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A brief overview of the baryon cycle in galaxies

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S. Veronese

Astrophysics of Galaxies · Submitted 4 Nov 2026 · 237 references

Pith's verdict: Solid review of the baryon cycle, well-written but entirely derivative; useful for newcomers, not a research paper.

T0 · AI review

How this review was made: An AI referee (deepseek/deepseek-v4-flash) read the paper on November 5, 2026 and answered a fixed list of questions about it. T0 is the starting level every paper gets. The levels describe how deeply Pith has checked the paper, never the authors.

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arXiv ID2611.06264 (version 1)
Pith ID7XWMI2OD, Pith's permanent record of this paper and its review
CitationS. Veronese. "A brief overview of the baryon cycle in galaxies." arXiv:2611.06264 (2026). https://pith.science/paper/7XWMI2OD
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The paper

The paper claims

This review summarises the baryon cycle—the continuous exchange of gas that governs galaxy formation—and points to key unresolved questions.

The reading

The baryon cycle framework describes how gas accretes from the cosmic web into galaxies, cools to form stars, and is partially returned to the circumgalactic medium through stellar and active galactic nucleus (AGN) feedback. This review synthesises the current understanding of each stage: gas accretion modes, the atomic-to-molecular transition that regulates star formation, the dual role of stellar feedback in suppressing and recycling gas, and the feeding and feedback of supermassive black holes. The paper identifies several open questions, including the lack of direct observational evidence for gas accretion in the nearby Universe, unresolved tensions between inferred accretion rates and star formation rates, the need to converge on a single physical model for molecular gas formation, and the untested applicability of chaotic cold accretion in disc galaxies. The overall conclusion is that the broad framework is well established but many critical details remain to be resolved by future simulations and multiwavelength surveys.

What carries the argument

The baryon cycle itself is the key conceptual machinery: a continuous flow of gas from the cosmic web into the circumgalactic medium (CGM), into the interstellar medium (ISM), where it forms stars, and back out via stellar and AGN feedback. The review also details specific mechanisms: cold-mode versus hot-mode accretion, the pressure-based, radiation-based, and opacity-based prescriptions for the molecular gas fraction, the Kennicutt–Schmidt relation, stellar winds and supernova feedback, and chaotic cold accretion as a pathway for fuelling supermassive black holes.

Extended reading notes

Core claim

The review consolidates the current picture of the baryon cycle, asserting that the interplay between gas accretion, star formation, and feedback is the central engine of galaxy evolution. It emphasises that while the cycle is conceptually coherent, direct observational confirmation of gas accretion at low redshift is still missing, the atomic-to-molecular transition bottleneck is governed by competing formalisms that have not converged, and the mechanisms that sustain interstellar turbulence are not uniquely identified. The paper further highlights that chaotic cold accretion, which links black hole feeding to a self-regulating feedback loop, has been tested only in elliptical galaxies and clusters, leaving its role in disc galaxies an open question.

Load-bearing premise

The review assumes that the ΛCDM model and the baryon cycle framework, as described in the cited literature, are correct; it does not test any particular assumption or present new data.

Editorial extensions

If this is right

  • If the baryon cycle framework is correct, upcoming deep HI surveys (SKA, DSA, ngVLA) should detect cold gas streams in the CGM of nearby galaxies and resolve the present mismatch between accretion rates and star formation rates at z~0.
  • The unresolved tension between theoretical models for the atomic-to-molecular transition implies that future simulations must adopt a unified prescription validated by observations across a range of metallicities and radiation fields.
  • If chaotic cold accretion operates in disc galaxies, it would provide a self-regulated link between SMBH feeding and AGN feedback, requiring a revision of current inflow models that rely primarily on bars and mergers.
  • The dual role of AGN feedback (suppressing star formation in some systems while triggering it in others) must be incorporated into cosmological simulations to accurately predict the galaxy mass function and the properties of massive galaxies.
  • The open question of what sustains ISM turbulence suggests that coordinated multiwavelength observations (e.g., HI, CO, Hα) combined with high-resolution simulations are needed to disentangle supernova, magnetic, gravitational, and accretion-driven contributions.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • The review implicitly prioritises cold-mode accretion as the dominant gas supply at low redshift, but a future detection of significant hot-mode accretion via X-ray observations of the CGM could shift the balance, particularly in massive haloes.
  • If the atomic-to-molecular transition is indeed the bottleneck for star formation, then the star formation law is fundamentally set by the microphysics of dust grain chemistry and UV radiative transfer, rather than by large-scale gravitational instability alone.
  • The emphasis on the lack of direct accretion detections suggests that the baryon cycle framework may be missing a diffuse, ionised gas component that is currently hard to observe but could carry a substantial fraction of the inflow budget.
Original abstract

Galaxy formation and evolution is governed by the continuous exchange of gas between the cosmic web, the circumgalactic medium (CGM), and the interstellar medium (ISM) of galaxies. This brief review summarises our current understanding of this baryon cycle, from the accretion of gas onto galaxies to its conversion into stars, and the feedback processes that regulate the growth of galaxies. Several open questions will be highlighted, that will encourage progresses in cosmological simulations, multiwavelength observations, and theory to resolve.

The compiled record

The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.

Compiled by classifier-1.13.0 via TypeSafe API, question bank abstract-p1-20260923, 2026-11-05: 1 request, 2,986 input tokens.

What the abstract itself states, as probabilities: claims first 0.02 · states limitation 0.15 · human participants 0.02 · controlled comparison 0.01 · randomized assignment 0.01 · states quantitative result 0.02 · compares with prior results 0.03 · mentions code or data availability 0.02.

The abstract, sentence by sentence

arXiv listing abstract v1. Results and the main claim are underlined; .

Galaxy formation and evolution is governed by the continuous exchange of gas between the cosmic web, the circumgalactic medium (CGM), and the interstellar medium (ISM) of galaxies. This brief review summarises our current understanding of this baryon cycle, from the accretion of gas onto galaxies to its conversion into stars, and the feedback processes that regulate the growth of galaxies. Several open questions will be highlighted, that will encourage progresses in cosmological simulations, multiwavelength observations, and theory to resolve.

background contribution method result interpretation main claim

What the classifier reports for each sentence. The number is the probability it gave the label.
#sentencerolethe authors' own workstates the main conclusionstrength
s0 Galaxy formation and evolution is governed by the continuous exchange of gas between the cosmic web, the circumgalactic medium (CGM), and the interstellar medium (ISM) of galaxies. background 1.00 0.05 0.14 asserted 1.00
s1 This brief review summarises our current understanding of this baryon cycle, from the accretion of gas onto galaxies to its conversion into stars, and the feedback processes that regulate the growth of galaxies. contribution 0.97 0.51 0.20 asserted 0.87
s2 Several open questions will be highlighted, that will encourage progresses in cosmological simulations, multiwavelength observations, and theory to resolve. future work 0.97 0.26 0.11 asserted 0.58

What kind of paper the classifier reports this is: review 1.00 · evidence from literature 1.00.

What was examined, and what was not

sources read
  • arXiv listing abstract v1
not examined
  • full text
  • figures and tables
  • references and citing sentences
  • relations to other papers
levels run
  • P abstract
not run
  • S sections
  • T body sentences
  • C citations
  • R cross-paper relations

Every quoted sentence on this record is the authors'. Every label is from Pith's closed vocabulary, with the probability shown beside it. Every number in the measurements table is the authors', checked by code. Record as JSON

Figures
Figure 1. Examples of H i complexes thought to trace accretion from the CGM and/or the cosmic web. The top two panels show gas clouds around M31 and NGC 891, whereas the bottom panels present the filamentary structures in NGC 2403 and M33. Image credit: [11]. inefficiently [29]. In more massive haloes, the gas is shock-heated to temperatures above 105 K. When the cooling time is shorter than the free-fall time, accretion proceeds mainly in a cold mode, in which streams and clouds of cool (𝑇 < 105 K) gas flow directly onto galaxies along cosmic filaments [30–36]. In contrast, when the cooling…
Figure 2. The left panel shows the redshift evolution of different baryonic components (baryons associated with galaxies in black, stars in maroon, H i in green, H2 in blue, and gas in orange) as presented by [25]. For each component the solid line is the best fit and the shaded region the 1𝜎 uncertainty. The right panel shows the Kennicutt–Schmidt relation from [65]. The density contours at the 40%, 80%, and 95% levels denote the spatially resolved regions of the galaxy sample for which both the SFR and molecular gas content were measured. The downward arrows correspond to 3𝜎 upper limits.…
Figure 3. The galactic wind in M82. The background-coloured image is from HST, while the ionised gas emission observed by the Subaru Telescope is shown in red. Image credit: STScI. (e.g. [106–108]). On galactic scales, stellar feedback regulates chemical evolution and contributes to the mass–metallicity relation [22, 106, 109–111]. Galactic winds also heat the ISM, preventing the efficient formation of GMCs and thereby suppressing further star formation [112, 113]. However, stellar feedback also has a positive impact on star formation. Typical wind velocities (< 100 km s−1 ) are lower than…
Figure 4. Sample of trajectories of CCA gas in the inner kpc region of a simulated galaxy [142]. and accretion flows [134–136]. Empirical studies [137–141] have not yet provided a conclusive answer, being largely limited by the difficulty of estimating how much power feedback processes inject into the ISM relative to other sources of turbulence. 5. AGN Feeding The gas accreted from the cosmic web and the CGM does not entirely convert into stars. A fraction of the inflowing material feeds the SMBH hosted at the centre of almost every galaxy [143–147], including the Milky Way [148]. However,…
Figure 5. A schematic cartoon of the two feedback modes from AGN [21]. In the left panel, radiatively inefficient accretion onto the SMBH leads to the launch of powerful jets of relativistic particles. The main effect of the jets is to heat the gas on scales ranging from the ISM to the intergalactic medium. In the right panel, radiatively efficient accretion leads to the release of most of the energy in the form of radiation, inducing photoionisation of the surrounding medium and launching outflows via radiation pressure on dusty gas. kpc down to pc scales. In hot, pressure-supported…

Figures reproduced from arXiv:2611.06264 by the authors.

References

Discussion

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