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Highly Ionized Oxygen in the Circumgalactic Medium of TNG100 Galaxies: Distributions, Thermal States, and Ionization Mechanisms

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Yossi Oren, Amiel Sternberg, Christopher F. McKee, Yakov Faerman

Astrophysics of Galaxies · Submitted 14 Sep 2026 · 81 references

Pith's verdict: Solid simulation-based work that clarifies ionization mechanisms of high oxygen ions in the CGM, with a real but containable caveat about ignoring local ionizing sources.

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arXiv ID2609.15298 (version 1)
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CitationAmiel Sternberg, Christopher F. McKee, Yakov Faerman, Yossi Oren. "Highly Ionized Oxygen in the Circumgalactic Medium of TNG100 Galaxies: Distributions, Thermal States, and Ionization Mechanisms." arXiv:2609.15298 (2026). https://pith.science/paper/UKUPPZDN
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The paper

The paper claims

A simple comparison of the halo virial temperature to the ion's collisional peak temperature determines whether O VI, O VII, and O VIII in the circumgalactic medium are produced by collisions or by the metagalactic radiation field.

The reading

This paper establishes a clean diagnostic for whether highly ionized oxygen (O VI, O VII, O VIII) in the circumgalactic medium (CGM) is produced by collisional ionization in hot gas or by photoionization from the metagalactic background. The diagnostic is simply whether the halo virial temperature lies below or above the temperature at which the ion's fraction peaks under collisional ionization equilibrium (CIE). When photoionization dominates, the ions still reside in the shock‑heated virialized gas at temperatures near the virial temperature, not in cool ∼10⁴ K gas—a result that runs counter to a common assumption. For Milky‑Way‑mass halos, the paper finds that O VI and O VII are collisionsally ionized while O VIII is photoionized, and all three are in the hot virialized phase. The paper validates this picture using TNG100 simulations and provides fitting formulae for mean column densities versus halo mass.

What carries the argument

The comparison of the halo virial temperature T_vir to the CIE peak temperature for each ion serves as the central diagnostic. The authors use the Illustris TNG100 simulation post‑processed with Cloudy to compute ion fractions combining collisional and photoionization. They also develop a power‑law model (PLM) for the radial density, temperature, and metallicity of the hot CGM gas, which allows analytic computation of mean column densities. The PLM is supplemented with an analytic correction for the intermediate‑temperature phase where collisional ionization dominates at high halo masses.

Extended reading notes

Core claim

The authors find that the dominant ionization mechanism for O VI, O VII, and O VIII in the CGM is determined by a single comparison: if the halo virial temperature T_vir is less than the CIE peak temperature for that ion, photoionization by the metagalactic radiation field dominates; if T_vir exceeds the CIE peak, collisional ionization dominates. However, when photoionization dominates, the ions are not in cool gas but in the shock‑heated virialized gas at temperatures near T_vir. The paper provides quantitative maps, radial profiles, and mean column density fits across halo masses from $10^{10}$.5 to $10^{14}$.5 M_⊙, and identifies the thermal phases (hot, intermediate, cool) that host the ions as a function of mass.

Load-bearing premise

The analysis assumes that the metagalactic radiation field is the only source of ionizing photons in the CGM, neglecting contributions from AGN, X‑ray binaries, and galactic winds, which could be important in the inner regions of massive halos.

Editorial extensions

If this is right

  • Observers can use the virial mass of a galaxy to predict whether detected O VI, O VII, or O VIII is predominantly collissionally or photoionized, guiding the interpretation of absorption‑line surveys such as COS‑Halos and CUBS.
  • The mean column density fitting formulae (Table 2) allow straightforward prediction of expected absorption‑line strengths for a given halo mass, facilitating comparison with observations from X‑ray (eROSITA, XMM‑Newton) and ultraviolet telescopes.
  • The conclusion that photoionized O VIII in MW‑mass halos resides in hot virialized gas rather than cool clumps implies that gas masses inferred from O VIII absorption under the assumption of cool photoionized gas could be significantly over‑ or underestimated.
  • The diagnostic rule separates the questions of ionization mechanism and gas thermal phase, resolving a long‑standing confusion in CGM studies.
  • The methodology can be directly extended to other high‑ions such as C IV, N V, and Ne VIII, providing a unified framework for interpreting the ionization state of the CGM.

Reading between the lines

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

  • If the metagalactic radiation field is the sole ionizing source, then in halos hosting active galactic nuclei (AGN) the local ionizing flux may boost photoionization for O VIII even in massive halos where collisions would otherwise dominate; this could shift the boundaries found here and is worth testing with TNG300 or other simulations that include AGN radiation.
  • The work implies that the common observational practice of assigning photoionized O VI to cool (T∼10⁴ K) gas is not always correct; photoionized oxygen can reside in gas at virial temperatures, affecting estimates of gas mass and metallicity from column densities.
  • The paper's framework suggests that the transition from photoionization‑dominated to collisional‑ionization‑dominated for each ion occurs over a narrow range in halo mass, providing a natural explanation for the observed scatter in O VI column densities at fixed stellar mass: the diagnostic is sharp, but halo‑to‑halo variations in temperature and density profiles produce scatter.
  • Because the analysis neglects local ionizing sources such as X‑ray binaries and galactic winds, the conclusions most robustly apply to the diffuse, outer CGM; the inner CGM (within ∼0.1 R_vir) may require a model that includes these additional sources.
Original abstract

We investigate the origin of the highly ionized oxygen species, O VI, O VII, and O VIII, in the circumgalactic medium (CGM) of galaxies at redshifts $z \approx 0$, by post-processing the outputs of the Illustris TNG100 simulation alongside analytic modeling. Our computations are broadly consistent with observations, and we provide convenient functional fits for the predicted mean oxygen ion column densities as functions of halo virial mass. We determine the relative roles of electron impact collisional ionization versus photoionization by the metagalactic radiation field in producing the ions for halo masses ranging from $10^{10.5}$ to $10^{14.5}$~M$_\odot$. We also determine the thermal gas phases, virialized hot, intermediate temperature, or cool, within which the ions reside. If the halo virial temperature is below the gas temperature at which the ion fractions are maximal for collisional ionization equilibrium (CIE), then photoionization dominates the overall CGM ion mass and column density. When photoionization dominates, the ions are produced primarily in the shock heated virialized phase. If the virial temperature is above the gas temperatures at which the ion fractions are maximal for CIE, then collisions dominate the ionization. However, if the virial temperature is much larger than the ion CIE peak temperature a substantial portion of the ion mass may reside within the intermediate temperature phase as opposed to the hot virialized gas. For MW-mass halos, the O VI and O VII are produced collisionally, and the O VIII by photoionization, all three in the hot, virialized phase.

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, 2026-09-23: 1 request, 7,217 input tokens.

What the abstract itself states, as probabilities: claims first 0.02 · states limitation 0.17 · human participants 0.01 · controlled comparison 0.05 · randomized assignment 0.01 · states quantitative result 0.43 · compares with prior results 0.62 · mentions code or data availability 0.03.

The abstract, sentence by sentence

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

We investigate the origin of the highly ionized oxygen species, O VI, O VII, and O VIII, in the circumgalactic medium (CGM) of galaxies at redshifts $z \approx 0$, by post-processing the outputs of the Illustris TNG100 simulation alongside analytic modeling. Our computations are broadly consistent with observations, and we provide convenient functional fits for the predicted mean oxygen ion column densities as functions of halo virial mass. We determine the relative roles of electron impact collisional ionization versus photoionization by the metagalactic radiation field in producing the ions for halo masses ranging from $10^{10.5}$ to $10^{14.5}$~M$_\odot$. We also determine the thermal gas phases, virialized hot, intermediate temperature, or cool, within which the ions reside. If the halo virial temperature is below the gas temperature at which the ion fractions are maximal for collisional ionization equilibrium (CIE), then photoionization dominates the overall CGM ion mass and column density. When photoionization dominates, the ions are produced primarily in the shock heated virialized phase. If the virial temperature is above the gas temperatures at which the ion fractions are maximal for CIE, then collisions dominate the ionization. However, if the virial temperature is much larger than the ion CIE peak temperature a substantial portion of the ion mass may reside within the intermediate temperature phase as opposed to the hot virialized gas. For MW-mass halos, the O VI and O VII are produced collisionally, and the O VIII by photoionization, all three in the hot, virialized phase.

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 We investigate the origin of the highly ionized oxygen species, O VI, O VII, and O VIII, in the circumgalactic medium (CGM) of galaxies at redshifts $z \approx 0$, by post-processing the outputs of the Illustris TNG100 simulation alongside analytic modeling. contribution 0.81 0.95 0.04 asserted 0.86
s1 Our computations are broadly consistent with observations, and we provide convenient functional fits for the predicted mean oxygen ion column densities as functions of halo virial mass. result 0.86 0.93 0.09 hedged possibility 0.72
s2 We determine the relative roles of electron impact collisional ionization versus photoionization by the metagalactic radiation field in producing the ions for halo masses ranging from $10^{10.5}$ to $10^{14.5}$~M$_\odot$. contribution 0.90 0.95 0.07 asserted 1.00
s3 We also determine the thermal gas phases, virialized hot, intermediate temperature, or cool, within which the ions reside. contribution 0.83 0.95 0.06 asserted 1.00
s4 If the halo virial temperature is below the gas temperature at which the ion fractions are maximal for collisional ionization equilibrium (CIE), then photoionization dominates the overall CGM ion mass and column density. result 0.93 0.85 0.31 asserted 0.99
s5 When photoionization dominates, the ions are produced primarily in the shock heated virialized phase. result 0.88 0.86 0.11 asserted 1.00
s6 If the virial temperature is above the gas temperatures at which the ion fractions are maximal for CIE, then collisions dominate the ionization. result 0.93 0.87 0.16 asserted 1.00
s7 However, if the virial temperature is much larger than the ion CIE peak temperature a substantial portion of the ion mass may reside within the intermediate temperature phase as opposed to the hot virialized gas. result 0.93 0.90 0.09 hedged possibility 1.00
s8 For MW-mass halos, the O VI and O VII are produced collisionally, and the O VIII by photoionization, all three in the hot, virialized phase. result 1.00 0.92 0.14 asserted 1.00

What kind of paper the classifier reports this is: simulation study 1.00 · evidence from simulation or computation 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. Top: Ion fractions assuming collisional ionization equilibrium (CIE) (Gnat & Sternberg 2007) for O VI (blue), O VII (orange), and O VIII (green) versus gas temperature𝑇. The upper 𝑥-axis is halo virial mass, 𝑀vir, assuming 𝑇vir = 𝑇. Bottom: Ion fractions assuming photoionization equilibrium (PIE) in the 𝑧 = 0 metagalactic radiation field (MGRF) at 𝑇 = 3 × 104 K. 𝑧 = 0. The temperature of 3 × 104 K is representative of gas in ther￾mal equilibrium with the MGRF). For these computations we made use of the Cloudy photoionization code (Ferland et al. 2017). For the assumed MGRF, 𝑛𝛾 =…
Figure 2. Ion fractions for O VI (left panel), O VII (central panel), and O VIII (right panel), calculated assuming collisional ionization by thermal electrons together with photoionization by the Haardt & Madau (2012) metagalactic radiation field at redshift 𝑧 = 0, as a function of hydrogen density, 𝑛𝐻, and gas temperature, 𝑇. Contours mark ion fractions in 1 dex increments between 10−5 and 10−1 . evolution of the universe from high redshift (𝑧 = 127) to the present day (𝑧 = 0) across 100 snapshots. At 𝑧 = 0, more than 6 × 106 dark matter halos are identified, containing a total of more than…
Figure 3. Our sample of 785 TNG100 halos selected for analysis in this work, presented on the specific star formation rate (𝑠𝑆𝐹𝑅) versus halo virial mass plane. Blue diamonds are halos with central star-forming galaxies. Red circles are halos with quenched galaxies, and red triangles are for galaxies with no star formation whatsoever. The black dashed line at 𝑠𝑆𝐹𝑅 = 10−11 yr−1 is the dividing line between star-forming and quenched systems. The gray points are the full TNG100 halo population within this mass range. 100 particles such that relative error is smaller than 10% (as is done by e.g. P
Figure 4. Observations of O VI column densities presented against the radially averaged O VI column densities from TNG100, in half-dex mass bins centered around log(𝑀vir/𝑀⊙ ) ∈ [11, 11.5, 12, 12.5, 13] (increasing from left to right, top to bottom). For TNG100, profiles are taken from maps of the 2Mpc cut (solid lines surrounded by shaded regions marking the 16-84 percentiles) and halo cat (dashed lines). Observations are shown as empty green circles for detections and faded green triangles for upper limits. Gray errorbars represent the median observed 𝑁O VI in 𝑏/𝑅vir bins of 0.25 dex. Median…
Figure 5. Observations of O VII (top panel) and O VIII (bottom panel) presented against the radially averaged column densities from TNG100 for halos with virial masses in the range log(𝑀vir/M⊙ ) = 12 ± 0.25. Solid lines show the TNG100 results from the 2Mpc cut and dashed lines show the halo cut. The 16-84 percentiles of the 2Mpc cut profiles are highlighted with shaded regions. Observations of the MW are shown as arrows pointing inward of errorbars placed at ∼ 100 kpc, whereas external observations are shown as error bars at the relevant impact parameters. In the bottom panel we show the…
Figure 6. The projected half mass impact parameters in units of the virial radius (𝑏1/2/𝑅vir) estimated from the halo cut maps (left panel) and the 2Mpc cut maps (central panel), and the spherical half mass radii in units of the virial radius (𝑟1/2/𝑅vir, right panel). The normalized 𝑏1/2 and 𝑟1/2 are presented for H (black lines), O VI (blue lines), O VII (orange lines), and O VIII (green lines). FSM20 half mass radii and half mass impact parameters are shown in the relevant panels, marked by black squares filled with the appropriate colors. halo cut maps at the lowest mass bin. The…
Figure 7. Left: The metallicity, 𝑍 ′ vir (relative to Solar) at the virial radius, as a function of 𝑀vir. Middle: the PLM metallicity index, 𝑎Z (see Eq. 12). Right: The metallicity 𝑍 ′ vir at the innermost CGM radius, 𝑥0 ≡ 𝑟0/𝑅vir. Black is the TNG100 mass weighted metallicity at 𝑥0 in solar units, and yellow is the PLM prediction, showing that the metallicity of the CGM in TNG100 galaxies is well approximated by a power law of the radius
Figure 8. Volume filling factors of the cool (blue), intermediate (green), and hot phase (red) of the CGM, as a function of halo mass and virial temperature (upper 𝑥-axis). Error bars show the standard deviations. The dotted lines show the linear fits between log( 𝑓V ) and log(𝑀vir). The vertical dashed black line marks the virial mass where 0.4𝑇vir = 3 × 104 K, below which there is no intermediate phase. We find, • log( 𝑓 cool V ) = −0.786 log(𝑀vir/1012 M⊙) − 2.541 • log( 𝑓 it V ) = 0.031 log(𝑀vir/1012 M⊙) − 1.444 • log( 𝑓 hot V ) = −0.003 log(𝑀vir/1012 M⊙) − 0.019 For the hot gas 𝑓
Figure 9. Mean column densities, O VI (left), O VII (middle) and O VIII (right), as functions of halo mass and virial temperature (upper 𝑥-axis). Top row: Solid curves are for our TNG100 halo sample, assuming collisional plus photoionization (black) or just collisional ionization equilibrium (blue). The black dashed curves are the hot phase PLM results (see text). Middle row: Shaded region is for the 16-84 percentile of the TNG100 (CI+PI) mean column densities, and the dashdot line is the piece-wise linear fit to the median. Bottom row: In addition to the TNG100 (CI+PI) black error-bars and…
Figure 10. The ratio, 𝑀CIE/𝑀CI+PI, of the CGM ion masses calculated assuming CIE to the ion masses calculated assuming CI+PI. Orange is O VI, blue is O VII, and green is O VIII. 6.2 ⟨𝑁ion⟩ with the PLM We now compute the mean column densities using our PLM for the hot phase. Comparison to the direct TNG100 results will show whether and when the intermediate temperature and cool gas com￾ponents contribute to the ion masses and mean columns. To calculate the mean column density using our PLM, we first estimate the ion density (cm−3 ) at a given radius using: 𝑛ion (𝑟) = 𝑛𝐻 (𝑟)𝑎⊙ 𝑍 ′ (𝑟) 𝑓ion [𝑛
Figure 11. Oxygen ion mass fractions in the hot (red) and intermediate temperature (yellow) gas phases, as functions of halo mass for our TNG100 sample, assuming CI+PI (see text)
Figure 12. Left: Mass weighted metallicity in the intermediate phase of the CGM in TNG100 (yellow) compared to the mass weighted metallicity in the hot phase of the CGM in TNG100 (solid red) and the the hot phase mass weighted metallicity obtained by our PLM (dashed red). Center: Oxygen ion fractions in the intermediate temperature range, ⟨ 𝑓ion,CIE⟩ it, assuming only CIE, as a function of temperature and its associated virial mass. Right: The mean column densities associated with the intermediate phase alone, ⟨𝑁ion ⟩ it, from TNG100 (solid lines) and our approximated approach (dashed lines).…
Figure 13. Phase diagrams showing the distributions of total CGM gas mass (colored bins) and O VI (left), O VII (center) and O VIII (right) mass fractions (black contours) in the 𝑛𝐻 versus 𝑇 plane for our three representative halos. Top row: low-mass halo with 𝑀vir = 1.3 × 1011 M⊙. Middle row: MW-mass halo with 𝑀vir = 1.2 × 1012 M⊙. Bottom row: massive halo with 𝑀vir = 8.9 × 1012 M⊙. The color scale shows the total gas mass per (𝑛𝐻 , 𝑇) bin, where a log(𝑛𝐻/cm−3 ) bin is 0.08 dex and a log(𝑇/K) bin is 0.05 dex. The black contours show where the fraction of the mass of a given ion per (𝑛𝐻 , 𝑇)…
Figure 14. Total CGM gas mass and O VI mass distributions within 𝑅vir in the 𝑇 versus 𝑡cool/𝑡ff planes (see text) for our three representative halos (left, middle, right). The gas mass fractions are indicated by the colored bins. The color scale shows the total gas mass per (𝑇, 𝑡cool/𝑡ff ) bin, where a log(𝑇/K) bin is 0.05 dex and a log(𝑡cool/𝑡ff ) bin is 0.07 dex. The black contours show the O VI mass fractions at the 5 × 10−5 , 5 × 10−4 and 5 × 10−3 levels. The vertical lines mark temperatures of 3 × 104 K and 0.4𝑇vir that separate the cool, intermediate and hot phases. all mass components,…

Figures reproduced from arXiv:2609.15298 by the authors.

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