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arxiv: 2511.03793 · v3 · pith:YPU6BG63new · submitted 2025-11-05 · 🌌 astro-ph.GA

Hot accretion onto spiral galaxies: the origin of extended and warped HI discs

Sriram Sankar , Jonathan Stern , Chris Power , Barbara Catinella , Drummond Fielding , Claude-Andr\'e Faucher-Gigu\`ere , Imran Sultan , Michael Boylan-Kolchin
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Joss Bland-Hawthorn
This is my paper

Pith reviewed 2026-05-18 00:47 UTC · model grok-4.3

classification 🌌 astro-ph.GA
keywords hot accretionHI discswarped discsgalaxy evolutionhydrodynamic simulationsspiral galaxiesgas condensationneutral hydrogen
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The pith

Hot gas from tilted galactic atmospheres condenses into extended, warped HI discs that fuel star formation in spirals.

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

Standard galaxy evolution predicts that massive spirals sit in hot atmospheres tilted relative to their stellar discs. Idealized simulations show that this hot gas flows inward from large scales and continuously condenses into cool discs whose size and tilt are set by the atmosphere's angular momentum. The resulting structures are more extended than the stellar disc and often warped, matching the neutral hydrogen observed around local spirals. If this process operates in nature, it supplies the gas needed for ongoing star formation while naturally producing the ubiquity of extended and warped HI without requiring external events.

Core claim

Using idealized hydrodynamic simulations, the paper shows that the central regions of hot galaxy atmospheres continuously condense into cool discs while being replenished by inflow from larger scales. The size and orientation of each condensed disc are fixed by the angular momentum of the surrounding atmosphere, so the disc is typically tilted and larger than the stellar component. Continuous smooth accretion from hot atmospheres therefore supplies fuel for star formation and accounts for the observed extended and warped HI discs around local spirals.

What carries the argument

Continuous condensation from a tilted hot atmosphere, where the incoming gas's angular momentum sets the radius and orientation of the resulting cool disc.

If this is right

  • Star formation in isolated spirals can be sustained by steady condensation from the hot halo rather than by mergers or cold streams.
  • HI warps should be common even in galaxies without recent interactions, with their outer edges set by the halo's angular momentum.
  • Cool HI observations trace the condensed product rather than the original source of the gas, consistent with the absence of 21 cm emission in halos.
  • Measurements of warp properties can be used to infer the angular momentum, accretion rate, and metallicity of the hot atmosphere.

Where Pith is reading between the lines

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

  • Galaxy formation models may need to treat smooth hot-mode accretion as a primary channel for disc growth at low redshift.
  • The alignment or misalignment between stellar discs and HI warps could serve as a diagnostic for the tilt history of hot halos.
  • If feedback disrupts condensation in some cases, the model predicts a subset of galaxies with truncated or aligned HI discs.

Load-bearing premise

The hot atmosphere maintains a persistent tilt relative to the stellar disc and condensation proceeds smoothly without major disruption from feedback or external events.

What would settle it

A survey of nearby spirals that finds no correlation between HI warp orientations and the expected angular momentum of their hot halos, or that detects substantial 21 cm emission from those halos, would contradict the model.

Figures

Figures reproduced from arXiv: 2511.03793 by Barbara Catinella, Chris Power, Claude-Andr\'e Faucher-Gigu\`ere, Drummond Fielding, Imran Sultan, Jonathan Stern, Joss Bland-Hawthorn, Michael Boylan-Kolchin, Sriram Sankar.

Figure 1
Figure 1. Figure 1: Illustration of the initial conditions in the simulations: a galactic disc surrounded by a hot rotating CGM with a tilted axis. The primed and non-primed coordinate systems respectively describe the CGM and disc ori￾entations, with 𝑧 and 𝑧 ′ along the rotation axes. The 𝑦 = 𝑦 ′ axis is oriented along the line of nodes, and the tilt angle is marked by 𝜃tilt. where we used eqn. (6) and the estimate of 𝑡cool/… view at source ↗
Figure 2
Figure 2. Figure 2: An Hi warp condensing out of a tilted hot CGM. Panels show edge-on projections of the 𝜃tilt = 30◦ simulation at 𝑡 = 400 Myr, after the hot inflow reached a steady state. Gas temperature is shown on the left, Hi column density in the centre, and velocity along the projected axis on the right. White streamlines in the left panel trace the velocity field in the projected plane. The pre-existing disc is appare… view at source ↗
Figure 3
Figure 3. Figure 3: Similar to [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Formation of Hi warps from rotating hot CGM with different tilt angles. Panels show edge-on projections of Hi column density in the 𝑡 = 400 Myr snapshots of the four simulations, with red streamlines tracing the velocity field in the projected plane. Tilt angles are noted on top and are evident from the symmetry axis of the streamlines. The hot inflow cools in the midplane of the tilted CGM in all four cas… view at source ↗
Figure 5
Figure 5. Figure 5: Angular momentum profiles of cool and hot gas in CGM inflows. Panels show the magnitude (top) and angle to the disc axis (bottom) of the mean specific angular momentum vector at each spherical radius, for both cool gas (𝑇 < 105 K, solid) and hot gas (𝑇 > 105 K, dashed). Colors denote the four simulations, which differ in CGM tilt angle, as evident in the hot gas profiles in the bottom panel at large radii.… view at source ↗
Figure 6
Figure 6. Figure 6: The longevity of Hi warps condensing out of a rotating hot CGM for different tilt angles. Each row shows a simulation with different 𝜃tilt, where the three left panels show the edge-on (𝑥𝑧) projections of snapshots at 𝑡 = 0, 400, and 800 Myr, respectively. The rightmost panel plots the angle of the mean specific angular momentum vector of cool gas as a function of radius and time. The size of the pre-exist… view at source ↗
Figure 7
Figure 7. Figure 7: Example trajectories of gas in a hot rotating CGM which cools onto the warp. We show trajectories which cool either at 𝜙105 K ≈ 0 ◦ (red) or at 𝜙105 K ≈ 180◦ (blue), corresponding to cooling at the maximum vertical offset of the warp. Thick curves show the median trajectories, with dashed (solid) segments corresponding to when the inflow is part of the hot CGM (hi warp). comparison that accounts for survey… view at source ↗
Figure 8
Figure 8. Figure 8: Polar and azimuthal angles at which the hot inflow cools from ≈ 106 K to ≈ 104 K, shown both in the disc frame (left) and in the halo frame (right) for the three simulations with 𝜃tilt = 15◦ (top), 𝜃tilt = 30◦ (middle), and 𝜃tilt = 60◦ (bottom). Each point denotes the cooling angles of a single accreting resolution element. In all simulations, most of the accreting gas cools in the halo midplane (𝜃 ′ 105 K… view at source ↗
Figure 9
Figure 9. Figure 9: Distribution of radii at which the hot inflow cools from ≈ 106 K to ≈ 104 K in the 𝜃tilt = 30◦ simulation. Color denotes whether cooling occurs in the CGM midplane (coral) or not (turquoise). The gray line denotes the PDF of circularization radii in the initial conditions. Note that almost all gas that cools beyond 4𝑅d = 10 kpc cools in the CGM midplane, and not farther out in the halo or in the plane of t… view at source ↗
Figure 10
Figure 10. Figure 10: Lagrangian evolution of hot rotating CGM with a tilted rotation axis. Panels show different properties of accreting gas as a function of time since cooling, in the Θ30 simulation. Lines and bands plot the mean and dispersion of properties of all gas which cools at 𝑅105 K > 15 kpc (see [PITH_FULL_IMAGE:figures/full_fig_p013_10.png] view at source ↗
read the original abstract

Gas accretion, hot ($\sim 10^6$ K) atmospheres, and a tilt between the rotation axes of the disc and the atmosphere are all common predictions of standard galaxy evolution theory for massive star-forming galaxies at low redshift. Using idealised hydrodynamic simulations, we demonstrate that the central regions of hot galaxy atmospheres continuously condense into cool ($\sim 10^4$ K) discs, while being replenished by an inflow from larger scales. The size and orientation of the condensed disc are determined by the angular momentum of the atmosphere, so the condensed disc is expected to often be tilted and more extended than the stellar disc. Continuous accretion from hot atmospheres can thus explain the ubiquity of extended and warped HI discs around local spirals, and also potentially provide the necessary fuel for star formation. This hot accretion scenario predicts the absence of significant HI from galaxy halos, consistent with recent 21 cm constraints on nearby spirals (the so-called `HI desert'). Moreover, our analysis indicates that observations of HI warps can be used to constrain the angular momentum, accretion rate, and gas metallicity of hot galaxy atmospheres, important parameters for disc galaxy evolution that are hard to determine by other means.

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 uses idealized hydrodynamic simulations to demonstrate that continuous condensation from tilted hot (~10^6 K) galaxy atmospheres, replenished by larger-scale inflow, forms cool (~10^4 K) discs whose size and orientation are set by the atmosphere's angular momentum. This produces extended, warped cool gas structures relative to the stellar disc, simultaneously supplying fuel for star formation and explaining the ubiquity of extended and warped HI discs around local spirals. The paper further argues that this hot-accretion channel implies cool-gas observations cannot trace the HI source (consistent with the 'HI desert') and that warp observations can constrain hot-atmosphere angular momentum, accretion rate, and metallicity.

Significance. If the mechanism holds, the work supplies a forward-modelled, parameter-free explanation (via standard hydrodynamics with stated initial conditions) for a common observational feature of disc galaxies. It links hot accretion to both star-formation fuel and observable HI morphology, and offers a route to constrain otherwise difficult-to-measure hot-atmosphere properties. The idealized setup isolates the angular-momentum transfer effect cleanly, which is a clear strength.

major comments (2)
  1. [Section 3] Simulation setup (Section 3): the central claim that the condensed disc remains extended and warped relies on the assumption that the hot atmosphere maintains a persistent tilt and that condensation proceeds smoothly. The idealized runs omit stellar/AGN feedback and external perturbations; because feedback is expected to inject turbulence or heat that could realign angular momentum or suppress net condensation, this omission is load-bearing for the link to observed HI ubiquity.
  2. [Section 5] Results and discussion (Section 5): the abstract and text assert that the mechanism explains the 'observed ubiquity' of extended and warped HI discs, yet no quantitative error analysis, statistical comparison to observed warp statistics, or direct matching to data is presented; the agreement therefore remains qualitative and does not yet substantiate the strength of the observational claim.
minor comments (2)
  1. [Abstract] Abstract: the phrase 'recent indications of a lack of 21 cm emission from the halos' would benefit from an explicit citation to the relevant observational work.
  2. [Figures] Figure captions: several panels lack explicit labels for the orientation of the angular-momentum vector or the radial extent of the condensed disc, making it harder to connect the visuals directly to the quantitative claims in the text.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their constructive and insightful comments on our manuscript. These have prompted us to clarify the scope of our idealized simulations and to moderate the strength of our observational claims. We address each major comment below and indicate the revisions made.

read point-by-point responses

  1. Referee: [Section 3] Simulation setup (Section 3): the central claim that the condensed disc remains extended and warped relies on the assumption that the hot atmosphere maintains a persistent tilt and that condensation proceeds smoothly. The idealized runs omit stellar/AGN feedback and external perturbations; because feedback is expected to inject turbulence or heat that could realign angular momentum or suppress net condensation, this omission is load-bearing for the link to observed HI ubiquity.

    Authors: We agree that the omission of feedback and external perturbations is a significant limitation of the current idealized setup. Our simulations were deliberately constructed to isolate the hydrodynamic condensation process from a tilted, rotating hot atmosphere that is continuously replenished, thereby demonstrating that this mechanism alone can generate extended and warped cool discs. We acknowledge that stellar and AGN feedback could inject turbulence, heat the gas, or realign angular momentum in real galaxies, potentially suppressing or disrupting the effect in some systems. However, the continuous replenishment from larger-scale inflows (as assumed in the model) may help sustain the conditions for net condensation. In the revised manuscript we have added an expanded discussion in Section 5 that explicitly addresses this limitation, notes the idealized nature of the runs, and outlines how feedback might interact with the proposed channel. We have also added a sentence in the abstract and conclusions to qualify that the results represent a baseline scenario. revision: partial

  2. Referee: [Section 5] Results and discussion (Section 5): the abstract and text assert that the mechanism explains the 'observed ubiquity' of extended and warped HI discs, yet no quantitative error analysis, statistical comparison to observed warp statistics, or direct matching to data is presented; the agreement therefore remains qualitative and does not yet substantiate the strength of the observational claim.

    Authors: We accept that the current comparison to observations is qualitative and that stronger claims would require quantitative statistics or direct model-data matching, which are beyond the scope of this idealized study. The manuscript's core contribution is the identification of a physically motivated, parameter-free hydrodynamic channel that naturally produces tilted and extended cool discs. In the revised version we have toned down the language in the abstract, introduction, and Section 5, replacing 'explains the observed ubiquity' with 'provides a plausible mechanism contributing to the observed ubiquity'. We have added references to existing observational warp statistics and a short paragraph discussing how future work with more realistic initial conditions or larger parameter surveys could enable quantitative comparisons. These changes better reflect the strength of the evidence while preserving the forward-modeling insight. revision: yes

Circularity Check

0 steps flagged

Forward hydrodynamic simulation with explicit initial conditions

full rationale

The paper obtains its central result by running idealized hydrodynamic simulations that start from stated initial conditions (hot atmosphere with persistent tilt relative to the stellar disc, continuous replenishing inflow, no feedback). The size, orientation, and warp of the condensed cool disc emerge directly as the numerical outcome of angular-momentum conservation and cooling under those inputs; no parameters are fitted to observed HI discs, no self-citation chain is invoked to forbid alternatives, and no ansatz is smuggled in. The derivation is therefore a self-contained numerical experiment rather than a reduction of the target claim to its own assumptions by construction.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The central claim rests on standard assumptions of ideal hydrodynamics and the existence of tilted hot atmospheres, without introducing new fitted parameters or postulated entities beyond those in the simulation initial conditions.

axioms (2)
  • standard math Idealized hydrodynamic equations with radiative cooling govern the gas evolution
    Invoked throughout the simulation description as the governing physics.
  • domain assumption Hot atmospheres around massive galaxies are common and possess angular momentum misaligned with the stellar disc
    Stated as a standard prediction of galaxy evolution theory in the abstract.

pith-pipeline@v0.9.0 · 5797 in / 1158 out tokens · 65988 ms · 2026-05-18T00:47:27.658559+00:00 · methodology

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Forward citations

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  2. A Universal Dance of Galactic Disks: Ubiquitous Precession and Its Implications

    astro-ph.GA 2026-05 unverdicted novelty 6.0

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