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arxiv: 2411.17811 · v2 · pith:INTDTKNEnew · submitted 2024-11-26 · 🌌 astro-ph.GA

Cosmic Ray Ionization of Low-Excitation Lines in Active Galactic Nuclei and Starburst Galaxies

E. Koutsoumpou , J. A. Fern\'andez-Ontiveros , K. M. Dasyra , L. Spinoglio This is my paper

Pith reviewed 2026-05-23 08:28 UTC · model grok-4.3

classification 🌌 astro-ph.GA
keywords cosmic raysAGNstarburst galaxiesBPT diagramsphotoionization modelinglow-ionization linesnebular emissionCLOUDY simulations
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The pith

Cosmic ray ionization creates a secondary low-ionization layer that enhances low-excitation lines and reproduces observed positions in BPT diagrams for AGN and starbursts.

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

The paper uses CLOUDY simulations to model how cosmic rays affect ionized nebular gas across a range of densities, ionization parameters, and cosmic ray ionization rates. Elevated rates above 10^{-13} per second, typical in AGN and strong starbursts, produce a deep secondary layer of low-ionization gas beyond the primary photoionized zone. This layer strengthens emission from transitions such as [N II], [S II], and [O I], which in turn shifts line ratios on diagnostic diagrams. AGN models with these high rates match the Seyfert loci without super-solar metallicities in the narrow-line region, while star-formation models populate the LINER domain. The work proposes revised maximum-starburst boundaries on BPT diagrams to separate AGN photoionization from star formation plus high cosmic-ray effects.

Core claim

Cosmic ray ionization rates of 10^{-13} s^{-1} and higher form a secondary low-ionization layer in nebular gas that boosts emission from low-excitation lines, allowing AGN simulations to reproduce Seyfert loci in BPT diagrams at solar metallicities and star-formation simulations to explain LINER line ratios.

What carries the argument

The secondary low-ionization layer produced by cosmic ray ionization beyond the photoionization front, which increases the contribution of low-excitation transitions to the total emission.

If this is right

  • AGN photoionization models that include high cosmic ray rates match observed Seyfert positions in BPT diagrams at solar metallicities.
  • Star formation models with high cosmic ray rates can account for line ratios in the LINER domain of diagnostic diagrams.
  • Line-ratio based estimates of metallicity and ionization parameter are altered once cosmic ray ionization is included.
  • New maximum starburst boundaries on BPT diagrams can separate regions dominated by AGN photoionization from those influenced by star formation plus high cosmic ray rates.

Where Pith is reading between the lines

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

  • Cosmic ray effects may need to be included when interpreting line ratios in other environments with dense molecular gas near supernova activity or outflows.
  • The secondary layer could reconcile some discrepancies between pure photoionization predictions and observations in composite systems.
  • Future observations targeting depth-dependent line ratios might directly test for the presence of this extra ionization component.

Load-bearing premise

The CLOUDY code accurately represents the combined thermal, ionization, and chemical effects of photoionization plus cosmic rays on the gas without missing important processes that would change the secondary layer.

What would settle it

Detection or non-detection of the predicted extra low-ionization emission component in regions with independently measured cosmic ray ionization rates above 10^{-13} s^{-1}, such as in the narrow-line region of Centaurus A or NGC 1068.

Figures

Figures reproduced from arXiv: 2411.17811 by E. Koutsoumpou, J. A. Fern\'andez-Ontiveros, K. M. Dasyra, L. Spinoglio.

Figure 1
Figure 1. Figure 1: The chosen apertures to extract spectra with MPDAF. The different shades of purple going from deep purple to pale lilac represent the ascending distance also noted with numbers. The apertures are drawn over Hα and [O iii]λ5007Å emission with the stellar continuum subtracted. In Centaurus A, the jets are depicted by use of VLA data at 8.4 GHz (Hardcastle et al. 2003; Tingay & Lenc 2009), in white contours a… view at source ↗
Figure 2
Figure 2. Figure 2: BPT emission-line fits, using Pyplatefit (Bacon, Roland et al. 2023), in the rest frame of Centaurus A. 4840 4850 4860 4870 4880 4890 4900 [Å] 50 100 150 200 250 F [1 0 2 0 e r g /(Å c m 2 s)] FLUX:685.38 H SNR:4.80 Aperture 3 Data Line Fit Continuum Fit (a) Centaurus A. 4850 4860 4870 4880 4890 4900 [Å] 500 1000 1500 2000 2500 3000 F [1 0 2 0 e r g /(Å c m 2 s)] FLUX:8155.94 H SNR:3.89 Aperture 8 Data Lin… view at source ↗
Figure 3
Figure 3. Figure 3: Hβ emission line in each galaxy’s rest frame fitted with Pyplatefit (Bacon, Roland et al. 2023). regions situated near star-forming regions and farther from the galactic center are presumed to be dominated by photoioniza￾tion from UV radiation produced by hot young stars. We intend to distinguish the corresponding contributions of photoioniza￾tion and CRs in various regions of each galaxy by collecting and… view at source ↗
Figure 4
Figure 4. Figure 4 [PITH_FULL_IMAGE:figures/full_fig_p006_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: BPT diagrams with the AGN photoionization models compared with the observations from the selected apertures in Centaurus A (Fig. 1a). On the left is the [N ii], in the middle is the [S ii], and on the right is the [O i] BPT diagram. The different shades of purple going from deep purple to pale lilac/white represent the ascending distance from the nucleus also noted with numbers, "N" being the closest apert… view at source ↗
Figure 6
Figure 6. Figure 6: BPT diagrams with the AGN photoionization models compared with the observations from the selected apertures in NGC 1068 (Fig. 1b). On the left is the [N ii], in the middle is the [S ii], and on the right is the [O i] BPT diagram. The different shades of purple going from deep purple to pale lilac/white represent the ascending distance from the nucleus also noted with numbers, "N" being the closest aperture… view at source ↗
Figure 7
Figure 7. Figure 7: BPT diagrams with the SF photoionization models compared with the observed line ratios from the selected apertures in NGC 253 (Fig. 1c). On the left is the [N ii], in the middle is the [S ii], and on the right is the [O i] diagram. The different shades of purple going from deep purple to pale lilac/white represent the ascending distance also noted with numbers, "1" being the most central aperture. Also, fr… view at source ↗
Figure 8
Figure 8. Figure 8: Temperature vs. depth in the simulated cloud for AGN and SF models, for a given initial density (see subcaption), three ζCR values, 10−14 s −1 (solid line), 10−13 s −1 (dash-dotted), 10−12 s −1 (dotted), and two log U values of −3.0 (green) and −2.0 (blue). The teal-shaded area indicates the approximate region where CR heating becomes dominant. 14 16 18 20 22 log(depth) [cm] −32 −30 −28 −26 −24 −22 −20 log… view at source ↗
Figure 9
Figure 9. Figure 9: Line emissivities vs. depth for AGN and SF models, for three ζCR values of 10−14 s −1 (solid line), 10−13 s −1 (dash-dotted), and 10−12 s −1 (dotted), and two log U values of −3.0 (green) and −2.0 (blue). The teal-shaded area indicates the region where CRs heating becomes dominant. Article number, page 11 of 24 [PITH_FULL_IMAGE:figures/full_fig_p011_9.png] view at source ↗
Figure 10
Figure 10. Figure 10: Nebular line emissivities as a function of depth for AGN and SF models, for three different CR ionization rate values of 10−14 s −1 (solid line), 10−13 s −1 (dash-dotted), and 10−12 s −1 (dotted), and two log U values of −3.0 (green) and −2.0 (blue). The shaded region (teal), starting from the lower inflection point of the log U = −3.0 and ζCR = 10−12 s −1 and extending to the right, indicates the approxi… view at source ↗
Figure 11
Figure 11. Figure 11: Thermal stability plot, depicting electron temperature as a function of the ionization parameter Ξ for different values CR ionization rate, 10−14 s −1 , 10−13 s −1 , 10−12 s −1 illustrated with purple, blue and green lines respectively. The upper x-axis shows the corresponding values for the ionization parameter U. (log U ≳ −5), the models corresponding to the three different CR rates behave similarly, wi… view at source ↗
Figure 12
Figure 12. Figure 12: BPT diagrams depicting the area covered by AGN and SF models with solar abundances for −3.5 ≤ log U ≤ −1.5, shown with purple and red contours, with 1 ≤ log nH ≤ 3.5 and, 1 ≤ log nH ≤ 3 respectively. The solid contour lines map regions containing 10%, 50%, and 90% of the models with ζCR = 10−12 s −1 in the [N ii] BPT diagram, and 10−13 s −1 in both the [S ii] and the [O i] panels. The gray hatched area st… view at source ↗
Figure 13
Figure 13. Figure 13: BPT diagrams depicting [N ii]/Hα, [S ii]/Hα, and [O i]/Hα ratios. Our proposed SFζ maximum starburst line is illustrated in red color, compared with the full observational dataset. Observations from Centaurus A, NGC 1068, NGC 253, and NGC 1320 are marked by cyan squares, green circles, magenta thin diamonds, and a red diamond, respectively. The Kewley and Schawinski lines are indicated with solid and dash… view at source ↗
read the original abstract

Cosmic rays (CRs) can significantly impact dense molecular clouds in galaxies, heating the interstellar medium (ISM) and altering its chemistry, ionization, and thermal properties. Their influence is particularly relevant in environments with high CR rates, such as starburst galaxies with supernova remnants or jets and outflows in active galactic nuclei (AGN). CRs transfer energy to the ionized phase of the ISM far from the ionization source, preventing gas cooling and driving large-scale winds. In this work, we use CLOUDY to explore the effect of CRs on nebular gas, a relatively underexplored area, mainly focused on cold molecular gas. Our models cover a broad range of density ($1 - 10^4\,\rm{cm^{-3}}$), ionization parameter ($-3.5 \leq \log U \leq -1.5$), and CR ionization rate ($10^{-16}\, \rm{s^{-1}} - 10^{-12}\, \rm{s^{-1}}$). These are compared to MUSE observations of two AGN, Centaurus A and NGC 1068, and the starburst NGC 253. We find that CR rates $\gtrsim 10^{-13}\, \rm{s^{-1}}$, typical of AGN and strong starburst galaxies, can significantly alter the thermal structure of the ionized gas by forming a deep secondary low-ionization layer beyond the photoionization-dominated region. This enhances emission from low-ionization transitions, such as [\ion{N}{ii}], [\ion{S}{ii}], and [\ion{O}{i}], affecting line-ratio diagnostics, metallicity, and ionization estimates. Unlike pure photoionization models, AGN simulations with high CR ionization rates reproduce the Seyfert loci in BPT diagrams without requiring super-solar metallicities for the narrow-line region. Additionally, star formation simulations with high CR ionization rates can explain line ratios in the LINER domain. We propose new maximum starburst boundaries for BPT diagrams to distinguish regions dominated by AGN photoionization from those that could be explained by star formation plus high CR ionization rates.

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 / 1 minor

Summary. The manuscript uses CLOUDY to model the effects of cosmic-ray ionization rates (10^{-16} to 10^{-12} s^{-1}) on nebular gas across densities 1–10^4 cm^{-3} and log U from -3.5 to -1.5. It claims that rates ≳10^{-13} s^{-1} (typical of AGN and strong starbursts) produce a deep secondary low-ionization layer that enhances [N II], [S II], and [O I] emission, allowing solar-metallicity AGN models to reproduce observed Seyfert loci in BPT diagrams without super-solar narrow-line-region metallicities and star-formation models to reach the LINER domain; revised maximum-starburst boundaries are proposed. The models are compared to MUSE data for Centaurus A, NGC 1068, and NGC 253.

Significance. If the modeling holds, the work supplies a physically motivated alternative to super-solar metallicity explanations for BPT positions in high-CR environments and could revise metallicity and ionization diagnostics for AGN and starburst narrow-line regions. The broad parameter grid and direct comparison to three galaxies provide directional support for the claim.

major comments (2)
  1. [Results/comparison to observations] The comparison to MUSE observations of Centaurus A, NGC 1068, and NGC 253 is described only qualitatively; no quantitative fit statistics (reduced χ², residual distributions, or line-ratio offsets with uncertainties) are reported, so the degree to which the high-CR models actually reproduce the observed loci versus pure-photoionization models remains unquantified.
  2. [Methods/modeling setup] The central result—that CR rates ≳10^{-13} s^{-1} produce a secondary low-ionization zone whose extra low-ionization-line emissivity shifts the models onto the Seyfert and LINER loci—depends on CLOUDY correctly computing ionization balance, secondary-electron heating, and net cooling in the partially ionized transition region; no validation against other codes, no exploration of additional cooling channels (e.g., enhanced molecular or fine-structure lines), and no sensitivity tests to CR penetration depth are presented.
minor comments (1)
  1. [Abstract] The abstract sentence “a relatively underexplored area, mainly focused on cold molecular gas” is ambiguous; clarify whether the clause modifies the present work or the existing literature.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the constructive report and the recommendation for major revision. We address each major comment below, indicating where we agree and will revise the manuscript.

read point-by-point responses

  1. Referee: The comparison to MUSE observations of Centaurus A, NGC 1068, and NGC 253 is described only qualitatively; no quantitative fit statistics (reduced χ², residual distributions, or line-ratio offsets with uncertainties) are reported, so the degree to which the high-CR models actually reproduce the observed loci versus pure-photoionization models remains unquantified.

    Authors: We agree that quantitative statistics would strengthen the comparison. In the revised manuscript we will add reduced χ² values for the key line ratios ([N II]/Hα, [S II]/Hα, [O I]/Hα), mean offsets, and uncertainties, computed for both the high-CR and pure-photoionization models against the MUSE data for the three galaxies. revision: yes

  2. Referee: The central result—that CR rates ≳10^{-13} s^{-1} produce a secondary low-ionization zone whose extra low-ionization-line emissivity shifts the models onto the Seyfert and LINER loci—depends on CLOUDY correctly computing ionization balance, secondary-electron heating, and net cooling in the partially ionized transition region; no validation against other codes, no exploration of additional cooling channels (e.g., enhanced molecular or fine-structure lines), and no sensitivity tests to CR penetration depth are presented.

    Authors: CLOUDY is the standard code for such nebular modeling and its CR implementation follows published treatments; the code already incorporates a broad set of atomic and molecular cooling channels. We will add a paragraph in the methods section explicitly stating these assumptions, noting the lack of cross-code validation and the uniform CR rate approximation, and acknowledging that sensitivity tests to penetration depth lie outside the present scope. Full validation against other codes would require substantial additional effort. revision: partial

Circularity Check

0 steps flagged

No circularity: forward modeling with CLOUDY compared to external observations

full rationale

The paper runs CLOUDY grids over stated ranges of density, log U, and CR ionization rate, then directly compares the resulting line ratios to MUSE data on Centaurus A, NGC 1068, and NGC 253. No equation or result is defined in terms of itself, no parameter is fitted to the target BPT loci and then re-predicted, and no load-bearing claim rests on a self-citation chain. The central finding (high CR rates shift models onto observed loci) is an output of the simulation, not a tautology. This is standard self-contained forward modeling against independent data.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The claim rests on the assumption that standard photoionization codes can be extended to include cosmic rays as an additional ionization and heating source without major missing physics; no new entities are postulated and no parameters are fitted to the target observations.

axioms (2)
  • domain assumption CLOUDY correctly computes the ionization and thermal balance when an additional cosmic-ray ionization rate is supplied as input.
    Invoked when the authors state they use CLOUDY to explore CR effects on nebular gas.
  • domain assumption The observed line ratios in the three galaxies can be directly compared to single-zone or simple multi-zone models without significant aperture or geometric effects.
    Implicit in the comparison of models to MUSE observations of Centaurus A, NGC 1068, and NGC 253.

pith-pipeline@v0.9.0 · 5941 in / 1459 out tokens · 22534 ms · 2026-05-23T08:28:27.383378+00:00 · methodology

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