{"id":"95462486-ce3a-4ac6-acb3-f5291042580a","arxiv_id":"2504.12863","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":5.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":1,"one_line_summary":"In both the GAEA semi-analytic model and the TNG hydrodynamical simulation, roughly half of galaxies entering Virgo-mass clusters never experience ram pressure strong enough to strip most of their gas on the first passage.","lead":"This paper asks what happens to a galaxy's gas when it first falls into a galaxy cluster. Using two different simulations, it finds that many galaxies, especially in smaller clusters, keep a large share of their cold gas and can continue forming stars after their first close pass.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"TNG and GAEA binding pressures are not computed on the same components, so the comparison used to dismiss TNG's stronger stripping may rest on an artifact.","rationale":"The reader's weakest assumption points to the binding-pressure profiles and the unproven attribution of TNG's behavior to numerical artifacts. My reading agrees that this is the central soft spot, but I locate the problem more concretely: the TNG and GAEA pressure estimates are not constructed from the same mass components. The TNG method includes hot gas in the restoring mass while GAEA does not. Since hot gas is extended, its inclusion artificially raises Pgrav at large radii, precisely the regime where the paper reports a TNG/GAEA discrepancy. The paper then uses that discrepancy to argue that TNG's stronger gas removal is artificial. If the discrepancy disappears after aligning the definitions, the independent support from TNG weakens substantially. The headline claim may still be correct, but it would rest on GAEA alone, so a conditional verdict remains appropriate. I do not see an internal inconsistency in the phase-space fits or the orbit tracing; the concern is specifically about the comparability of the two model measurements and the role that comparison plays in the final argument.","tokens_in":18523,"tokens_out":13140,"duration_ms":143387,"concrete_test":"Recompute the TNG Pgrav profiles shown in Figure 1 at z~0.5 using only cold gas and stars in Sigma_gs and Mgs, matching the GAEA definition, and overlay them on the GAEA curves. If the TNG profiles at large radii drop to or below the GAEA values, the paper's higher-TNG-Pgrav result and the artificial-RPS argument are artifacts; if they remain higher, the argument is supported.","verdict_should_be":"UNCHANGED","load_bearing_attack":"In Section 3.1, the paper compares gravitational binding pressures from GAEA and TNG and finds that TNG profiles are higher at large radii. In Section 4, this is combined with the fact that TNG cluster galaxies are gas-poorer to argue that RPS in TNG is artificially enhanced and that GAEA's longer stripping timescales are preferred. This argument is load-bearing because the central claim that most cluster galaxies retain gas after the first pericentric passage is presented as robust largely because GAEA and TNG agree. The comparison, however, is not apples-to-apples. For GAEA, Equation (1) uses the surface density of cold gas and stars as the gravitating component. For TNG, the text states that the projected surface density includes 'gas (including hot and cold gas)' in the same equation, and Equation (3) uses the total gas plus stellar mass within radius r. Hot gas is diffuse and vertically extended, so including it in the thin-disk restoring-pressure formula inflates Pgrav, especially at large radii, which is exactly where the TNG profiles are reported to be higher. If the TNG/GAEA gap is mostly due to this component mismatch, the artificial-enhancement argument collapses: TNG could be gas-poor simply because its baryonic restoring pressure is lower than GAEA's once hot gas is excluded. The independent support from TNG for the headline conclusion would then disappear, leaving a single semi-analytic model with its own assumptions behind the claim.","agreement_with_reader":"partial"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper uses the semi-analytic model GAEA and the Illustris TNG simulation to ask whether cluster galaxies are strongly ram-pressure stripped during their first pericentric passage. The authors compute gravitational binding pressure profiles for galaxies in three stellar mass bins at z~0.5 and z~1, derive retained cold-gas fractions as a function of ram pressure from GAEA alone, and use these to define strong, moderate, weak, and no RPS zones in cluster phase space, with analytic fits for the zone boundaries (Eqs. 4-6). Tracing satellite orbits from 2.5 Rvir to the first apocenter, they measure how long galaxies spend in each RPS zone. Their main conclusion is that in Virgo-like halos a large fraction of low-mass galaxies avoid strong RPS during the first passage in both GAEA and TNG, while in Coma-like halos strong RPS can strip low-mass galaxies but more massive galaxies retain 10-20% of their gas. They argue that the consistency between GAEA and TNG supports the view that hydrodynamical simulations may artificially enhance ram-pressure stripping.","tokens_in":18786,"tokens_out":8254,"duration_ms":84363,"significance":"If the conclusions hold, the paper is a useful counterweight to previous cosmological hydrodynamical-simulation studies that predict near-total gas removal at the first pericenter, and the analytic phase-space RPS-zone fits could be valuable for interpreting observational phase-space diagrams. The paper has concrete strengths: it validates the simplified ram-pressure calculation against a direct gas-cell method in TNG (Section 3.2, Fig. 4), tests the convergence of pericenter/apocenter measurements from interpolated orbits (Appendix A), and draws on two independent modeling techniques. However, the quantitative center of the paper is carried by GAEA: the retained-fraction curves are computed only from GAEA, and the RPS thresholds are calibrated using GAEA's gravitational binding pressure. The TNG comparison is currently qualitative and, as discussed below, is affected by an inconsistency in the way the binding pressure is computed in the two models. The central claim is plausible but needs a stronger and more consistent cross-model test before it can be regarded as robust.","major_comments":[{"comment":"The comparison of gravitational binding pressures between GAEA and TNG in Fig. 1 is not apples-to-apples. For GAEA, Eq. (1) is evaluated with Sigma_gs being the surface density of cold gas plus stars, whereas for TNG the text states that the projected surface density includes \"gas (including hot and cold gas)\" within radius r, and Eq. (3) uses the total gas plus stellar mass within r. Hot gas is diffuse and vertically extended, so including it in a thin-disk restoring-pressure formula inflates Pgrav at large radii, which is exactly where the TNG profiles are reported to be higher than GAEA in Fig. 1. This matters because Section 4 uses the combination of higher TNG Pgrav and gas-poorer TNG galaxies to argue that RPS in TNG is artificially enhanced and that GAEA's longer stripping timescales are preferred. If the TNG/GAEA gap is largely due to this component mismatch, that argument collapses and the paper loses the independent TNG support claimed in the abstract. I ask the authors to recompute the TNG Pgrav with a definition consistent with the GAEA calculation (for example, excluding hot gas from the gravitating surface density), or to demonstrate explicitly that the hot-gas contribution is negligible, and then to re-evaluate the Section 4 reasoning.","section":"Sec. 3.1, Eq. (1) and Sec. 4"},{"comment":"The RPS-zone thresholds (\"strong log Pram > -10.5,\" \"moderate -12 < log Pram < -10.5,\" etc.) are derived from GAEA's gravitational binding pressure profiles in Fig. 2, and the same thresholds are then used to classify first-passage orbits and to measure the time spent in each zone for both GAEA and TNG in Section 3.3 (Figs. 5-7). This is partially circular for GAEA and inconsistent for TNG, whose own Pgrav differs from GAEA's (Fig. 1). The headline claim that a substantial fraction of galaxies in Virgo-like halos did not suffer strong RPS in both GAEA and TNG therefore depends on thresholds calibrated with a single model. The retained-fraction curves, which are the physical basis for these thresholds, are also computed only from GAEA. I recommend that the authors quantify the sensitivity of the zone fractions to the assumed thresholds (for example, by varying log Pram by +/-0.5 dex) or, preferably, compute the strong-RPS fraction for TNG using TNG-specific Pgrav profiles and retained fractions. If the conclusion survives such a test, it should be stated with the new quantitative support.","section":"Secs. 3.1-3.3, Figs. 2 and 7"}],"minor_comments":[{"comment":"There are several typos: \"semi-analtyic\" in the Section 2.1 heading and \"hydro-simulation simulation TNG\" in the first paragraph of Section 4 should be corrected.","section":"Sec. 2.1 and Sec. 4"},{"comment":"The phrase \"within the r brand radius r90\" after Eq. (1) appears garbled; please clarify whether \"break radius\" or \"r-band radius\" is intended.","section":"Sec. 3.1"},{"comment":"The text refers to \"Equation: 6\" and \"Eqn 6\" but the zone fits are given by Eqs. (4)-(6); please correct the references and state the redshift and halo-mass range over which these fits are valid.","section":"Sec. 3.2 and Sec. 3.3"},{"comment":"The claim that GAEA and TNG phase-space ram-pressure distributions are \"consistent\" is not supported by a direct quantitative comparison; Fig. 3 shows GAEA and Fig. 4 shows TNG separately. An overlay or a statistical comparison of the two maps would strengthen the claim.","section":"Sec. 3.2"},{"comment":"The statement that \"more massive galaxies have similar orbits as lower-mass ones\" is not shown or quantified; a figure or a brief quantitative statement would make this assertion easier to verify.","section":"Sec. 3.3"}],"recommendation":"major_revision","confidential_remarks":"The central issue is the component mismatch in the TNG Pgrav calculation and the GAEA-calibrated thresholds, both of which are load-bearing for the claim of cross-model agreement. These are fixable in revision: the authors could recompute TNG Pgrav excluding hot gas, rerun the zone classification with TNG-specific thresholds, and present the sensitivity of their main fractions to reasonable threshold variations. I would not recommend rejection, because the orbital analysis and the gas-cell validation are well executed and the paper addresses an important discrepancy in the literature."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Short version: the paper makes a specific, quantitative claim—about half of Virgo-like cluster galaxies avoid strong ram-pressure stripping on first pericenter—and backs it up with a two-model comparison and useful phase-space fits. That claim is plausible and worth taking seriously. But the two-model consistency is not as clean as the paper presents, because the TNG binding pressure is computed differently from GAEA's, and the gap is used to dismiss TNG's stronger stripping.\n\nWhat is genuinely new: the phase-space zone fits (Eqs. 4-6), which let you read off strong/moderate/weak/no RPS zones as a function of redshift and halo mass; and the explicit test of a simplified ram-pressure estimator against TNG's gas-cell calculation (Fig. 4). The authors are also careful in places—they check orbital interpolation convergence (Appendix A), they acknowledge orphan galaxy caveats, and they present the retained fractions as lower limits. The GAEA result alone is a solid model prediction that contradicts several hydro-simulation studies, and the observational comparisons to Boselli et al. and Wang et al. are appropriate.\n\nWhere it gets soft. First, the strong/moderate/weak thresholds are defined from GAEA's own binding pressure, then used to compute the retained fractions and to bin the phase space. That is partly circular, though the authors do offer the TNG comparison and observed gas disk radii as external checks. Second, and more serious: the TNG binding pressure in Fig. 1 includes hot gas in the gravitating mass, while GAEA's does not. The TNG calculation explicitly projects 'gas (including hot and cold gas)' and Eq. 3 uses total gas plus stars within radius r. Hot gas is diffuse and vertically extended, so including it inflates the restoring pressure at large radii—exactly where TNG profiles rise above GAEA. The paper uses that gap to argue TNG's more efficient stripping is a numerical artifact. If you redo TNG with cold gas only, the gap might shrink or flip, and then TNG would be gas-poorer because its baryonic restoring pressure is genuinely lower, not because RPS is artificially enhanced. That weakens the 'independent support' from TNG, though it doesn't erase it: TNG phase-space ram pressures still show ~40% of galaxies avoiding the strong zone even with the GAEA-derived threshold.\n\nBottom line: the GAEA-based result and the phase-space fits are worth publishing, and the authors are honest about assumptions. The referee should ask for a TNG binding pressure calculation excluding hot gas, and for a sensitivity test of the zone thresholds. I'd send this to peer review; with those two additions it could be solid.","headline":"A useful, mostly honest model comparison that overstates the TNG/GAEA agreement because the TNG binding pressure is computed with hot gas included; still deserves peer review.","tokens_in":735,"tokens_out":2035,"would_cite":true,"duration_ms":64074,"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":"Most cluster galaxies keep their gas after first close pass","keywords":["galaxies: evolution","galaxies: star formation","galaxies: ISM","galaxies: interactions","galaxies: haloes","ram pressure stripping","galaxy clusters","satellite galaxies"],"falsifier":"Measure gravitational restoring pressure profiles for a sample of resolved cluster galaxies in a Virgo-like cluster and compare them with the ram pressure implied by each galaxy's phase-space position; if typical central restoring pressures fall below $10^{-10.5}\\,\\mathrm{g\\,cm^{-1}s^{-2}}$ for low-mass galaxies, the strong-stripping fraction would be much higher than the paper's roughly-half estimate. Equivalently, an HI census of recently accreted cluster members selected from phase space should show about half retaining a substantial gas reservoir if the claim is right, and nearly universal HI deficiency would falsify it.","tokens_in":18317,"feed_emoji":"🌌","tokens_out":12090,"duration_ms":101802,"temperature":0.7,"pith_summary":"Satellite galaxies falling into massive cluster halos are often assumed to lose their cold gas in one decisive event at first closest approach. This paper tests that assumption using two independent modeling techniques, the semi-analytic model GAEA and the hydrodynamical simulation TNG, by following galaxy orbits from $2.5R_{\\rm vir}$ through the first pericentric passage. The authors compare the ram pressure felt along each orbit with the gravitational pressure binding each galaxy's gas disk, and divide the cluster phase-space diagram into strong, moderate, weak, and no stripping zones. They find that in Virgo-like halos about half of low-mass galaxies never enter the strong stripping zone during their first passage, while in Coma-like halos almost all low-mass galaxies do; galaxies above $10^{10} M_\\odot$ retain some gas even there. The conclusion is that the first pericenter is not a universal gas-removal event for cluster galaxies, and rapid stripping in some cosmological simulations may be artificially strong.","feed_headline":"Most cluster galaxies keep their gas after first close pass","feed_subtitle":"Two independent models agree: first passage strips only low-mass galaxies in the most massive clusters.","key_machinery":"The load-bearing comparison is between ram pressure, $P_{\\rm ram} = \\rho_{\\rm ICM} v^2$ (Gunn–Gott), and the gravitational binding pressure of the gas disk, $P_{\\rm grav}(r) = 2\\pi G \\Sigma_{gs}(<r)\\,\\Sigma_g(r)$ in GAEA's exponential-disk approximation and a cell-based equivalent in TNG. Binding pressure profiles fix the thresholds that define the stripping zones: $10^{-10.5}$, $10^{-12}$, and $10^{-13.5}\\,\\mathrm{g\\,cm^{-1}s^{-2}}$, corresponding roughly to strong, moderate, and weak stripping. These thresholds become separating lines in the phase-space diagram (normalized cluster-centric distance versus relative velocity), fitted as functions of redshift and final halo mass. Galaxy orbits are interpolated to 0.02 Gyr resolution and overlaid on these zones; the time each galaxy spends with ram pressure above each threshold, from infall at $2.5 R_{\\rm vir}$ to first apocenter, is the quantity that carries the argument.","core_discovery":"The paper's central claim is that ram pressure during the first pericentric passage removes a significant fraction of cold gas only from galaxies with $\\log M_\\star/M_\\odot < 9.5$ in halos with $\\log M_h/M_\\odot > 15$. In the more common Virgo-like halos ($\\log M_h/M_\\odot \\sim 14$), about half of low-mass galaxies do not experience strong ram pressure at all during that passage, and typical low-mass satellites retain at least 10 percent of their cold gas afterward. The claim is built by estimating gravitational binding pressure profiles of model disks, translating three characteristic pressures ($10^{-10.5}$, $10^{-12}$, $10^{-13.5}\\,\\mathrm{g\\,cm^{-1}s^{-2}}$) into retained gas fractions, and mapping those thresholds onto the phase-space diagram. Orbital tracks from $2.5R_{\\rm vir}$ to the first apocenter then show how long galaxies spend in each zone; GAEA and TNG agree that the strong zone is usually avoided in Virgo-like halos and reached by nearly all galaxies in Coma-like halos. Although TNG central galaxies show higher binding pressures at large radii, TNG cluster galaxies are still gas-poorer, which the authors attribute to artificially enhanced ram pressure in the hydrodynamical run. The authors conclude that most cluster galaxies keep a notable gas reservoir and continue forming stars after the first passage.","pith_inferences":["A testable extension: if the first passage rarely empties a galaxy's gas, resolved HI or CO observations of a Virgo-like cluster should find that galaxies on first-infall orbits retain substantial gas, with jellyfish tails concentrated among orbits that dive into the strong zone.","The paper's interpretation of the TNG discrepancy implies that numerical mixing of interstellar and intracluster gas, plus resolution-limited feedback, inflates stripping in hydrodynamical runs; rerunning such simulations with finer gas resolution or gentler feedback is a concrete way to see whether retained fractions move toward the GAEA values.","The phase-space zone fits could be applied to survey data to build predicted ram-pressure maps of clusters and compare them with observed star-forming fractions as a function of phase-space position, extending the model-only analysis to observations.","If most galaxies keep dense central gas, star formation should persist in central disks even while outer HI is removed; resolved molecular-gas observations of recently accreted cluster members would provide a direct check."],"forward_implications":["In Virgo-like clusters, about half of low-mass galaxies are never strongly stripped on first passage, so HI-poor galaxies and jellyfish morphologies should be rarer among first-infall systems than among older, repeatedly stripped satellites.","In Coma-like clusters, low-mass galaxies are almost all stripped of their cold gas, but galaxies with $\\log M_\\star/M_\\odot > 10$ keep 10–20 percent of their central gas, meaning first-passage ram pressure alone cannot fully quench them.","Because most first-passage stripping is moderate rather than strong, the finite (roughly gigayear) stripping timescale matters: galaxies spend only about 400 Myr in the strong zone, too short for complete removal.","The agreement of GAEA and TNG on orbits and ram-pressure distribution implies that the remaining difference between their gas contents lies mostly in how efficiently hydrodynamics removes stripped gas, not in the pressure exerted.","The fitted phase-space zone boundaries give a direct way to classify a galaxy's expected stripping state from its projected radius, velocity, cluster mass, and redshift."],"supporting_citations":[{"why":"Supplies the ram pressure formula used throughout and the criterion that stripping acts when ram pressure exceeds the disk restoring force.","marker":"Gunn & Gott 1972"},{"why":"Establishes the two-phase stripping timescale, roughly 10 Myr of disturbance followed by up to 1 Gyr of continuous stripping, used to judge whether first-passage durations suffice.","marker":"Roediger & Hensler 2005"},{"why":"Shows that stripping duration depends on ram pressure history and restoring pressure, supporting the paper's use of cumulative residence time in each zone.","marker":"Roediger & Brüggen 2006"},{"why":"Provides the alternative volume-based gravitational binding pressure formula applied to TNG galaxies.","marker":"McCarthy et al. 2008"},{"why":"The GAEA environmental model (strangulation, hot-gas stripping, and cold-gas ram pressure) and the HI/H2 calibration that this work extends.","marker":"Xie et al. 2020"},{"why":"Supports tracking infall from 2.5 virial radii to avoid back-splashed and pre-processed galaxies.","marker":"Bahé et al. 2013"},{"why":"A previous orbit-based analysis concluding that galaxies lose their entire gas reservoir near first pericenter, the position this paper directly challenges.","marker":"Oman et al. 2021"},{"why":"Hydrodynamical results showing first-orbit quenching of galaxies in massive halos, used as the contrasting simulation-based claim.","marker":"Wright et al. 2022"},{"why":"Provides the local ICM shell method used as the refined ram pressure estimate in TNG.","marker":"Ayromlou et al. 2019"}],"fun_headline_variants":["First pass strips only small galaxies in biggest clusters","Most cluster galaxies keep gas through first close pass","Ram pressure harsh only for dwarfs in giant halos","GAEA and TNG agree: first passage spares most galaxies","Cluster galaxies retain gas after first pericentric pass"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The central claim assumes that the gravitational binding pressures computed from the modeled gas and stellar disks, which ignore dark matter and black hole gravity, are close to the real restoring forces holding gas in cluster galaxies.","fun_headline_variants_meta":{"raw":{"variants":["First pass strips only small galaxies in biggest clusters","Most cluster galaxies keep gas through first close pass","Ram pressure harsh only for dwarfs in giant halos","GAEA and TNG agree: first passage spares most galaxies","Cluster galaxies retain gas after first pericentric pass"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000224,"raw_usage":{"total_tokens":1600,"prompt_tokens":1225,"completion_tokens":375,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":841,"completion_tokens_details":{"reasoning_tokens":298}},"tokens_in":841,"tokens_out":375,"duration_ms":4516,"temperature":1.0,"reasoning_tokens":298,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-16T12:20:32.091824+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Measure gravitational restoring pressure profiles for a sample of resolved cluster galaxies in a Virgo-like cluster and compare them with the ram pressure implied by each galaxy's phase-space position; if typical central restoring pressures fall below $10^{-10.5}\\,\\mathrm{g\\,cm^{-1}s^{-2}}$ for low-mass galaxies, the strong-stripping fraction would be much higher than the paper's roughly-half estimate. Equivalently, an HI census of recently accreted cluster members selected from phase space should show about half retaining a substantial gas reservoir if the claim is right, and nearly universal HI deficiency would falsify it.","supporting_citations":[{"cited_title":"& Br ¨uggen, M","cited_arxiv_id":null,"evidence_quote":"Shows that stripping duration depends on ram pressure history and restoring pressure, supporting the paper's use of cumulative residence time in each zone."},{"cited_title":"A., Bah´ e, Y","cited_arxiv_id":null,"evidence_quote":"A previous orbit-based analysis concluding that galaxies lose their entire gas reservoir near first pericenter, the position this paper directly challenges."},{"cited_title":"An orbital perspective on the starvation, stripping, and quenching of satellite galaxies in the EAGLE simulations","cited_arxiv_id":"2205.08414","evidence_quote":"Hydrodynamical results showing first-orbit quenching of galaxies in massive halos, used as the contrasting simulation-based claim."},{"cited_title":"A New Method to Quantify Environment and Model Ram-Pressure Stripping in N-Body Simulations","cited_arxiv_id":"1903.01988","evidence_quote":"Provides the local ICM shell method used as the refined ram pressure estimate in TNG."}],"review_version":1}