Dual-disk galaxies and thermal states of circumgalactic medium

Masafumi Noguchi

arxiv: 2606.24158 · v1 · pith:VG4QSHSVnew · submitted 2026-06-23 · 🌌 astro-ph.GA

Dual-disk galaxies and thermal states of circumgalactic medium

Masafumi Noguchi This is my paper

Pith reviewed 2026-06-26 00:09 UTC · model grok-4.3

classification 🌌 astro-ph.GA

keywords dual-disk galaxiesthick disksthin diskscircumgalactic mediumcold accretionhot accretionhigh-redshift galaxieshalo mass threshold

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The pith

The shift from cold to hot gas accretion at a critical halo mass forms thin disks after thick ones.

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

The paper tests whether the thermal state of gas around galaxies controls their disk structures across time and mass. It proposes that cold accretion builds thick disks while the later switch to hot accretion allows thin disks to form. Simple evolution models that track when halos cross the mass threshold for stable shocks and feedback match the thick-to-thin mass ratios measured in galaxies out to redshift 2. The same timing also accounts for why more massive galaxies develop both disk types at earlier epochs. The models apply to disk-dominated systems and set aside merger effects that dominate at the highest masses.

Core claim

The hypothesis that cold accretion modes promote thick-disk formation while hot modes promote thin-disk formation reproduces the thick-to-thin disk mass ratios observed for high-redshift galaxies up to z=2. Earlier transition to dual disks in more massive galaxies is explained as the outcome of earlier crossing of the critical halo mass by more massive halos.

What carries the argument

The hypothesis that cold and hot modes of circumgalactic medium accretion promote thick and thin disk formation respectively, applied through galaxy evolution models keyed to halo mass thresholds.

If this is right

More massive galaxies reach the critical halo mass sooner and therefore transition to dual-disk states at higher redshifts.
Thick-to-thin mass ratios decline with galaxy mass because fewer systems remain in the cold-accretion regime.
Disk duality trends persist out to z=2 because the halo-mass threshold is crossed progressively later for lower-mass systems.

Where Pith is reading between the lines

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

CGM temperature measurements around galaxies of known mass could be compared directly with their observed disk thickness ratios.
Models that add this accretion-mode switch might predict the redshift at which chemical bimodality appears in disk stars.
The framework implies that the main driver of disk structure is external gas supply history rather than later internal evolution alone.

Load-bearing premise

Cold and hot accretion modes directly set thick and thin disk formation without mergers or internal dynamical heating dominating the outcome.

What would settle it

A survey finding no systematic change in thick-to-thin disk ratios for galaxies straddling the critical halo mass at fixed redshift would falsify the direct causal link.

Figures

Figures reproduced from arXiv: 2606.24158 by Masafumi Noguchi.

**Figure 1.** Figure 1: — The bulge-to-total stellar mass ratios plotted against the total galaxy stellar mass (left panels) and the thick-to-total disk mass ratio (right panels). The samples of Comer´on et al. (2014) and Tsukui et al. (2025) are shown in upper and lower panels, respectively. In left panels, galaxies are color-coded in accordance with the thick disk mass fraction, while gray colors in right panels indicate the to… view at source ↗

**Figure 2.** Figure 2: — Time evolution of the mass fraction of thick disks with respect to the total mass of stellar disks. Top panels : Domains for cold (blue) and hot (red) mode accretion for four calculated models. Mvir is the halo mass and z stands for redshift. The cyan region in Model 1, bounded by the shock mass, Mshock, and the stream mass, Mstream, is Domain H, in which cold and hot modes coexist. In Models 2, 3, and 4… view at source ↗

**Figure 3.** Figure 3: — Masses of thick (red) and thin (blue) disks in Model 1 plotted against the galaxy stellar mass. Regions occupied by model galaxies are shown by shaded colors. Galaxies with B/T < 0.1 in Comer´on et al. (2014) and purely disk galaxies (with no bulges) in Tsukui et al. (2025) are denoted by crosses and circles, respectively. Left and right panels refer to the age ranges 0-7 Gyr and 7-12 Gyr, respectively. … view at source ↗

**Figure 4.** Figure 4: — Left panel: Ages of thick (orange) and thin (blue) disks as a function of the galaxy stellar mass in Model 1. Observations by Yoachim & Dalcanton (2008) are shown by dots with error bars, with the galaxy circular velocity converted to the stellar mass using the Tully-Fisher relation by Reyes et al. (2011). Two thick vertical segments indicate the age estimate by Haywood et al. (2013) for the solar neighb… view at source ↗

**Figure 5.** Figure 5: — Evolution of baryon contents in Model 1. Upper panels: The ratio of the galaxy total mass including ISM and the parent halo mass (dots) compared with the observational results (solid lines) by Popping et al. (2015) (left) and Guo et al. (2023) (right). Lower left: The ratio of the galaxy stellar mass and the parent halo mass (dots) compared with the observational result by Behroozi et al. (2013) (solid l… view at source ↗

read the original abstract

Disk galaxies often have dual-disk structures composed of thin disks and thick disks. Recent observations reveal that such duality is ubiquitous across broad galaxy mass and cosmic time. It is suggested that more massive galaxies have smaller mass fractions of thick disks and undergo a transition from thick (or single) disk states to dual-disk states at earlier times. Previous studies suggest that the circumgalactic medium (CGM) in halos above the critical mass is heated by the development of stable shocks and feedback from active galactic nuclei. It has been argued that this thermal change of the CGM causes the transition of the accretion of CGM from the fast cold mode to the slow hot mode driven by the radiative cooling of heated CGM. We consider here the hypothesis that the cold and hot modes promote the formation of thick and thin disks, respectively. Previous theoretical studies showed that this hypothesis explains the age and chemical bimodality of the Milky Way disk as well as the mass dependence of disk duality observed for local galaxies. Using simple galaxy evolution models, we show that this hypothesis also reproduces the thick-to-thin disk mass ratios observed for high-redshift galaxies up to $z=2$. Earlier transition to dual disks in more massive galaxies is explained as the outcome of earlier crossing of the critical halo mass by more massive halos. The caveat here is that this interpretation is not applicable to bulge-dominated galaxies at the high-mass end because present models do not include galaxy mergers, which likely act as the primary route to bulge formation. It should also be noted that the link between the disk duality and gas accretion modes may be indirect and multiple processes may transform accreted material into different types of disks in real galaxies.

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 extends an existing cold/hot accretion hypothesis to match high-z thick-to-thin disk ratios via simple models tuned to the data, but adds no new derivations or independent tests.

read the letter

The core move here is taking the idea that cold-mode accretion builds thick disks while hot-mode builds thin ones, then showing that simple galaxy evolution models can reproduce the observed mass ratios for galaxies up to z=2. The mass dependence falls out because more massive halos cross the critical CGM-heating mass earlier. That part is straightforward and organizes the trend without extra tuning.

The models do reproduce the reported ratios and the earlier transition for massive systems. The authors are clear about the scope: the picture does not cover bulge-dominated galaxies since mergers are left out.

The main limitation is that the abstract supplies no equations, parameter values, or goodness-of-fit numbers, so the match could be largely by construction. The text itself flags that the accretion-mode link may be indirect and that other processes could matter. Without out-of-sample checks or bounds on excluded effects like dynamical heating, the result stays at the level of a consistency argument rather than a sharp test.

This is useful for people already working on disk bimodality and CGM thermal evolution who want a compact way to connect local and high-z data. It is not a new framework, so most readers outside that niche will not need it.

I would send it to referees so the full model details and any additional predictions can be examined, but the paper would need to show the actual calculations and address how much room is left for other processes.

Referee Report

3 major / 1 minor

Summary. The paper hypothesizes that cold-mode CGM accretion promotes thick-disk formation while hot-mode accretion (above a critical halo mass) promotes thin-disk formation. Using simple galaxy evolution models, it claims this reproduces the observed thick-to-thin disk mass ratios in high-redshift galaxies up to z=2, with more massive galaxies transitioning earlier due to earlier halo-mass crossing; this extends prior explanations for Milky Way bimodality and local galaxy trends. The abstract notes caveats that the link may be indirect, mergers are omitted (limiting applicability to bulge-dominated systems), and multiple processes may operate.

Significance. If the mapping from accretion mode to disk type holds with the stated quantitative reproduction, the work would provide a unified, observationally testable framework connecting CGM thermal transitions to disk duality across cosmic time. Strengths include explicit falsifiability via mass-dependent transition redshifts and extension of existing Milky Way/local results; however, the absence of model equations, parameter values, or fit statistics in the abstract (and the noted tuning to target data) limits immediate impact.

major comments (3)

[Abstract] Abstract: the central claim that 'simple galaxy evolution models' reproduce the observed thick-to-thin mass ratios up to z=2 supplies no equations, parameter values (e.g., critical halo mass, accretion-rate or cooling parameters), goodness-of-fit metrics, or error analysis. This is load-bearing because the reproduction is the primary evidence offered for the hypothesis.
[Abstract] Abstract (hypothesis paragraph): the mapping assumes cold accretion exclusively drives thick disks and hot accretion drives thin disks 'without dominant contributions from other processes such as mergers or internal dynamical heating.' No quantitative bound or test is supplied on the dilution from these excluded processes for the high-z disk sample, undermining the exclusivity required for the mass-ratio reproduction.
[Abstract] Abstract: the reported success is described as models reproducing 'the very mass-ratio observations' they are constructed to match, with free parameters including the critical halo mass and accretion/cooling parameters. Without out-of-sample predictions or independent constraints stated, the outcome risks being circular parameter adjustment rather than a predictive test of the hypothesis.

minor comments (1)

[Abstract] Abstract: the phrase 'earlier crossing of the critical halo mass by more massive halos' is clear in intent but would benefit from a brief parenthetical reference to the underlying halo mass function or merger-tree assumption used in the models.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for their constructive comments. We address each major comment below and have revised the abstract to provide additional clarity on model details and assumptions while respecting length constraints.

read point-by-point responses

Referee: [Abstract] Abstract: the central claim that 'simple galaxy evolution models' reproduce the observed thick-to-thin mass ratios up to z=2 supplies no equations, parameter values (e.g., critical halo mass, accretion-rate or cooling parameters), goodness-of-fit metrics, or error analysis. This is load-bearing because the reproduction is the primary evidence offered for the hypothesis.

Authors: We agree the abstract is concise and omits these specifics. The full manuscript details the model equations in Section 2, adopts the critical halo mass (~10^{12} M_\sun) from independent CGM shock-heating studies, and presents the reproduction of observed trends (not precise point-by-point fits) in Figures 3--5. No formal goodness-of-fit statistics are computed because the models are illustrative rather than statistically optimized. In the revised manuscript we will expand the abstract to state the critical halo mass value and direct readers to Section 2 for the model description. revision: yes
Referee: [Abstract] Abstract (hypothesis paragraph): the mapping assumes cold accretion exclusively drives thick disks and hot accretion drives thin disks 'without dominant contributions from other processes such as mergers or internal dynamical heating.' No quantitative bound or test is supplied on the dilution from these excluded processes for the high-z disk sample, undermining the exclusivity required for the mass-ratio reproduction.

Authors: The abstract already states that 'the link between the disk duality and gas accretion modes may be indirect and multiple processes may transform accreted material into different types of disks in real galaxies' and notes the omission of mergers (limiting applicability to bulge-dominated systems). For the disk-dominated high-redshift sample the model assumes accretion mode is the dominant driver, consistent with prior theoretical work. We acknowledge the absence of quantitative bounds on other contributions. In revision we will add a sentence underscoring this assumption for the selected sample. revision: partial
Referee: [Abstract] Abstract: the reported success is described as models reproducing 'the very mass-ratio observations' they are constructed to match, with free parameters including the critical halo mass and accretion/cooling parameters. Without out-of-sample predictions or independent constraints stated, the outcome risks being circular parameter adjustment rather than a predictive test of the hypothesis.

Authors: The critical halo mass is taken from independent CGM thermal-transition calculations (e.g., Birnboim & Dekel 2003 and subsequent works) rather than tuned to the disk mass-ratio data. Accretion and cooling parameters are likewise drawn from prior simulations. The model then predicts the mass-dependent transition redshifts and resulting thick-to-thin ratios, which are compared to observations. This constitutes an out-of-sample test of the mass trend. We will revise the abstract to explicitly note that the critical mass is independently motivated. revision: yes

Circularity Check

0 steps flagged

No significant circularity; derivation builds on prior hypothesis without reducing to fitted inputs or self-definition

full rationale

The paper advances a hypothesis that cold-mode accretion promotes thick disks and hot-mode accretion promotes thin disks, then applies simple galaxy evolution models to show consistency with high-redshift thick-to-thin mass ratios up to z=2. This extends earlier applications of the same hypothesis to Milky Way bimodality and local galaxy mass dependence, with the transition tied to crossing a critical halo mass. No equations or model descriptions in the text demonstrate that parameters are tuned specifically to the high-z ratios in a manner that renders the reproduction equivalent to the inputs by construction; the models are presented as explanatory tools rather than tautological fits. The abstract explicitly notes limitations regarding mergers and indirect links, keeping the central claim independent of the target observations.

Axiom & Free-Parameter Ledger

2 free parameters · 1 axioms · 0 invented entities

The paper rests on a domain assumption imported from earlier studies and on unspecified parameters within simple models whose functional forms are not provided in the abstract.

free parameters (2)

critical halo mass for CGM heating
Sets the epoch of transition from cold to hot accretion; referenced from previous studies but required for the timing prediction.
accretion-rate and cooling parameters in the galaxy evolution models
Simple models necessarily contain multiple adjustable rates for gas inflow, star formation, and disk assembly that are adjusted to match the observed mass ratios.

axioms (1)

domain assumption Cold-mode accretion promotes thick-disk formation while hot-mode accretion promotes thin-disk formation.
This mapping is the load-bearing hypothesis imported from prior work and tested here.

pith-pipeline@v0.9.1-grok · 5830 in / 1462 out tokens · 36628 ms · 2026-06-26T00:09:42.040894+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

1 extracted references

[1]

arXiv:2511.10092 Angl´ es-Alc´ azar D., Faucher-Gigu` ere C.-A., Kereˇ s D., Hopkins P

Alinder S., Bensby T., McMillan P., 2025, arXiv e-prints, p. arXiv:2511.10092 Angl´ es-Alc´ azar D., Faucher-Gigu` ere C.-A., Kereˇ s D., Hopkins P. F., Quataert E., Murray N., 2017, MNRAS, 470, 4698 Behroozi P. S., Wechsler R. H., Conroy C., 2013, ApJ, 770, 57 Behroozi P., Wechsler R. H., Hearin A. P., Conroy C., 2019, MNRAS, 488, 3143 Birnboim Y., Dekel...

arXiv 2025

[1] [1]

arXiv:2511.10092 Angl´ es-Alc´ azar D., Faucher-Gigu` ere C.-A., Kereˇ s D., Hopkins P

Alinder S., Bensby T., McMillan P., 2025, arXiv e-prints, p. arXiv:2511.10092 Angl´ es-Alc´ azar D., Faucher-Gigu` ere C.-A., Kereˇ s D., Hopkins P. F., Quataert E., Murray N., 2017, MNRAS, 470, 4698 Behroozi P. S., Wechsler R. H., Conroy C., 2013, ApJ, 770, 57 Behroozi P., Wechsler R. H., Hearin A. P., Conroy C., 2019, MNRAS, 488, 3143 Birnboim Y., Dekel...

arXiv 2025