How Do Disk Galaxies Form?

arXiv:2602.04950 · astro-ph.GA, astro-ph.CO · Submitted 2026-02-04 · Read on arXiv

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Introduction to the show: ident: Astrophysics Radio. Generated commentary on the latest astrophysics papers.

Vera: I'm Vera, and with me are Jocelyn and Subrahmanyan, guest researcher.

Jocelyn: Today's paper: "How Do Disk Galaxies Form?".

Vera: Disk galaxies account for most of star formation in today's Universe,

Jocelyn: First, who's behind it and why it matters.

Title and authors: Vera: Now, looking at the title and the authors of "How Do Disk Galaxies Form?", it really sets up this investigation into the physical mechanism behind galaxy morphology evolution. It’s about understanding what drives that shift toward disks in today's Universe.

Jocelyn: The paper is authored by Semenov, Ferreira et al., de Graaff et al., Robertson et al., and Danhaive et al., which shows it’s a collaborative effort pulling from various simulation and observational findings.

Subrahmanyan: And those collaborators bring different strengths to the analysis; we get theoretical insights from the simulations and observational constraints on how this process unfolds in practice.

Vera: What that means for us is that they are trying to provide a unified analytical framework, using dark matter halo properties like mass and concentration as inputs to predict disk formation.

Jocelyn: It’s about moving beyond just looking at the final shape and trying to pinpoint the exact physical conditions—like the potential's steepness—that trigger that shape change.

Subrahmanyan: The implication here is that we might be able to use these halo properties to forecast galaxy evolution much more reliably, giving us a clearer picture of when and why disks begin to dominate.

Vera: So, instead of just seeing a population trend, this paper aims to give us the underlying physics that dictates that trend across cosmic time.

Jocelyn: It sounds like they are trying to establish a formal rule based on halo concentration and mass that governs disk formation.

The paper's summary: Vera: To summarize what this paper is doing, it’s developing an analytical model where galaxy morphology hinges on whether the gravitational potential steepens enough due to baryon accumulation in the center of a dark matter halo.

Jocelyn: That steepening is what triggers disk formation, and they link this directly to how concentrated the host halo is through its concentration parameter.

Subrahmanyan: They establish a criterion by comparing two spatial scales: the circularization radius, where baryons settle into rotational support, and the turbulence confinement radius.

Vera: And they find that a disk forms when that scale for settling baryons becomes smaller than the scale on which turbulent motions are contained by gravity, which they state is r circ/r turb > one <ref:2602.04950#pg0,the scale on which turbulent motions>.

Jocelyn: This inequality is equivalent to saying the rotational velocity divided by the velocity dispersion must be greater than one, or v rot/sigma > one which simplifies things nicely for comparison with existing data <ref:2602.04950#pg0>.

Subrahmanyan: Furthermore, they tie the disk size to the halo's spin parameter, suggesting that angular momentum is largely retained on average during this process.

Vera: And they also set a threshold for minimum galaxy mass by requiring that the circular velocity at half-mass radius exceeds the virial velocity of the host halo, specifically v c(r b)/v vir > one <ref:2602.04950#pg0>.

Jocelyn: It’s a very concrete set of conditions they’ve derived, and it allows us to test these morphological predictions against cosmological simulations like TNG50.

The paper's improvements: Vera: One significant improvement the authors introduce is this formal criterion for disk formation, which links morphology to basic properties of the dark matter halo, specifically its mass, concentration, and spin parameter.

Jocelyn: They go beyond just describing what happens; they create a predictive framework that uses these halo properties and baryonic fractions to classify simulated galaxies as either "non-disk" or "disk."

Subrahmanyan: This allows for the derivation of critical physical parameters, specifically the minimal baryon mass fraction in the center, denoted as f b, and the minimum halo concentration, c disk, required for disk formation.

Vera: They’ve shown how these critical thresholds evolve with redshift by linking them directly to cosmological structure formation models.

Jocelyn: It's really about creating a model that predicts how these critical values will change over cosmic time, which is crucial since the density of matter in the universe has been evolving so much.

Subrahmanyan: They also provide a mechanism to distinguish between different size regimes for galaxies before and after disk formation, separating sizes set by turbulent confinement from those set by halo spin.

Vera: And they’ve even proposed that the primary driver for the observed increase in the critical halo mass as redshift goes up is actually the evolution of halo concentration, rather than just weakly evolving baryonic mass fractions.

Conclusion: Jocelyn: So, wrapping up, this paper provides a formal criterion for disk formation based on v c/v vir > one evaluated at the half-mass radius r b <ref:2602.04950#pg0>. This gives us a clear metric to check galaxy shapes.

Vera: And they’ve shown that this transition is driven by increasing halo concentration, and that the typical halo mass where disks start forming increases by roughly an order of magnitude from redshift zero to three.

Subrahmanyan: From my perspective, this confirms the theoretical expectation that environmental and structural changes in a halo are primary drivers for these large-scale morphological shifts in galaxies.

Jocelyn: It’s fascinating how they’ve managed to capture the diversity in galaxy properties by showing how scatter in baryonic mass fractions and halo concentrations dictates when disk formation happens versus when mergers might destroy it.

Vera: Ultimately, this paper on "How Do Disk Galaxies Form?" gives us a predictive tool to classify galaxies based on their dark matter environment and internal structure.

Subrahmanyan: It’s a solid piece of analytical work that connects microphysics—like the competition between rotation and turbulence—to macro-scale cosmological trends in galaxy populations.

Jocelyn: It really frames the observational puzzles we see in surveys like TNG50 with a more precise theoretical lens, and I think this will be very useful for interpreting future data from instruments like ALMA.

Center for Astrophysics | Harvard & Smithsonian

astro-ph.GA, astro-ph.CO

Submitted: 2026-02-04

Updated: 2026-10-02

Comments: 26 pages, 11 figures. Accepted for publication in ApJ

License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/

Importance score: 77/100

The gist: Disk galaxies account for most of star formation in today's Universe, and this paper develops an analytical model to elucidate the physical origin of the mass threshold for disk galaxy formation and

Key concepts

Circularization Radius (rcirc)
This is the radius where gas settles into rotational support. The model assumes this scale is set by the halo's spin parameter, meaning it relates to how angular momentum is retained during structure formation. This radius helps define the initial size of a potential disk.
Turbulence Confinement Radius (rturb)
This scale defines where turbulent motions are contained by gravity. Disk formation occurs when the radius where baryons settle into rotation (rcirc) becomes smaller than this confinement radius, leading to a condition that favors rotational support over turbulence.
Disk Formation Criterion (vc/vvir > 1)
The formal criterion for forming a disk is when the circular velocity at the galaxy's half-mass radius exceeds the virial velocity of its host halo. This condition signifies that the gravitational potential has become steep enough to overcome turbulent support and force baryons into a rotationally supported disk state.

Terminology

Summary

Disk galaxies account for most of star formation in today's Universe, and this paper develops an analytical model to elucidate the physical origin of the mass threshold for disk galaxy formation and its evolution with redshift.

The gist

Our model predicts that galaxy morphology is governed by the competition between rotational support and turbulence in a gravitational potential, forming a disk when the potential steepens due to baryon accumulation, and finds remarkable agreement with TNG50 simulation results by linking disk formation to the evolution of halo concentration.

How it works

The core mechanism for disk formation is defined by comparing two spatial scales: the radius on which baryons settle into rotational support, denoted as the circularization radius, and the scale on which turbulent motions are contained by gravity, denoted as the turbulence confinement radius. A disk forms when this scale on which baryons settle into rotational support becomes smaller than the scale on which turbulent motions are contained by gravity, leading to a criterion of "rcirc/rturb > 1, which is equivalent to vrot/σ > 1."

The paper develops a criterion for disk formation linking galaxy morphology to basic properties of its dark matter halo: halo mass, concentration, and spin parameter. The model assumes that angular momentum is retained on average, setting the disk size by the halo's spin parameter: rcirc ∼ λrvir. The minimal mass threshold for disk formation is then determined by requiring that the potential becomes sufficiently steep. This steepening is achieved when the circular velocity at the galaxy's half-mass radius, vc(rb), exceeds the virial velocity of the host halo: "vc(xb)/vvir > 1."

Key Model Assumptions

The analytical framework relies on several key assumptions to derive a testable criterion. First, it is assumed that gas inherits its angular momentum from large-scale structure formation and retains it on average, fixing the constant angular momentum line based on the halo spin parameter λ. Second, to find the minimal halo mass threshold at which disk formation can be satisfied in the limit of high velocity dispersion (σ ∼ vvir), it assumes that the galaxy size in this regime is approximately equal to rturb, meaning vc(rturb) = σ = vvir. Finally, the model assumes that the only way to trigger disk formation is by developing a sufficiently steep potential in the center, requiring vc(r) at the scale of the galaxy size to equal vvir.

Transition and Evolution

The transition from non-disks (vrot/σ 1) can occur in two qualitatively different ways: either by decreasing σ relative to vvir, or by making the potential steep, resulting in a peaked rotation curve. The paper focuses on the latter as the main driver at the threshold when vrot/σ ∼ 1 is met.

The model predicts that for MW-like galaxies in TNG50, disk formation is primarily driven by the increase of the halo concentration, as baryonic mass fraction evolves only weakly. This shift from non-disks to disks occurs when "vc/vvir > 1, and this critical condition depends on the halo concentration (Equation 9). The model also predicts that the typical halo mass at which disks start to form increases by roughly an order of magnitude from z ∼ 0 to 3," driven by the evolution of halo concentrations.

Validation against Simulations

The model is validated against TNG50 cosmological simulations. In Section 3.2, the model describes the emergence of disks in MW-like analogs remarkably well, and in Section 3.3, it explains the threshold halo mass for disk formation and its evolution with redshift across the entire population of star-forming galaxies at z ≤ 3. The results show that the transition to disks at Mvir ∼ 2 × 10 11 M⊙ occurs as a result of increasing c in agreement with Equation (10).

Disk Settling and Diversity

After disk formation, the potential remains steep, and the disk settles into a rotationally supported state. The cooldown of turbulent motions is suggested to be caused by the cold-to-hot accretion mode transition, leading to a subsonic cooling flow that drives lower velocity dispersions in the ISM. Diversity in galaxy properties arises from scatter in baryonic mass fractions and halo concentrations, which can cause variations in disk formation timing and destruction via mergers. The model successfully captures this diversity by showing how the critical line for disk formation predicted by our model separates non-disk galaxies from disks based on their fb and c values.

Disk Formation Criterion

The formal criterion for disk formation is defined as: "vc/vvir > 1." This ratio is evaluated at the scale of the galaxy's baryon half-mass radius, rb.

Improvements for AI systems

As a fastidious researcher, I have analyzed this paper, How Do Disk Galaxies Form? by Semenov et al., focusing on its analytical model linking disk formation to gravitational potential steepening and halo concentration.

Here are the specific improvements for AI systems and what the improved system can achieve:

)

  • Improve the accuracy of galaxy morphology prediction in cosmological simulations (e.g., TNG50) by incorporating a formal, analytically derived criterion for disk formation:

  • Implement a predictive framework that uses halo properties (mass, concentration, spin parameter) and baryonic fractions to classify simulated galaxies as either non-disk or disk based on the condition:

  • Derive the critical physical parameters—specifically the minimal baryon mass fraction in the center (fb) and the minimum halo concentration (cdisk)—required for disk formation as a function of redshift, linking them directly to cosmological structure formation models:

  • Develop a model that predicts the evolution of these critical thresholds across cosmic time:

  • Predict the evolution of galaxy size before and after disk formation, distinguishing between dispersion-dominated sizes (set by turbulent confinement) and rotationally supported sizes (set by halo spin):

  • Create a system capable of identifying the physical driver behind the observed trend that the critical halo mass for disk formation increases with redshift:

  • Integrate feedback mechanisms into simulations to test how they influence disk timing and thickness, allowing for a study of the cooldown process (transition from thick to thin disks):

  • Develop an AI capable of evaluating different theoretical models (e.g., potential deepening vs. central mass concentration steepening) against simulation data to assess which physical mechanism is the dominant driver for disk formation in specific halo mass regimes:

  • Improve the ability of AI systems to distinguish between different definitions of diskiness (e.g., vrot/σ vs. stellar fraction) by analyzing the full parameter space defined by fb and xb:

The improved AI system can perform the following specific tasks:

  1. Predict the fate of a galaxy in a cosmological simulation based on its dark matter halo properties, providing a probabilistic classification of whether it will form a disk or remain dispersion-dominated.

  2. Determine the minimum required concentration of an evolving dark matter halo needed to trigger disk formation for galaxies with specific baryonic mass fractions, accounting for the redshift evolution of the concentration-mass relation.

  3. Forecast the typical characteristic size (virial radius) of a galaxy in two distinct evolutionary phases: one dominated by turbulent support and one settled into rotational support, based on its current potential steepness.

  4. Analyze observational data (like TNG50 samples) to determine the underlying physical driver for observed trends—specifically quantifying how much of the increase in critical halo mass at high redshift is attributable to the evolution of halo concentration versus changes in feedback-driven velocity dispersions.

  5. Evaluate the causal connection between potential steepening and disk formation, allowing researchers to test hypotheses about whether disk formation causes potential steepening or vice versa within a given simulation context.

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