispersion-supported galaxies have experienced active merger histories. A further robustness claim is that the power-law slopes relating IHSC mass, central-galaxy mass, and satellite stellar mass are essentially independent of the user-set phase-space density threshold; only the normalization changes.","pith_inferences":["If the $V/\\sigma$ correlation survives in observational samples, surveys that already measure galaxy kinematics could statistically infer halo mass fractions without directly detecting the faint diffuse light.","The same phase-space definition could be applied to other cosmological hydrodynamical simulations; a testable prediction is that the near-unity slope of the IHSC mass versus satellite mass relation is robust across feedback implementations, while the normalization carries information about the subgrid physics.","The paper's tentative flat evolution at cluster scales suggests a concrete forecast: with dozens of simulated clusters, the intracluster-light fraction should remain within 10 to 20 percent with no monotonic redshift trend, whereas strong evolution would indicate missing physics or a resolution effect.","Current Milky Way-mass observational samples are dominated by rotationally supported galaxies, so the apparent tension with the simulated relation may disappear once ellipticals with similar stellar masses are measured and found to have systematically higher diffuse fractions."],"forward_implications":["At a fixed total stellar mass below $10^{12}\\,M_\\odot$, measuring the central galaxy's $V/\\sigma$ predicts the diffuse stellar fraction: high rotation means under 0.1 percent, low rotation about 5 percent.","The large observed scatter in stellar-halo mass fractions of Milky Way-like galaxies, about 2 dex, is not pure noise but encodes distinct accretion histories, and should correlate with kinematic morphology once dispersion-supported galaxies are included.","At group and cluster scales the diffuse component reaches 10 to 20 percent of all stars by $z\\sim1$ and then stays nearly constant, so the intracluster-light fraction should be a weak function of cluster mass at late times; the paper flags this as tentative because of the small simulated cluster sample.","Because the slope between IHSC mass and satellite stellar mass is about unity and is independent of the phase-space threshold, observational measurements made with different surface-brightness limits should recover the same slope and differ mainly in normalization."],"supporting_citations":[{"why":"Supplies the six-dimensional phase-space galaxy-finding algorithm whose unlinked particles define the IHSC.","marker":"Cañas et al. (2019)"},{"why":"Provides the Horizon-AGN simulation used for all measurements.","marker":"Dubois et al. (2014)"},{"why":"Observed stellar-halo mass fractions for Milky Way-like galaxies used to calibrate the phase-space density threshold and to compare the relation.","marker":"Merritt et al. (2016)"},{"why":"Independent stellar-counts measurements of halo fractions used as a second calibration and comparison point.","marker":"Harmsen et al. (2017)"},{"why":"Observed intracluster-light fractions that set the high-mass normalization of the relation.","marker":"Morishita et al. (2017)"},{"why":"Introduces the original phase-space structure finder that the improved galaxy finder builds on.","marker":"Elahi et al. (2011)"},{"why":"Shows that dry mergers are the main route to low-$V/\\sigma$ galaxies, linking kinematic morphology to the merger histories invoked to explain the IHSC fraction.","marker":"Lagos et al. (2018b)"},{"why":"Provides the accreted stellar-mass-fraction relation whose shape the $f_{M_*,\\mathrm{IHSC}}$-$M_*$ relation is compared with and interpreted through.","marker":"Rodriguez-Gomez et al. (2016)"}],"fun_headline_variants":["Spin predicts diffuse stellar halo share","Galaxy rotation sets its halo's diffuse stars","V/sigma anti-correlates with diffuse halo mass","Diffuse stellar fraction: a galaxy spin story"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The load-bearing premise is that the chosen phase-space density threshold identifies the same physical component that observers measure as stellar halos and intracluster light, because the threshold is calibrated to match those observations rather than derived independently.","fun_headline_variants_meta":{"raw":{"variants":["Spin predicts diffuse stellar halo share","Galaxy rotation sets its halo's diffuse stars","V/sigma anti-correlates with diffuse halo mass","Diffuse stellar fraction: a galaxy spin story"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000308,"raw_usage":{"total_tokens":1964,"prompt_tokens":1350,"completion_tokens":614,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":966,"completion_tokens_details":{"reasoning_tokens":556}},"tokens_in":966,"tokens_out":614,"duration_ms":7791,"temperature":1.0,"reasoning_tokens":556,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-14T14:29:04.688079+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Measure $V/\\sigma$ and the diffuse stellar fraction for a sample of $M_{*,\\mathrm{tot}}\\sim10^{11}\\,M_\\odot$ galaxies: the paper predicts that rotation-supported galaxies stay below $f_{M_*,\\mathrm{IHSC}}\\sim0.1\\%$, so finding many high-$V/\\sigma$ systems with diffuse fractions above 1% would falsify the claimed anti-correlation.","supporting_citations":[],"review_version":1}