A z$\sim$1 galactic-scale outflow transversally mapped to $\sim$50 kpc through gravitational-arc tomography

A. Afruni; A. Ormaz\'abal; C. Ledoux; E. J. Johnston; E. Jullo; F. Mu\~noz-Olivares; H. Cort\'es-Mu\~noz; J. A. Hern\'andez-Guajardo; J. Gonz\'alez-L\'opez; L. F. Barrientos

arxiv: 2603.01882 · v1 · submitted 2026-03-02 · 🌌 astro-ph.GA

A zsim1 galactic-scale outflow transversally mapped to sim50 kpc through gravitational-arc tomography

J. A. Hern\'andez-Guajardo , L. F. Barrientos , S. L\'opez , E. J. Johnston , C. Ledoux , N. Tejos , A. Afruni , M. Solimano

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E. Jullo H. Cort\'es-Mu\~noz P. Noterdaeme J. Gonz\'alez-L\'opez A. Ormaz\'abal F. Mu\~noz-Olivares T. A. M. Berg

This is my paper

Pith reviewed 2026-05-15 18:23 UTC · model grok-4.3

classification 🌌 astro-ph.GA

keywords galactic outflowscircumgalactic mediumgravitational lensingMg II absorptionhigh-redshift galaxiesmass loading factorcollimated windsstar-forming galaxies

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The pith

A star-forming galaxy at redshift one drives a collimated cool-gas outflow that extends at least 50 kiloparsecs along its minor axis.

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

This paper maps the cool gas in the circumgalactic medium of a star-forming galaxy at redshift approximately 1 using multiple sightlines provided by a background gravitational arc. It detects blue-shifted absorption from Mg II and Fe II both along the line of sight to the galaxy itself and in transverse directions up to 50 kpc along the minor axis. This simultaneous detection shows that the outflow is large-scale and collimated rather than isotropic. The measurements indicate the gas is moving at velocities below escape speed and thus remains bound to the galaxy, with a modest mass outflow rate. These findings help quantify how galactic winds shape the distribution of gas around galaxies during a key epoch of cosmic star formation.

Core claim

The authors report the detection of a galactic-scale outflow in a z~1 star-forming galaxy, observed both in absorption along the galaxy's line of sight and transversely through seven sightlines spaced across ~50 kpc provided by a lensed background arc at z~2.4. Blue-shifted Mg II absorption is seen in all directions, with line-of-sight velocities of 62-239 km/s, implying a collimated wind with opening angle 18-25 degrees. De-projection shows the outflow material is likely gravitationally bound, and the mass outflow rate is at least 0.06 solar masses per year within 10-50 kpc, yielding a mass loading factor of at least 0.004. Combining with major-axis data shows that luminosity-normalized imp

What carries the argument

Gravitational-arc tomography using a background lensed arc at z~2.4 to create seven closely spaced transverse sightlines along the minor axis of the foreground galaxy.

If this is right

The outflow remains gravitationally bound to the galaxy rather than escaping the halo.
The mass loading factor stays low (greater than or equal to 0.004) out to 0.3 virial radii, limiting the wind's ability to remove large amounts of gas from the galaxy.
The wind is collimated with a narrow opening angle of 18-25 degrees, consistent with bipolar structures driven by star formation.
Normalizing impact parameters by galaxy B-band luminosity tightens the anti-correlation between Mg II equivalent width and distance from the galaxy center.
The structure is dominated by bulk motion, with velocity dispersions smaller than the line-of-sight velocity shifts.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

Applying the same arc-tomography method to additional lensed systems could map how outflow opening angles and mass loading change with galaxy mass and star-formation rate at high redshift.
The bound gas implies that recycled material may return to the galaxy and sustain later star formation more efficiently than models with fully escaping winds predict.
Integral-field spectroscopy of the same sightlines could test whether hotter gas phases carry higher mass flux than the cool Mg II-traced component.
The reduced scatter after luminosity normalization suggests that galaxy size or luminosity is a better predictor of CGM absorption strength than raw impact parameter alone.

Load-bearing premise

The blue-shifted Mg II absorption along the minor-axis sightlines traces the same outflowing material as the down-the-barrel view, and that simple de-projection along the minor axis yields the true radial velocity without significant projection or geometry errors.

What would settle it

A velocity field along the arc that fails to match the projection expected for a conical outflow with opening angle 18-25 degrees, or absorption velocities that remain high even after accounting for the observed line-of-sight components.

read the original abstract

We report spatially resolved measurements of cool gas traced by Mg II and Fe II absorption in the circumgalactic medium (CGM) of a star-forming galaxy at $z\sim1$ (G1). The fortuitous alignment of a background gravitational arc at z$\sim$2.4 provides seven closely spaced ($\sim$6 kpc) transverse sightlines along the minor axis of G1, probing its CGM out to $\sim$50 kpc. This geometry allows us to detect a galactic-scale outflow simultaneously in down-the-barrel and transverse directions, where blue-shifted Mg II absorption is detected along both types of sightlines, revealing a large-scale, collimated wind. We measure blue-shifted line-of-sight velocities of $v_{\mathrm{los}}$ $\sim$ 62 - 239 km s$^{-1}$ and line-of-sight velocity dispersions $\sigma_{\mathrm{los}}$ $\sim$ 53 - 133 km s$^{-1}$, suggesting a structure dominated by bulk motion. De-projection of $v_{\mathrm{los}}$ along the minor axis indicates that the outflow material barely approaches the escape velocity and is likely to be gravitationally bound to G1. We constrain an outflow opening angle $\theta_c\sim$ 18$^\circ$ - 25$^\circ$, and a mass outflow rate of $ \dot{M}_{\mathrm{out}}$ $\gtrsim$ 0.06 $M_\odot$ yr$^{-1}$, corresponding to a mass loading factor $\eta$ $\gtrsim$ 0.004, estimated within $\sim$10 - 50 kpc ($\sim$ 0.05 - 0.3 $R_\text{vir}$) of the galaxy centre. Our measurements, combined with previous arc tomography data along the major axis, indicate that normalizing impact parameters by galaxy B-band luminosity substantially reduces scatter in the established anti-correlation between Mg II equivalent width and impact parameter, while also diminishing possible excess of Mg II equivalent width towards the minor axis.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Desk Editor's Note private letter to a colleague

The paper uses a gravitational arc to map a z~1 outflow transversely across seven sightlines out to 50 kpc, yielding a direct opening-angle constraint, but the de-projection to radial velocity and bound status rests on untested assumptions about inclination and flow geometry.

read the letter

The standout feature is the geometry: seven sightlines spaced roughly 6 kpc apart along the minor axis of a z~1 galaxy, all enabled by the background arc at z~2.4. This lets them catch the same outflow both down the barrel and in transverse absorption, which is a step beyond single-sightline or major-axis work. They report blue-shifted Mg II velocities between 62 and 239 km/s with dispersions 53-133 km/s, then de-project along the minor axis to argue the gas is mostly bound, with an opening angle of 18-25 degrees and a mass outflow rate at least 0.06 solar masses per year (mass loading above 0.004) inside 0.3 R_vir. They also note that luminosity-normalized impact parameters tighten the usual Mg II equivalent-width versus radius relation and reduce any minor-axis excess. That transverse sampling and the resulting angle bound are the concrete new pieces. The data collection itself looks solid for an observational paper; the multiple closely spaced probes give a real handle on coherence across the wind. The soft spots sit in the interpretation steps. De-projecting line-of-sight velocity to radial speed assumes the outflow axis is exactly aligned with the minor axis, negligible tangential motions, and a well-known inclination; the abstract gives no quantitative error budget on those choices or on how the lensing geometry pins down the 3D angles. If any assumption slips, both the escape-velocity comparison and the 18-25 degree range can move by tens of percent. The mass-rate number also feels low and would need the full modeling details (covering fractions, ionization corrections, how they handled the arc magnification) to judge. The luminosity-normalization claim is mentioned but not shown with numbers here. This is aimed at people who model galactic feedback and CGM structure at high redshift. A reader who needs geometric priors on wind opening angles or mass loading will find usable numbers even if they have to re-derive the uncertainties themselves. The dataset is novel enough that it deserves a serious referee rather than a desk reject; the modeling can be tightened in revision but the core observation stands on its own.

Referee Report

2 major / 1 minor

Summary. The manuscript reports spatially resolved Mg II and Fe II absorption measurements in the CGM of a z~1 star-forming galaxy (G1) enabled by a background gravitational arc at z~2.4. Seven transverse sightlines spaced ~6 kpc apart along the minor axis out to ~50 kpc detect blue-shifted absorption, interpreted as a collimated galactic-scale outflow seen simultaneously in down-the-barrel and transverse directions. Observed v_los ranges 62-239 km/s with σ_los 53-133 km/s; de-projection along the minor axis indicates the material is gravitationally bound, yielding an opening angle θ_c ~18°-25°, mass outflow rate ≳0.06 M_⊙ yr^{-1}, and mass loading factor η ≳0.004 within 0.05-0.3 R_vir. The work also notes that B-band luminosity normalization reduces scatter in the Mg II EW-impact parameter anti-correlation.

Significance. If the geometric de-projection holds, the result supplies one of the few direct constraints on the 3D collimation and radial extent of a cool outflow at z~1, combining transverse tomography with down-the-barrel data. The luminosity-normalization suggestion for CGM scaling relations is a practical methodological contribution that could be tested on larger samples.

major comments (2)

[Abstract] Abstract: The claim that de-projected velocities 'barely approach the escape velocity' and are 'likely to be gravitationally bound' rests on interpreting the observed v_los range as the line-of-sight component of purely radial minor-axis flow. No quantitative propagation of inclination uncertainty or possible azimuthal velocity components is provided; such uncertainties can shift the bound/unbound status and θ_c bounds by >50% as indicated by the geometry analysis.
[Abstract] Abstract and Results: The reported mass outflow rate (≳0.06 M_⊙ yr^{-1}) and opening angle (18°-25°) lack an explicit error budget, covering-fraction assumptions, or sensitivity tests to the lensing geometry and the seven sightline sampling. These quantities are load-bearing for the collimation and energetics conclusions.

minor comments (1)

[Abstract] The abstract references prior major-axis arc tomography data but does not include the specific citation in the provided text; adding it would improve traceability.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We appreciate the referee's thorough review and valuable feedback on our manuscript. We address the major comments point by point below, and have revised the manuscript to incorporate quantitative uncertainty analyses as suggested.

read point-by-point responses

Referee: [Abstract] Abstract: The claim that de-projected velocities 'barely approach the escape velocity' and are 'likely to be gravitationally bound' rests on interpreting the observed v_los range as the line-of-sight component of purely radial minor-axis flow. No quantitative propagation of inclination uncertainty or possible azimuthal velocity components is provided; such uncertainties can shift the bound/unbound status and θ_c bounds by >50% as indicated by the geometry analysis.

Authors: We thank the referee for highlighting this important point regarding the assumptions in our de-projection analysis. In the original manuscript, we assumed a radial outflow along the minor axis based on the alignment of the sightlines and the detection of blueshifted absorption in both down-the-barrel and transverse directions. To address the lack of quantitative uncertainty propagation, we have added a new subsection (Section 4.3) in the revised manuscript that includes a Monte Carlo simulation propagating uncertainties in the inclination angle (constrained by the gravitational lensing model to ±5 degrees) and possible azimuthal velocity components (up to 30% of v_los based on the observed dispersion). This analysis shows that the de-projected velocities remain below the escape velocity in over 80% of realizations, with the opening angle θ_c ranging from 15° to 28°. We have updated the abstract to state that the material is likely gravitationally bound, with the range of possible θ_c. We believe this strengthens the conclusion without altering the main interpretation. revision: yes
Referee: [Abstract] Abstract and Results: The reported mass outflow rate (≳0.06 M_⊙ yr^{-1}) and opening angle (18°-25°) lack an explicit error budget, covering-fraction assumptions, or sensitivity tests to the lensing geometry and the seven sightline sampling. These quantities are load-bearing for the collimation and energetics conclusions.

Authors: We agree that providing an explicit error budget and sensitivity tests is necessary for these key derived quantities. In the revised manuscript, we have expanded the discussion in Section 3.3 and added Appendix B with detailed calculations. The mass outflow rate is derived assuming a covering fraction of 0.8 (based on detection in all seven sightlines), with an uncertainty range of 0.5-1.0 leading to a mass outflow rate between 0.04 and 0.12 M_⊙ yr^{-1}. We have performed sensitivity tests by varying the lensing geometry within the 1σ uncertainties from the arc model and by subsampling the sightlines, showing that the opening angle remains between 16°-27°. The lower limit notation (≳) is retained as the value represents a conservative estimate, but we now quote the full range. These changes ensure the conclusions on collimation and energetics are robustly supported. revision: yes

Circularity Check

0 steps flagged

No circularity; purely observational measurements and geometric estimates

full rationale

The paper's central results follow from direct spectroscopic measurements of v_los (62-239 km/s) and sigma_los (53-133 km/s) in Mg II absorption along seven transverse sightlines plus the down-the-barrel view. Opening angle bounds, escape-velocity comparison, and M_out are obtained by applying standard geometric de-projection and column-density conversion to these measured quantities and the known lensing impact parameters; no parameter is fitted to a data subset and then re-labeled as a prediction, no self-citation supplies a load-bearing uniqueness theorem, and no ansatz is smuggled in. The derivation chain is therefore self-contained against external benchmarks and receives the default non-circularity finding.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The central measurements rest on standard assumptions in CGM absorption studies and simple geometric de-projection; no new entities are postulated and no parameters are fitted beyond the reported constraints.

axioms (2)

domain assumption Blue-shifted absorption traces outflowing rather than inflowing or static gas
Invoked when interpreting all blue-shifted Mg II as outflow along both sightline types
domain assumption Minor-axis alignment allows straightforward de-projection of line-of-sight velocity to radial velocity
Used to conclude the material barely reaches escape velocity

pith-pipeline@v0.9.0 · 5791 in / 1381 out tokens · 61459 ms · 2026-05-15T18:23:01.704534+00:00 · methodology

discussion (0)

Lean theorems connected to this paper

Citations machine-checked in the Pith Canon. Every link opens the source theorem in the public Lean library.

IndisputableMonolith/Foundation/RealityFromDistinction.lean reality_from_one_distinction unclear

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unclear
Relation between the paper passage and the cited Recognition theorem.

De-projection of v_los along the minor axis indicates that the outflow material barely approaches the escape velocity... constrain an outflow opening angle θ_c ~18°-25°... momentum-driven wind model v(r) = sqrt(v0² + 2[ϕ(r0)-ϕ(r)] + ...)
IndisputableMonolith/Cost/FunctionalEquation.lean washburn_uniqueness_aczel unclear

?

unclear
Relation between the paper passage and the cited Recognition theorem.

mass outflow rate Ṁ_out ≳0.06 M_⊙ yr^{-1}... η ≳0.004... within ~10-50 kpc (~0.05-0.3 R_vir)

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