pith. sign in

arxiv: 2602.11280 · v2 · submitted 2026-02-11 · 🌌 astro-ph.GA

First statistical constraints on galactic scale outflows properties traced by their extended Mg II emission with MUSE

Ismael Pessa , Lutz Wisotzki , Tanya Urrutia , Nicolas F. Bouch\'e , Floriane Leclercq , Ramona Augustin , Yucheng Guo , Daria Kozlova
show 2 more authors
Haruka Kusakabe John Pharo
This is my paper

Pith reviewed 2026-05-16 02:05 UTC · model grok-4.3

classification 🌌 astro-ph.GA
keywords galactic outflowsMg II emissioncircumgalactic mediumstar-forming galaxiesMUSE integral field spectroscopyoutflow kinematicsstellar mass dependencefeedback processes
0
0 comments X

The pith

Galactic outflows traced by Mg II halos accelerate linearly from 60 km/s launch speeds to maxima of 490 km/s that scale with stellar mass.

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

This paper reports the first statistical analysis of extended Mg II emission around 47 star-forming galaxies at redshifts 0.7 to 2.0 observed with MUSE. The data are modeled as outflows consisting of radially accelerating shells, revealing that the winds are driven by star formation and show linear velocity increase with radius. Galaxies hosting these outflows display elevated star formation rates, specific star formation rates, and younger stellar ages compared to the broader population. Halo sizes peak near 5 kpc half-light radius but extend to 20 kpc, with compact halos tied to stellar mass while larger ones are not. The study supplies population-level constraints on outflow velocity, opacity, and geometry that single-object studies could not establish.

Core claim

Deep MUSE integral-field observations of 47 galaxies show extended Mg II halos tracing galactic-scale outflows. Modeling these as an ensemble of radially accelerating shells indicates that the winds launch at velocities around 60 km/s and accelerate linearly outward, reaching maximum velocities up to 490 km/s that correlate positively with host-galaxy stellar mass. The inner regions remain highly opaque, opening angles tend to be wide in lower-mass systems but span a wider range in higher-mass galaxies, and the half-light radii of the halos cluster around 5 kpc with a tail to larger sizes.

What carries the argument

Outflow modeling framework that represents the circumgalactic gas as an ensemble of radially accelerating shells, now applied statistically across the sample after prior single-object validation.

If this is right

  • Maximum outflow velocities increase with stellar mass, reaching higher terminal speeds in more massive galaxies.
  • Compact Mg II halos with half-light radii below 8 kpc correlate with stellar mass, whereas extended halos show no such correlation.
  • Galaxies with detected Mg II outflows have systematically higher star-formation rates, specific star-formation rates, and younger stellar populations.
  • Central optical depth shows a tentative positive trend with stellar mass.
  • Outflow opening angles are predominantly wide at low stellar mass but display both wide and narrow values at higher mass.

Where Pith is reading between the lines

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

  • A sustained driving mechanism such as continuous radiation pressure or cosmic-ray feedback would be needed to produce the observed linear acceleration rather than a single impulsive ejection event.
  • The lack of stellar-mass correlation for the largest halos raises the possibility that some extended structures are powered by AGN activity, which could be tested with X-ray or radio observations.
  • These velocity and geometry constraints can be used to calibrate sub-grid feedback implementations in cosmological simulations so that predicted outflow speeds and covering fractions better match observed Mg II halos.

Load-bearing premise

The analysis assumes outflows can be represented as ensembles of radially accelerating shells without separate confirmation of shell geometry across the full sample.

What would settle it

Spatially resolved Mg II velocity maps or line profiles from additional galaxies that show constant-velocity or decelerating flows instead of the linear radial acceleration required by the shell model.

read the original abstract

Galaxies evolve within vast gaseous halos that fuel star formation and carry signatures of feedback-driven outflows. Deep integral field data have enabled the study of MgII halos, which trace galaxy-scale outflows in emission, but their faintness has limited studies to single-object analyses. Here, we present the first statistical study of MgII-emitting halos using deep MUSE observations of 47 star-forming galaxies at $0.7<z<2.0$. Building on our previous work, where we developed and applied an outflow modeling framework for a single MgII halo, we now extend this approach to a larger sample, enabling robust population-level insights on the properties of circumgalactic outflows traced by their extended MgII emission. We detect extended emission out to tens of kiloparsecs and model the outflows as an ensemble of radially accelerating shells. Galaxies with MgII outflows tend to have higher SFRs, sSFRs, and younger stellar populations, consistent with star-formation-driven winds. The observations are consistent with winds that accelerate linearly with radius, from launching velocities of ~60 km/s up to maximum velocities that correlate with stellar mass and reach ~490 km/s. Their inner regions are highly opaque, and we find a tentative trend between stellar mass and central optical depth. The opening angle of the outflow shows some dependency on the host-galaxy stellar mass, with less massive galaxies showing primarily wide opening angles, and more massive galaxies showing a broader range of values, with both wide and narrow opening angles. The distribution of the spatial extent of MgII halos exhibits a clear peak at half-light radius (HLR) of ~5 kpc, with an extended tail of larger HLR values, up to ~20 kpc. Compact halo sizes (HLR $< 8$ kpc) correlate with stellar mass, but extended halos do not, which could suggest a difference in the powering mechanism between compact and extended halos.

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.

Referee Report

2 major / 2 minor

Summary. The manuscript presents the first statistical study of extended Mg II emission tracing galactic outflows in 47 star-forming galaxies at 0.7 < z < 2.0 observed with MUSE. Extending a prior single-object modeling framework, the authors model the halos as ensembles of radially accelerating shells and report consistency with linear acceleration from launch velocities of ~60 km/s to maximum velocities reaching ~490 km/s that correlate with stellar mass. They identify associated trends with SFR, sSFR, stellar population age, central optical depth, outflow opening angle, and halo half-light radii (peaking at ~5 kpc with a tail to ~20 kpc), including a distinction between compact and extended halos.

Significance. If the modeling framework and its assumptions are validated, the work supplies the first population-level constraints on outflow kinematics and geometry from Mg II emission, moving the field beyond individual case studies toward statistical trends that can be compared with simulations of star-formation-driven winds. The sample size and forward-modeling approach are clear strengths.

major comments (2)
  1. [§3] §3 (outflow modeling framework): the central claim that the data are consistent with radially accelerating shells whose velocity increases linearly with radius rests on extending the single-object shell geometry without reported comparisons to alternative profiles (constant-velocity, decelerating, or non-shell geometries). This assumption directly determines the reported launch velocity (~60 km/s), maximum velocity (~490 km/s), and linear radial dependence; if other profiles fit equally well, these specific quantitative statements do not follow uniquely from the observations.
  2. [Results] Results section (velocity and correlation trends): the reported maximum velocities, stellar-mass correlations, and compact vs. extended halo distinctions lack visible error bars, uncertainty quantification, or robustness checks against sample selection and post-hoc HLR cuts (HLR < 8 kpc). This undermines assessment of whether the trends are statistically significant or sensitive to modeling choices.
minor comments (2)
  1. [Abstract] Abstract and §4: the post-hoc separation into compact and extended halos would benefit from a pre-specified size threshold or sensitivity test to confirm it does not drive the reported differences in powering mechanisms.
  2. [Sample] Sample description: additional details on selection criteria, completeness, and any biases in the 47-galaxy sample would clarify the generality of the population trends.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their constructive review and positive assessment of the work's significance. We address each major comment point by point below, providing the strongest honest defense of the manuscript while acknowledging where revisions are warranted.

read point-by-point responses

  1. Referee: [§3] §3 (outflow modeling framework): the central claim that the data are consistent with radially accelerating shells whose velocity increases linearly with radius rests on extending the single-object shell geometry without reported comparisons to alternative profiles (constant-velocity, decelerating, or non-shell geometries). This assumption directly determines the reported launch velocity (~60 km/s), maximum velocity (~490 km/s), and linear radial dependence; if other profiles fit equally well, these specific quantitative statements do not follow uniquely from the observations.

    Authors: The modeling framework was developed and validated through detailed fitting in our prior single-object study, where the linear radial acceleration provided the best match to the observed Mg II kinematics and surface brightness profiles. For the current statistical sample of 47 galaxies, we extended this same framework to derive population-level constraints. While we agree that explicit comparisons to constant-velocity or decelerating profiles are not presented here, such alternatives were not favored in the original validation and would require substantial additional computational resources for the full sample. In revision, we will add a dedicated paragraph in §3 discussing the physical motivation (e.g., consistency with radiation-pressure-driven wind models) and note the limitations of not testing alternatives in this work. revision: partial

  2. Referee: [Results] Results section (velocity and correlation trends): the reported maximum velocities, stellar-mass correlations, and compact vs. extended halo distinctions lack visible error bars, uncertainty quantification, or robustness checks against sample selection and post-hoc HLR cuts (HLR < 8 kpc). This undermines assessment of whether the trends are statistically significant or sensitive to modeling choices.

    Authors: We agree that the results section would be strengthened by explicit uncertainty quantification. The reported velocities and correlations are derived from the forward-modeling posteriors, but these were not visualized with error bars in the submitted version. In the revised manuscript, we will add error bars to all velocity and correlation plots, report the median uncertainties from the modeling, and include a new subsection with robustness tests against sample selection effects and the HLR < 8 kpc cut (e.g., repeating the trends with varied thresholds and reporting p-values or Spearman coefficients with uncertainties). This will allow readers to assess the statistical significance directly. revision: yes

Circularity Check

0 steps flagged

No significant circularity in derivation chain

full rationale

The paper applies a forward-modeling framework (ensemble of radially accelerating shells) developed in prior single-object work to new MUSE data on 47 galaxies at 0.7<z<2.0. Reported quantities such as launching velocities (~60 km/s), maximum velocities (~490 km/s), linear radial acceleration, optical depths, and opening angles are direct outputs of fitting this model to the observed Mg II surface-brightness and kinematics profiles. No step renames a fitted parameter as an independent prediction, invokes a self-citation uniqueness theorem, or reduces the central statistical claims to the inputs by construction. The self-citation serves only to reference the method; the population-level constraints remain data-driven and falsifiable against the current sample. The derivation is therefore self-contained.

Axiom & Free-Parameter Ledger

4 free parameters · 2 axioms · 0 invented entities

Central claims rest on the domain assumption that extended MgII traces star-formation-driven outflows modeled as radially accelerating shells, with several fitted parameters for velocity, opacity, and geometry derived from the MUSE data.

free parameters (4)
  • launching velocity = ~60 km/s
    Starting speed of the accelerating shells fitted to match observed emission profiles.
  • maximum velocity = ~490 km/s
    Terminal speed of the wind, reported to correlate with stellar mass.
  • central optical depth
    Opacity parameter showing tentative stellar-mass trend.
  • opening angle
    Angular width of the outflow cone, varying with stellar mass.
axioms (2)
  • domain assumption MgII emission traces galactic-scale outflows driven by star formation
    Standard assumption in the field, invoked to interpret the extended halos as wind signatures.
  • domain assumption Outflows can be modeled as an ensemble of radially accelerating shells
    Carried forward from the authors' prior single-object framework and applied to the statistical sample.

pith-pipeline@v0.9.0 · 5705 in / 1527 out tokens · 204869 ms · 2026-05-16T02:05:54.008748+00:00 · methodology

discussion (0)

Sign in with ORCID, Apple, or X to comment. Anyone can read and Pith papers without signing in.