ALMA Central molecular zone Exploration Survey (ACES) VI: ALMA Large Program Reveals a Highly Filamentary Central Molecular Zone

Adam Ginsburg; Alex Lazarian; \'Alvaro S\'anchez-Monge; Alyssa Bulatek; Anika Schmiedeke; Ashley T. Barnes; Cara Battersby; Christian Henkel; Christoph Federrath; Daniel L. Walker

REVIEW 3 major objections 3 minor 3 references

ALMA Central molecular zone Exploration Survey (ACES) VI: ALMA Large Program Reveals a Highly Filamentary Central Molecular Zone

T0 review · 3 major / 3 minor · reviewed 2026-08-02 · deepseek-v4-flash

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Pith's one-line read The Milky Way's central molecular zone is shown to be threaded with filaments from 0.1 to tens of parsecs, with the largest tracing the gas streams orbiting the galactic center.

desk verdict A genuinely new high-resolution view of the CMZ in HNCO, with a useful if preliminary filament taxonomy; the shock-tracer caveat is real but fixable, and the survey result stands. read the letter →

arxiv 2602.20262 v2 pith:AL5S4YFX submitted 2026-02-23 astro-ph.GA

Cara Battersby , Miriam G. Santa-Maria , Dani Lipman , Dylan M. Paré , Rachel R. Lee , Pablo García , Izaskun Jiménez-Serra , Xing Pan

show 39 more authors

Daniel L. Walker Jack Sullivan Danya Alboslani H Perry Hatchfield Yue Hu Alex Lazarian Jennifer Wallace Qizhou Zhang Xing Lu Elisabeth A.C. Mills Adam Ginsburg Ashley T. Barnes Pei-Ying Hsieh Jonathan D. Henshaw Steven N. Longmore John Bally Laura Colzi Paul T. P. Ho Maya A. Petkova Mattia C. Sormani N. Bijas Alyssa Bulatek Natalie O. Butterfield Christoph Federrath Simon C. O. Glover Mark D. Gorski Savannah R. Gramze Christian Henkel Janik Karoly Ralf S. Klessen Sergio Martín Francisco Nogueras-Lara Jaime E. Pineda Denise Riquelme-Vásquez Víctor M. Rivilla Álvaro Sánchez-Monge Anika Schmiedeke Yoshiaki Sofue Volker Tolls

This is my paper · ORCID

classification astro-ph.GA

keywords CentralMolecularZonegalacticcenterfilamentsinterstellarmediumposition-velocityspacemagneticfieldalignmentturbulencelinesurvey

verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

Using a new high-resolution millimeter-wave survey of the entire Central Molecular Zone (CMZ) — the inner ~400 parsecs of the Milky Way that funnels gas toward the supermassive black hole — the paper aims to establish that the molecular gas there is ubiquitously filamentary on scales from about 0.1 pc to tens of parsecs. It identifies two families of elongated structures: large-scale filaments (LFs), over 10 pc long, which appear to be contiguous pieces of the orbital streams feeding the CMZ, and small-scale filaments (SFs), about 1 pc long, which pervade the region and are coherent in position-position-velocity space. The paper argues that LFs offer a new observable for mapping the 3D dynamics and mass flow of the CMZ, while SFs may be produced by turbulence and shear, consistent with numerical simulations. If correct, this replaces the picture of the CMZ as a collection of discrete giant clouds with one of a continuous, tangled web of filaments, reshaping how gas reaches the galactic center and how stars form there.

What carries the argument

The central object is the survey's high-resolution (0.1 pc) cube of HNCO 4(0,4)-3(0,3) emission covering the entire Central Molecular Zone. The analysis hinges on constructing integrated moment-0 maps over velocity ranges of at least 5 km/s (to suppress velocity-crowding artifacts), and on hand-drawn masks of six structures that are parameterized with filament-tracing algorithms (spines, widths, curvature). The paper combines this with a position-position-velocity decomposition to situate the filaments relative to known CMZ orbital streams, and with alignment statistics (Projected Rayleigh Statistic and Alignment Measure) comparing filament orientations to magnetic-field pseudo-vectors from

What would settle it

Construct synthetic observations of a magnetohydrodynamic simulation of the CMZ (with matching turbulence and orbital motions) and run the same filament-identification procedure; if the simulated data do not produce a comparable population of small-scale coherent filaments at 0.1 pc resolution, or if the observed filaments disappear when imaged in a higher-density tracer that mitigates velocity crowding, the ubiquity claim would be falsified.

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Extended reading notes

Core claim

The paper reports that the survey's Band 3 observations of the HNCO 4(0,4)-3(0,3) line, covering the whole CMZ at 0.1 pc resolution, reveal a pervasive filamentary morphology in the molecular gas. Through visual inspection, the authors identify at least two classes of elongated structures: Large-scale Filamentary structures (LFs) with projected lengths of 10-49 pc and widths of 0.16-0.69 pc, and Small-scale Filamentary structures (SFs) with lengths of 1.3-3.4 pc and widths of 0.08-0.14 pc. For three objects of each class, they show that the structures are largely coherent in both position-position and position-velocity space, that two of the LFs lie directly on the CMZ orbital streams, that

Load-bearing premise

The load-bearing premise is that the small-scale filaments are real spatial density structures rather than artifacts of velocity crowding in position-position-velocity space; if superposition of unrelated gas along the line of sight creates the appearance of filaments, the claim of ubiquity would be overstated.

Editorial extensions

If this is right

If the CMZ's dense gas is ubiquitously filamentary, models of gas inflow and star formation in the galactic center must treat filaments as the fundamental geometry rather than isolated clouds.
The large-scale filaments' close correspondence to orbital streams means they can serve as dynamical tracers, potentially constraining the CMZ's 3D structure and the flow of gas toward the central supermassive black hole.
The pervasive small-scale filaments, if confirmed as turbulence and shear products, would indicate that the CMZ is a natural laboratory for studying filament formation in extreme conditions, linking observations to magnetohydrodynamic turbulence theories.
The systematic chemical differences between LFs and SFs (e.g., higher HNCO/SiO in SFs) imply different shock strengths or histories, offering a route to observationally differentiate formation mechanisms.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

If the SF population is truly universal, one would predict that the same filaments appear in other dense-gas tracers at comparable resolution (e.g., CS or HCO+), and that their orientation distribution relative to local magnetic fields is bimodal (parallel vs perpendicular) corresponding to turbulent vs shock-compressed origins; this can be tested with the full survey dataset.
The claim that LFs trace orbital streams implies that filaments could be used as proxies for 3D orbital flow in external galaxies or simulations, potentially circumventing the need for full 3D kinematic data.
The small sample (three per class) means the reported widths and axis ratios may not be representative; a systematic census with an automated filament-finding algorithm would reveal whether there is a continuum of filament sizes between the LF and SF populations, a possibility the paper itself acknowledges.
The anti-correlation between HNCO and H13CO+ seen in SF 1 hints at excitation or opacity effects that could be disentangled with multi-transition observations, connecting the filaments' chemistry to their kinematics.

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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Desk Editor's Note

read the letter

Here's the short version. This is the first contiguous ALMA 3" map of the whole CMZ in a dense-gas tracer, and it delivers: the CMZ is full of filamentary structures in HNCO, spanning roughly 0.1 to 50 pc, and the LF/SF split is a sensible starting taxonomy. The authors are also unusually honest about the limitations—six visually selected objects, hand-drawn masks, no complete sample, no rigorous class definition. That honesty doesn't hurt them; the paper is a clear first look, not a finished classification.

The caveat the stress-test flagged is legitimate. Virtually all the filament identification rests on HNCO 4(0,4)–3(0,3), which the authors themselves describe as a tracer of low-velocity shocks and dense gas. The correlation table in Section 4.4 shows the strongest relationships are with other shock-sensitive lines (SO, HC3N), while H13CO+ and H13CN—cleaner dense-gas tracers—are weakly correlated or uncorrelated, especially for SF2 and SF3. So the robust claim is that the CMZ's HNCO-bright gas is highly filamentary. The jump to "CMZ molecular gas" in the abstract is a generalization the data don't yet support. That's fixable, but it matters, because the whole 'ubiquity' argument depends on it.

What the paper does well: the PPV coherence analysis is careful, the LFs are cross-identified with an independent RHT method reaching ~70% mask overlap, the magnetic field comparison is honest about the poor resolution match and the few vectors overlapping SFs, and the systematic error discussion in Section 5.5.1 is refreshingly candid. Data products and code are public. No free parameters, no fitted claims—circularity burden is low.

The other soft spots are the small sample and projection effects, both acknowledged. The magnetic field conclusions are weak but presented as weak, which is fine.

Who should read it: anyone working on the CMZ, molecular filaments, or Galactic center dynamics. It deserves a serious referee. I'd send it out and ask for a revision that either adds one comparative tracer map for another SF or tempers the abstract to 'HNCO-bright gas.'

Referee Report

3 major / 3 minor

Summary. This paper presents ACES, an ALMA Large Program Band 3 survey of the Central Molecular Zone, and reports that HNCO 4(0,4)–3(0,3) maps at ~0.1 pc resolution reveal pervasive filamentary structures. The authors visually identify two classes: Large-scale Filamentary Structures (LFs, ≳10 pc, three examples) and Small-scale Filamentary Structures (SFs, ~1 pc, three examples). For these six objects they measure morphology, kinematics, and line widths; compare the structures to the Walker et al. (2025) orbital model in PPV space; examine magnetic field alignment using FIREPLACE 214 μm polarimetry; and compute HNCO-to-other-tracer line ratios and Spearman correlations. They conclude that LFs trace CMZ orbital streams, LF 2 resembles an Extended Velocity Feature, and SFs are ubiquitous and likely result from turbulence and shear. The paper explicitly disclaims complete samples or rigorous class definitions and frames the work as highlighting the discovery and presenting a small representative sample.

Significance. The ACES data are a major new resource: a contiguous, high-resolution (3″/0.1 pc) spectral line map of the entire CMZ at 3 mm, with public data products and reproducible reduction on GitHub. If the ubiquity claim holds, this is an important step in characterizing the structure of CMZ molecular gas, with implications for gas inflow, turbulence, and star formation near the Galactic center. The paper also makes concrete connections to orbital models and magnetic fields, and it is transparent about its limitations, including the small hand-selected sample and the lack of a complete classification. The agreement with an independent RHT-based identification in a companion paper (70% mask overlap) is a genuine strength, as is the use of published data (FIREPLACE, Herschel column density, Walker et al. orbital model) for comparison rather than for internal fitting.

major comments (3)

[§3.1; §6] The headline claim that CMZ 'molecular gas' is 'highly filamentary' rests almost entirely on a single tracer, HNCO 4(0,4)–3(0,3), which the authors themselves note traces low-velocity shocks as well as dense gas (§2.1). The multi-tracer validation is limited: detailed spatial/kinematic comparison is shown only for SF 1 (§4.4), and Table 3 reports essentially zero Spearman correlations for SF 2 (all six coefficients between −0.19 and 0.20) and weak correlations for SF 3 with H13CO+ (0.12) and H13CN (0.19). These data do not demonstrate that the filamentary morphology is a general property of the underlying density field rather than a shock-selective chemical effect. Please either present multi-tracer maps or line-intensity comparisons for the other five structures, or qualify the abstract and conclusions to state that the filaments are traced by HNCO (shock-enhanced dense gas). This is no
[null] The 'ubiquitous population of small-scale filamentary structures' is asserted from visual inspection. The paper explicitly disclaims complete samples, class definitions, and a rigorous classification scheme (§3.1), and the conclusions repeat the term 'omnipresent' (§6). No quantitative measure—fraction of HNCO emission in filaments, number density of SFs, or comparison with the RHT sample of Paré et al. (2025a)—is provided to support 'ubiquitous' or 'omnipresent.' Given that this is the paper's central claim, please either include a simple quantitative estimate from the ACES data or soften the wording to 'pervasive in the ACES HNCO maps' so that the stated result matches the demonstrated evidence.
[null] The mitigation of velocity crowding relies on integrating over channel widths ≥5 km s−1 (§3.2). This is a reasonable necessary condition, but it is not sufficient to establish that the SFs are physically coherent spatial density structures rather than line-of-sight/velocity superpositions. The weak multi-tracer correlations for SF 2 and SF 3 (Table 3) and the absence of a direct comparison with the Herschel-derived N(H2) maps (already used in §3.3) leave this ambiguity open. The statement in §3.1 that the SFs 'are not due to velocity crowding effects' would be more convincing if tested explicitly—for example, by comparing the HNCO SF masks with the Herschel column density maps or with an independent velocity-decomposed dense-gas tracer. As it stands, the physical reality of the SF population is an assumption that should be flagged as such in the main text.

minor comments (3)

[null] Typo: 'ACES datset' should be 'ACES dataset.'
[null] The caption reads 'rightpanels' and 'leftpanels'; please insert spaces for readability.
[null] The 'Magnetic Angle' column entries such as '90°⊥' and '0°∥' are clear in context, but the table notes do not define these symbols; a one-line explanation would help.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the filamentary-structure claims are direct observational measurements, and self-citations are used as external benchmarks rather than as load-bearing derivations.

full rationale

The paper is an observational morphology survey, not a derivation. Its central claim—that ACES Band 3 HNCO emission reveals ubiquitous filamentary structures—is a direct result of new ALMA data (§3.1, §4.1) and is not fitted to any model or defined in terms of the conclusion. The six analyzed structures are identified by visual inspection and characterized with standard, publicly available tools (FilFinder, RadFil); the lengths, widths, velocity gradients, and line widths in Table 2 are measurements, not predictions. Comparisons to the Henshaw et al. (2016a) Mopra HNCO decomposition and Walker et al. (2025) orbital model are consistency checks against previously published, independent data; neither was fitted to the ACES filaments, so agreement constitutes evidence rather than a tautology. The magnetic-field analysis uses external FIREPLACE data with standard AM/PRS statistics. No fitted parameter is renamed as a prediction, no uniqueness theorem is invoked, and no ansatz is smuggled in through citation. The main caveat—that the ubiquity claim is based on a single shock-sensitive tracer (HNCO)—is explicitly acknowledged in §2.1 and §5.5.1 and is a scientific interpretation limitation, not a circularity.

Assumptions & free parameters 0 free parameters · 4 assumptions · 0 invented entities

The paper introduces no new free parameters or entities. It relies on standard astrophysical assumptions (distance, tracer fidelity, velocity-caustic mitigation, line-of-sight projection) and external data products.

assumptions (4)

domain assumption The distance to the Galactic Center is 8.2 kpc (Reid et al. 2019; GRAVITY Collaboration et al. 2021).
Used to convert angular sizes to physical lengths and widths (Section 2.1); a different distance would scale all measured sizes.
domain assumption HNCO 4(0,4)-3(0,3) traces dense molecular gas and low-velocity shocks, and its distribution is representative of the dense gas in the CMZ.
The paper uses this single tracer to identify filaments, and notes in Section 5.5.1 that properties could vary with tracer.
domain assumption The integrated moment maps over velocity ranges >=5 km/s suppress velocity-caustic artifacts sufficiently for real structures to be identified.
Section 3.2 discusses velocity caustics and argues that using channel widths exceeding the turbulent velocity dispersion mitigates the effect.
domain assumption FIREPLACE 214 um polarization pseudovectors trace the same gas as the filaments despite possible foreground contamination.
Section 5.5.2 discusses the possibility that magnetic fields originate from foreground structures; the paper argues it is likely tracing the same dense gas.

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Cite this review

Pith. "Pith review of ALMA Central molecular zone Exploration Survey (ACES) VI: ALMA Large Program Reveals a Highly Filamentary Central Molecular Zone." pith.science (2026). https://pith.science/paper/AL5S4YFX

@misc{pith2026260220262,
  author       = {Pith},
  title        = {Pith review of: ALMA Central molecular zone Exploration Survey (ACES) VI: ALMA Large Program Reveals a Highly Filamentary Central Molecular Zone},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/AL5S4YFX}},
  note         = {Machine review of arXiv:2602.20262}
}

read the original abstract

The Central Molecular Zone (CMZ) of the Milky Way is the way station that primarily controls how much gas flows from the disk of the Galaxy towards the central nucleus. While this region is well documented to have extreme gas properties that clearly distinguish it from the rest of the Galaxy, the properties of the bulk molecular gas at high angular resolution are relatively unexplored. Band 3 data from the ALMA (Atacama Large Millimeter/Submillimeter Array) large program ACES (ALMA CMZ Exploration Survey) reveal the highly filamentary nature of CMZ molecular gas at high resolution (3" or 0.1pc) across the entire CMZ. Visual inspection of these data suggests that there are at least two general classes of elongated structures, which we identify as: i) large-scale (10 pc) filamentary structures (LFs) and ii) a ubiquitous population of small-scale (about 1 pc) filamentary structures (SFs). We present detailed morphological and kinematic properties towards three structures in each category, as well as their association with magnetic fields and the correlation of HNCO 4(0,4)-3(0,3) with other molecular species. Our investigation reveals that these structures are largely coherent in position-position-velocity space. The alignment with the magnetic field structure is mixed, with some parallel, some perpendicular, and some intermediate alignments. We find that LFs likely trace pieces of contiguous CMZ orbital structures and are a manifestation of global CMZ dynamics. The second class, SFs, are pervasive and may be the result of complicated turbulence and shearing dynamics in the CMZ gas flows, as seen in numerical simulations.

Figures

Figures reproduced from arXiv: 2602.20262 by the authors.

**Figure 1.** The ACES HNCO 4(0,4)–3(0,3) data reveal the ubiquity of molecular filamentary structures in the CMZ on scales from tens to tenths of pc. The central panel shows a peak intensity map of HNCO 4(0,4)–3(0,3) from ACES and our sample selection in the colored boxes. The top three zoom-ins show the three selected Large-scale Filamentary Structures (LFs) in HNCO, integrated over the velocity range of each individual structu… view at source ↗

**Figure 2.** Small-scale filamentary structures (SFs) are ubiquitous in molecular gas in the CMZ as traced with HNCO 4(0,4)–3(0,3) by ACES. This image shows a zoom-in on the three cyan SF fields displayed in [PITH_FULL_IMAGE:figures/full_fig_p005_2.png] view at source ↗

**Figure 3.** A morphological comparison of the three LFs and SFs is presented in the left panels, which highlights both their similarities (extended filamentary structures coherent in PPV space) and their differences (size scales). The right panels show position-velocity diagrams extracted along the spines (shown as the white line along each filament in left panels, described in Section 3.3), and show largely coherent structures… view at source ↗

Figures from the paper (6 more)

**Figure 4.** Figure 4: The LFs identified in this work appear to trace the CMZ orbital streams, while the SFs are embedded within CMZ cloud structures. This image shows the LFs and SFs identified in this work on top of the kinematic decomposition of Mopra HNCO data created by Henshaw et al. …

**Figure 5.** Figure 5: A qualitative comparison of POS magnetic field orientations from FIREPLACE (yellow lines) with the LFs shows that LF 1 (top) is mostly perpendicular, LF 2 (middle) is mostly parallel, and LF 3 (bottom) is mixed. Grayscale shows the HNCO 4(0,4)–3(0,3) moment 0 map (inte…

**Figure 6.** Figure 6: Few FIREPLACE POS magnetic field orientations overlap with the three SFs (SF 1 at top, SF 2 in the middle, and SF 3 on the bottom), however those that do show relative angles of about 30◦ (SF 2) and 90◦ (SF 3). HNCO 4(0,4)–3(0,3) moment 0 is shown in grayscale with the…

**Figure 7.** Figure 7: A quantitative comparison of structure alignment with magnetic field shows a range of orientations between the LFs and FIREPLACE magnetic field pseudovectors. Plots of the Alignment Measure (AM) calculated for LF 1 (top left), LF 2(top right), and LF 3(bottom left). Th…

**Figure 8.** Figure 8: A comparison between HNCO 4(0,4)–3(0,3) and a variety of molecular species seen with ACES in SF 1 largely shows good correlation, but with some interesting exceptions and potential trends. Integrated intensity maps of HNCO 4(0,4)–3(0,3) (background) from −5 km s−1 to +…

**Figure 9.** Figure 9: A comparison of the HNCO 4(0,4)–3(0,3) position-velocity diagram across SF 1 (from [PITH_FULL_IMAGE:figures/full_fig_p017_9.png]

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Works this paper leans on

3 extracted references · 1 linked inside Pith

[1]

S., Hoang T., 2024, MNRAS, 531, 5012 Anderson L

Akshaya M. S., Hoang T., 2024, MNRAS, 531, 5012 Anderson L. D., et al., 2020, ApJ, 901, 51 Andersson B. G., Lazarian A., Vaillancourt J. E., 2015, ARA&A, 53, 501 André P., et al., 2010, A&A, 518, L102 Astropy Collaboration et al., 2013, A&A, 558, A33 Astropy Collaboration et al., 2018, AJ, 156, 123 Astropy Collaboration et al., 2022, apj, 935, 167 Balança...

2024
[2]

Astronomy, Astrophysics Source Code Library, record ascl:2103.031 Denis-AlpizarO.,StoecklinT.,GuilloteauS.,DutreyA.,2018,MNRAS,478, 1811 Denis-AlpizarO.,StoecklinT.,DutreyA.,GuilloteauS.,2020,MNRAS,497, 4276 Duarte-Cabral A., Dobbs C. L., 2017, MNRAS, 470, 4261 Faure A., Lique F., Wiesenfeld L., 2016, MNRAS, 460, 2103 Federrath C., et al., 2016, ApJ, 832,...

arXiv doi:10.48550/arxiv.2203.09562 2018
[3]

This paper has been typeset from a TEX/LATEX file prepared by the author

Astronomy, University of Manchester, Oxford road, M13 9PL, United Kingdom. This paper has been typeset from a TEX/LATEX file prepared by the author. MNRAS000, 1–24 (2025)

2025

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Reviewed August 2, 2026 · model on record in the stance chip above.

[1] [1]

S., Hoang T., 2024, MNRAS, 531, 5012 Anderson L

Akshaya M. S., Hoang T., 2024, MNRAS, 531, 5012 Anderson L. D., et al., 2020, ApJ, 901, 51 Andersson B. G., Lazarian A., Vaillancourt J. E., 2015, ARA&A, 53, 501 André P., et al., 2010, A&A, 518, L102 Astropy Collaboration et al., 2013, A&A, 558, A33 Astropy Collaboration et al., 2018, AJ, 156, 123 Astropy Collaboration et al., 2022, apj, 935, 167 Balança...

2024

[2] [2]

Astronomy, Astrophysics Source Code Library, record ascl:2103.031 Denis-AlpizarO.,StoecklinT.,GuilloteauS.,DutreyA.,2018,MNRAS,478, 1811 Denis-AlpizarO.,StoecklinT.,DutreyA.,GuilloteauS.,2020,MNRAS,497, 4276 Duarte-Cabral A., Dobbs C. L., 2017, MNRAS, 470, 4261 Faure A., Lique F., Wiesenfeld L., 2016, MNRAS, 460, 2103 Federrath C., et al., 2016, ApJ, 832,...

arXiv doi:10.48550/arxiv.2203.09562 2018

[3] [3]

This paper has been typeset from a TEX/LATEX file prepared by the author

Astronomy, University of Manchester, Oxford road, M13 9PL, United Kingdom. This paper has been typeset from a TEX/LATEX file prepared by the author. MNRAS000, 1–24 (2025)

2025