{"id":"746b444b-8f4b-469f-bc15-1bd223ec59fe","arxiv_id":"2506.08508","paper_version":2,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":3,"one_line_summary":"In 13 FIRE-2 Milky Way-mass simulations, all progenitors pass through transient elongated phases, while their present-day stellar populations are symmetric disks or spheroids, implying many observed high-redshift elongated galaxies are short-lived.","lead":"Using 13 simulated Milky Way-mass galaxies, this paper shows that galaxy progenitors frequently pass through elongated, pickle-shaped phases in the early universe, then relax into round, disk-like configurations by today. It suggests JWST and HST elongated early galaxies are usually brief stages, not stable structures, and some may become Milky Way-like systems.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The JWST/HST comparison in Section 4 is not forward-modeled: the 3D r^-2-weighted shape estimator differs from observed 2D light-weighted shapes, so the 'roughly consistent' elongated fractions and the transitory-phase interpretation of observed galaxies are unsupported as stated.","rationale":"The reader's weakest assumption identifies the unforward-modeled comparison to observations, and I agree that this is the most load-bearing issue for the central claim. I extend it in two ways: the shape estimator itself (r^{-2}-weighted reduced-mass eigentensor) is not equivalent to observed light-weighted 2D shapes, so the mismatch is not merely projection/dust/selection; and the abstract's 25-45% elongated fraction is not derived in the body, making the comparison hard to audit. These concerns do not invalidate the simulation-side finding: the visual projections in Figures 3 and A1 independently show elongated stellar distributions, and the transient nature is consistent with previous Vela simulations (Ceverino et al. 2015; Tomassetti et al. 2016), which is independent support. The paper's explicit caveat in Section 2.3 shows good faith, but the Section 4 consistency claim overreaches without forward-modeling. Therefore, the appropriate verdict remains CONDITIONAL, pending a forward-modeled comparison or a toned-down observational claim. No change to the reader's verdict is needed.","tokens_in":14283,"tokens_out":9487,"duration_ms":111195,"concrete_test":"Project each of the 13 simulated galaxies at every snapshot along many random sight lines; generate g-band images with a stellar population synthesis model, optionally including dust; measure 2D axis ratios using the same pipeline as Pandya et al. (2024) or van der Wel et al. (2014); apply the same stellar-mass and redshift selection; and compare the elongated fraction to both the intrinsic 3D fraction and the observed fractions. If the projected fraction differs from the 3D fraction by more than ~20% or falls outside the observed range, the Section 4 \"roughly consistent\" claim fails and the transitory-phase interpretation of observed galaxies would need to be downgraded to a suggestion.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The paper's central interpretive claim, that observed elongated galaxies at z>0.5 are transitory phases of Milky Way-mass assembly, depends on the comparison in Section 4 between the simulated elongated fraction and observed fractions (e.g., Pandya et al. 2024). The simulated fractions are computed using three-dimensional axis ratios from the reduced-mass eigentensor of Equation (1), which includes the r^{-2}_{ell} weighting, measured within 0.1 rvir and scaled to retain 90% of the mass. Observed shapes are two-dimensional projected light distributions, usually characterized by Sersic or moment-based axis ratios, with dust, PSF, and selection effects. The r^{-2} weighting is designed to suppress outer particles and can move a galaxy across the elongated boundary compared to a light-weighted measurement; projection can either round or elongate apparent shapes depending on orientation. The authors acknowledge this gap in Section 2.3 (\"This is a quasi-observable shape measurement; however, measurements of observed galaxies are projected and affected by dust and other factors\"), but Section 4 still makes the quantitative statement that the simulated elongated fraction is \"roughly consistent\" with observations, and the abstract quotes 25-45% without a derivation or uncertainties in the body. This is a load-bearing assumption: if forward-modeled projected fractions differ from the 3D fractions, the observational interpretation is unsupported, even though the simulation-side finding (all progenitors experience elongated phases) could remain valid.","agreement_with_reader":"partial"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper studies the 3D shape evolution of thirteen Milky Way-mass galaxies from the FIRE-2 zoom-in simulations. It uses an iterated reduced-mass eigentensor with r^{-2}_{ell} weighting (Eq. 1) measured within 0.1 rvir, classifies shapes as elongated, disky, or spheroidal following Zhang et al. (2019), and tracks main progenitors from z=7 to z=0. Shapes are measured for all stars, young stars (<500 Myr), g-band luminosity-weighted stars, and dark matter, and are complemented by an 'archaeological' decomposition of present-day stars in 500 Myr age bins. The central findings are that all progenitors pass through elongated 'pickle' phases that oscillate with spheroidal phases on Gyr timescales; that these phases are transitory, since the same stellar populations appear axisymmetric about their minor axes at z=0; that z=0 stars separate into thin-disk, thick-disk, and flattened-spheroid components; and that the simulated elongated fraction is claimed to be roughly consistent with observed JWST/HST elongated fractions, supporting the interpretation that observed high-redshift elongated galaxies are statistically common but transitory phases.","tokens_in":14527,"tokens_out":8827,"duration_ms":89396,"significance":"If the simulation-side results are correct, the paper offers a coherent resolution to a current observational puzzle: the numerous 'pickle'-shaped galaxies at z>0.5 are not a stable, distinct class of objects but an ordinary, recurrent phase in Milky Way-mass assembly. The paper has genuine strengths: the shape estimator is standard and explicitly defined; Section 2.4 provides a particle-count convergence test that motivates the 500 Myr young-star cutoff; the results for all 13 galaxies are documented in Appendix B; and Section 2.3 explicitly flags the quasi-observable nature of the luminosity-weighted shapes. The strongest evidence is internal: Figures 2-4 show the same stellar population evolving from elongated at formation to axisymmetric at z=0, which directly supports the transitory-phase claim without recourse to observed data. The population-level comparison to observations is not on the same footing: it lacks forward modeling of projection and dust, the abstract-quoted fractions are not derived in the body, and the abstract asserts measurements (velocity dispersion anisotropy, dark-matter fraction non-correlation) that do not appear in the text.","major_comments":[{"comment":"The abstract states that 25-45% of the population have elongated luminosity-weighted shapes at any given time at z=0.5-8.5, but this statistic is never derived in the body: there is no figure or table of the elongated fraction versus redshift, no statement of how the fraction is computed (per snapshot or averaged), and no uncertainty estimate despite the small sample of 13 galaxies with time-correlated snapshots. In addition, the quoted range extends to z=8.5, while Section 2.3 states that measurements cover 18 snapshots 'spaced approximately 750 Myr apart, starting from z=7 to z=0.' Please add a derivation of the elongated fraction as a function of redshift with binomial or bootstrap uncertainties, and reconcile the redshift ranges.","section":"Abstract and Section 4"},{"comment":"The claim that the simulated elongated fraction is 'roughly consistent' with observations compares the 3D reduced-mass eigentensor shapes of Eq. (1), measured within 0.1 rvir with r^{-2}_{ell} weighting and no projection, to observed 2D projected light distributions affected by dust, PSF, and selection effects. Section 2.3 acknowledges this ('This is a quasi-observable shape measurement; however, measurements of observed galaxies are projected and affected by dust and other factors'), but Section 4 and the abstract convert it into a quantitative consistency statement that supports the interpretive claim that observed JWST/HST elongated galaxies are transitory phases. That interpretation is load-bearing and is not supported without forward-modeled mock observations (projection, dust, selection) or, at minimum, an explicit statement that the comparison is illustrative rather than quantitative. Please either add forward modeling or soften the claim.","section":"Section 4 (Pandya et al. comparison)"},{"comment":"The abstract claims that 'during their transient elongated phases, our galaxies have anisotropic velocity dispersion ellipsoids directed along their spatial major axis' and that galaxy shapes 'do not correlate with their dark matter fraction nor with the shapes and orientations of their underlying dark matter halos.' Neither measurement appears in the paper: no velocity dispersion tensor is defined or computed, no dark matter fraction is computed, and halo-stellar alignment is not tested. Section 3.2 reports only visual inspection of axis ratios in Figure 5, and Section 4 explicitly defers the velocity-dispersion anisotropic-support question to future work ('It will be interesting to ask whether this is ubiquitous among our systems'). Please either add these measurements or remove the unsupported claims from the abstract.","section":"Abstract vs. body"}],"minor_comments":[{"comment":"The boundaries separating the elongated, disky, and spheroidal regions are shown only graphically in Figure 1; please state the numerical thresholds on C/A and B/A in the text or caption so that the classification is reproducible.","section":"Section 2.2 / Figure 1"},{"comment":"The statement that stellar and dark-matter elongation do not correlate is based on visual inspection of the bottom row of Figure 5; a Spearman rank correlation coefficient between stellar B/A and dark-matter B/A, with a significance estimate, would make the claim testable and would better justify the abstract's stronger wording.","section":"Section 3.2"},{"comment":"The 500 Myr young-star cutoff is motivated by the 1000-particle convergence limit in Section 2.4; the paper should state explicitly in the main text that this is a resolution-driven choice rather than a physically special timescale, so that readers do not interpret 500 Myr as physically significant.","section":"Section 2.3"},{"comment":"The text should state explicitly whether m12z, the merging galaxy that is excluded from the 'Milky Way analogs' label, is included in the population-level statements and in the elongated fractions quoted in the abstract.","section":"Section 4"},{"comment":"The convergence test is performed on z=0 galaxies; a sentence explaining why the 1000-particle threshold is assumed to apply at high redshift, where the spatial and dynamical distribution of star particles is different, would strengthen the methodology.","section":"Section 2.4"}],"recommendation":"major_revision","confidential_remarks":"The paper is well suited to a galaxy-evolution journal such as ApJ or MNRAS. The simulation-side result (all Milky Way-mass progenitors experience transitory elongated phases, with the same stellar populations becoming axisymmetric by z=0) is convincing, clearly presented, and supported by appendices for all 13 galaxies; the main publication-blocking issues are the abstract-body mismatch (the 25-45% fraction, the velocity-dispersion anisotropy claim, and the dark-matter non-correlation claim are not derived in the text) and the un-forward-modeled comparison to observed elongated fractions. On novelty: the method is adopted from Chua et al. (2019) and Klein et al. (2025), with a shared author on the latter, and the authors should state explicitly in the introduction what is incremental relative to the FIREbox z=0 shape results of Klein et al. (2025)."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"The real news here is systematic: thirteen FIRE-2 Milky Way-mass galaxies, all tracked from z~7 to z=0, and every one of them passes through an elongated phase. That extends the earlier Vela results (Ceverino 2015, Tomassetti 2016) from single objects to a small but coherent population, and the mono-age population tracking from birth to present is a genuinely useful way to show that early pickles relax into axisymmetric spheroids and disks. The paper also does something rare: it reports a null result for the dark-matter connection that earlier work emphasized, and it flags that as something to explore later rather than over-interpreting it.\n\nThe method is standard and the convergence test (Section 2.4) is the right kind of sanity check. The caveats in Section 2.3 about the quasi-observable nature of the luminosity-weighted shapes are explicit, and the limitation section is honest about the drivers being future work. I believe the central claim—that elongated shapes are transitory phases in simulated MW-mass assembly—is well supported by the figures and the appendix plots.\n\nThe soft spots are real but not fatal. The 25–45% elongated fraction quoted in the abstract has no uncertainties and is derived from a 3D, r^-2-weighted, luminosity-weighted measurement that has not been pushed through projection, dust, or selection effects. Section 4's 'roughly consistent' with JWST/HST observations is therefore weaker than the abstract's phrasing suggests. The stress-test note lands: if you forward-model those effects, the simulated fractions could shift. But the authors never claim the comparison is exact, and the transitory-phase interpretation for observed galaxies does not need the fractions to match precisely—it needs the simulated galaxies to be elongated often enough that JWST/HST would catch some. The small sample (13 galaxies, one of which is a merger) limits population statistics but is not disqualifying for a first systematic look.\n\nThe citation pattern is fine: self-citation to Klein et al. 2025 is appropriate given the shared method, and the prior Vela work is credited clearly.\n\nThis paper deserves a serious referee. The referee should push for a forward-modeled projection of simulated shapes, error bars on the fractions, and a sharper separation between the simulation-side result and the observational comparison. But the core finding will survive that process.\n\nI'd bring it to reading group and would cite it for the simulation-side claim; the observational comparison I'd cite with a caution flag.","headline":"A solid simulation-side result on transient elongation in Milky Way progenitors; the comparison to observed galaxy fractions is weaker than the abstract implies, but the core finding and the writing's honesty hold up.","tokens_in":15157,"tokens_out":988,"would_cite":true,"duration_ms":14855,"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":"This paper argues that the pickle-shaped, elongated galaxies seen by JWST and HST in the early universe are a temporary but common phase that Milky Way-like galaxies pass through.","keywords":["galaxy shapes","triaxial ellipsoids","Milky Way progenitors","galaxy formation","high-redshift galaxies","JWST","stellar populations","galaxy evolution"],"falsifier":"Forward-model the simulated galaxies at $z = 1$--$4$ through dust attenuation and random projection, run the resulting mock images through the same two-dimensional shape-fitting pipeline used on JWST/HST data, and compare the mock elongated fraction with the observed one; if the mock fraction is much lower than $25$--$45\\%$ or fails to reproduce the observed redshift trend, the central 'roughly consistent' comparison fails.","tokens_in":14041,"feed_emoji":"🥒","tokens_out":10306,"duration_ms":113712,"temperature":0.7,"pith_summary":"This paper tries to establish that the elongated, pickle-shaped galaxies JWST and HST see at high redshift are not stable structures but a common, temporary phase in the assembly of Milky Way-like galaxies. Analyzing thirteen simulated Milky Way-mass galaxies, it finds that every progenitor goes through elongated phases in the early universe, often oscillating between spheroidal and elongated shapes on billion-year timescales, with about $25$--$45\\%$ of the population elongated at any given time at $z = 0.5$--$8.5$. It also shows that the same stellar populations that were elongated at birth end up symmetric about their minor axes at $z = 0$, meaning their shapes changed significantly over time. If this is right, many of the elongated galaxies seen in deep surveys are likely ordinary young Milky Way progenitors caught in a fleeting configuration.","feed_headline":"Pickle-shaped galaxies are a passing phase for Milky Way-like systems","feed_subtitle":"In 13 simulated Milky Way-mass galaxies, every progenitor goes through elongated phases; most relax into disks by today.","key_machinery":"The paper's central tool is the reduced-mass eigentensor, an iteratively computed shape tensor that weights each star particle by the inverse square of its elliptical radius relative to the current best-fit ellipsoid. Diagonalizing it gives axis lengths $A \\ge B \\ge C$, and the ratios $C/A$ and $B/A$ classify galaxies as disky, spheroidal, or elongated. This tool matters because it lets the authors measure shapes of the same stellar cohort both at formation and at $z = 0$, and because it allows luminosity-weighted shapes to be compared directly to observed galaxy populations.","core_discovery":"Every one of the thirteen simulated Milky Way-mass progenitors passes through phases when its stars are arranged in an elongated configuration, and at $z = 0.5$--$8.5$ some $25$--$45\\%$ of the population has elongated luminosity-weighted shapes at any given snapshot. The shape is not stable: the same single-age stellar populations that formed in elongated or triaxial configurations at early times are, at $z = 0$, members of thin disks, thick disks, or flattened spheroids that are all symmetric about the minor axis, with $B/A \\sim 0.9$. The paper reads this as evidence that the elongated galaxies observed in the early universe are statistically common but transient, and that some of them may evolve into present-day Milky Way-like disk galaxies.","pith_inferences":["A testable extension: if the elongated phase is transient and common, integral-field spectroscopy of high-$z$ elongated galaxies should reveal velocity-dispersion anisotropy aligned with the major axis, as the paper reports for its simulated galaxies.","A direct forward-modeling test with dust, projection, and selection effects would put the paper's 'roughly consistent' comparison on firmer ground, since the current comparison uses quasi-observable 3D shapes rather than mock 2D images.","If the elongated phase tracks merger or filamentary accretion activity, the observed fraction of elongated galaxies at fixed mass and redshift could serve as a statistical tracer of merger activity in the early universe.","The paper's result implies that the Milky Way's oldest stars, now sitting in a minor-axis-symmetric spheroid, may preserve faint relics of an elongated formation phase in their orbital anisotropy, a signature that could be searched for in high-precision stellar kinematics."],"forward_implications":["At $z \\simeq 0.5$--$8.5$, roughly one-quarter to nearly half of Milky Way-mass progenitors should appear elongated at any given time, so deep surveys should expect many pickle-shaped galaxies at these masses and redshifts.","An individual high-redshift elongated galaxy is not necessarily a rare or exotic object; if it is a Milky Way progenitor, it can later settle into a disk, so its present-day descendant should be a flattened, minor-axis-symmetric spheroid rather than an elongated structure.","Present-day roundness of old stellar populations does not imply they formed round: the same stars were often elongated at birth, so galactic archaeology needs to account for shape relaxation over cosmic time.","Because the single $z = 0$ elongated galaxy in the sample is undergoing a late merger, and because elongated phases do not track dark-matter halo shape, mergers or merger-driven accretion are a plausible, but not proven, driver of the elongated phase."],"supporting_citations":[{"why":"Provides the JWST-observed elongated/prolate galaxy fractions at $z = 0.5$--$8$ that the simulated population is compared against.","marker":"Pandya et al. (2024)"},{"why":"Establishes the observational trend that elongated fractions rise toward high redshift and low mass, the baseline the simulations address.","marker":"van der Wel et al. (2014)"},{"why":"Earlier zoom simulations finding $z \\sim 2$ galaxies often prolate, providing the direct precedent for elongation phases.","marker":"Ceverino et al. (2015)"},{"why":"Earlier simulations showing such elongation is transient and tied to filamentary assembly; the paper tests and contrasts its dark-matter correlation claim.","marker":"Tomassetti et al. (2016)"},{"why":"Supplies the reduced-mass eigentensor method used to measure all galaxy shapes.","marker":"Chua et al. (2019)"},{"why":"Supplies the elongated/disky/spheroidal classification scheme used to categorize shapes.","marker":"Zhang et al. (2019)"},{"why":"Defines the feedback physics used in all thirteen simulated galaxies.","marker":"Hopkins et al. (2018)"}],"fun_headline_variants":["Simulations: Elongated galaxies are a fleeting phase on the road to disks","Milky Way analogs spend 25-45% of early time as pickle-shaped","Every Milky Way progenitor gets elongated, but disks win out eventually","Early-universe elongated galaxies are short-lived, simulations show","Milky Way ancestors spend up to 45% of early life as pickles"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The comparison between simulated and observed elongated fractions assumes that the luminosity-weighted 3D shapes measured from star particles inside $0.1\\,r_{\\rm vir}$, without dust, projection, or selection effects, can stand in for the 2D projected shapes that JWST/HST actually measure.","fun_headline_variants_meta":{"raw":{"variants":["Simulations: Elongated galaxies are a fleeting phase on the road to disks","Milky Way analogs spend 25-45% of early time as pickle-shaped","Every Milky Way progenitor gets elongated, but disks win out eventually","Early-universe elongated galaxies are short-lived, simulations show","Milky Way ancestors spend up to 45% of early life as pickles"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000759,"raw_usage":{"total_tokens":3408,"prompt_tokens":1019,"completion_tokens":2389,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":635,"completion_tokens_details":{"reasoning_tokens":2292}},"tokens_in":635,"tokens_out":2389,"duration_ms":17638,"temperature":1.0,"reasoning_tokens":2292,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-07T05:09:18.496797+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Forward-model the simulated galaxies at $z = 1$--$4$ through dust attenuation and random projection, run the resulting mock images through the same two-dimensional shape-fitting pipeline used on JWST/HST data, and compare the mock elongated fraction with the observed one; if the mock fraction is much lower than $25$--$45\\%$ or fails to reproduce the observed redshift trend, the central 'roughly consistent' comparison fails.","supporting_citations":[{"cited_title":"R., Faber, S","cited_arxiv_id":null,"evidence_quote":"Supplies the elongated/disky/spheroidal classification scheme used to categorize shapes."}],"review_version":1}