arxiv: 2604.10595 · v2 · submitted 2026-04-12 · 🌌 astro-ph.CO

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Low-ionization Metal Absorption at 0.7 lesssim z lesssim 2 Confronting Cosmological Simulations with Observations

Ivan Rapoport , Ehud Behar , Vincent Desjacques

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Pith reviewed 2026-05-10 15:53 UTC · model grok-4.3

classification 🌌 astro-ph.CO

keywords low-ionization absorptionMgIIIllustrisTNGcolumn density PDFequivalent widthcircumgalactic mediumUVBredshift evolution

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The pith

IllustrisTNG reproduces low-ionization metal column density PDFs but underestimates strong MgII absorber incidence at higher redshifts.

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 IllustrisTNG cosmological simulation can reproduce observed absorption from low-ionization metals in quasar spectra, which trace cool gas in and around galaxies. Using a grid-based method to calculate MgI, MgII and FeII column densities under collisional and photo-ionization models, the authors find that the resulting probability distributions match observational data across column densities from 10^11.4 to 10^16 cm^{-2}. However, the model with a uniform ultraviolet background correctly predicts the redshift evolution of weak absorbers but falls short for strong ones with equivalent widths above 1 Å and does not capture their increasing numbers toward z approximately 2.

Core claim

The authors compute MgI, MgII and FeII column densities from IllustrisTNG gas cells on a grid using both a purely collisional ionization model and one that includes photo-ionization by a uniform UVB. The resulting column density PDFs broadly reproduce the observed distributions from HIRES, UVES, SDSS and DESI samples across 10^11.4 ≲ N ≲ 10^16 cm^{-2}. The UVB model yields dN/dz values that match observations for MgII equivalent widths W0^2796 below 0.6 Å but underestimates the incidence at W0^2796 above 1 Å and fails to reproduce the observed rise in numbers toward z ∼ 2.

What carries the argument

Grid-based post-processing to derive ionic column densities from the simulation's gas distribution under uniform UVB photo-ionization plus collisional ionization equilibrium, followed by construction of PDFs and equivalent-width incidence statistics.

If this is right

The simulation captures the dominant drivers of low-ionization absorption for column densities up to 10^16 cm^{-2}.
Photo-ionized low-density gas in the outer CGM is adequately modeled for weak equivalent-width absorbers below 0.6 Å.
High-EW MgII systems arise from denser gas whose abundance is underproduced in TNG, especially at z approaching 2.
The mismatch at strong absorbers indicates that current galaxy-formation prescriptions miss some physics affecting cool dense gas.

Where Pith is reading between the lines

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

The discrepancy points to a need for sub-grid treatments of cold gas clumps or local ionization sources to improve strong-absorber statistics.
Comparing the same statistics across different simulations with varying feedback strengths could isolate which missing process drives the high-EW shortfall.
The success for weak absorbers but failure for strong ones implies that outer CGM gas is better constrained than inner, denser regions.

Load-bearing premise

The IllustrisTNG gas distribution combined with a uniform UVB and standard collisional ionization prescriptions faithfully captures the physical conditions that produce the observed low-ionization absorption without needing local radiation fields or unresolved cold-gas structures.

What would settle it

Re-running the column-density and dN/dz analysis on a higher-resolution TNG variant or with added local stellar radiation that produces more high-column systems and an increase in strong MgII absorbers toward z=2.

Figures

Figures reproduced from arXiv: 2604.10595 by Ehud Behar, Ivan Rapoport, Vincent Desjacques.

**Figure 1.** Figure 1: Average log NMg II in bins of W2796 0 for the HIRES/UVES absorption components using b values measured by Churchill et al. (2020). range of absorption strengths, ranging from optically thin to highly saturated systems, with rest-frame EW for the Mg II λ2796 transition in the range 0.006 ˚A ≤ W2796 0 ≤ 6.23 ˚A. Churchill et al. (2020) employed simultaneous Voigt profile fitting and the full instrumental re… view at source ↗

**Figure 2.** Figure 2: Left: Steady state fractional abundance of Mg II (χ 1 Mg) as a function of ne, T, including the UVB field at z = 1. The horizontal branch above T = 104 K is where collisional ionization and radiative recombination dominate (both scale with ne), whereas at lower temperatures UVB photo-ionization balanced by radiative recombination determines the Mg II population, which peaks around ne ≈ 0.1 cm−3 . Right: … view at source ↗

**Figure 3.** Figure 3: Three-dimensional two-point autocorrelation functions ξ(r) of Mg II and the total gas distributions at z = 1, measured from TNG as a function of comoving separation r. The Mg II mass exhibits a much stronger autocorrelation, consistent with its association with dense regions. 1992), where each particle is smoothed over two layers of grid cells (R = 2), corresponding to (2R + 1)2 = 25 pixels on the square… view at source ↗

**Figure 5.** Figure 5: Systemic examination of the Mg II column density PDF, normalized to 1012 ≤ NMg II ≤ 1018 cm−2 . The solid line shows results for the xy plane of TNG50-1 at redshift z = 1 with grid size Ngrid = 70000, integration over full z axis. The red line shows the same results but when all column densities are multiplied by 0.5. The dashed line shows Ngrid = 70000 again but when only the upper half of the box is in… view at source ↗

**Figure 6.** Figure 6: Column density PDFs for the three ions examined in the paper. Each panel shows four redshift bins (see Sec §4.1). Squares represent results from the HIRES/UVES catalog; the insets indicate the average redshift ⟨z⟩ and the total number of absorbers (num) used to compute the PDFs. Open squares denote upper limits. Solid and dashed lines show results for the Coll+UVB and Coll models, respectively. Since the … view at source ↗

**Figure 7.** Figure 7: Comparison of the measured Mg II absorber incidence with TNG. Each panel shows the cosmic incidence in a different EW bin as quoted, and these are compared to TNG when mapped into the approximate corresponding column density bins (see §4.2), with the solid black lines and dashed lines corresponding to the Coll+UVB, Coll models respectively. The grey shades represent the model uncertainties, due to the adop… view at source ↗

**Figure 8.** Figure 8: PDFs of the rest-frame EW of the Mg II λ2796 line for absorbers in the redshift range 0.5 < z < 2, normalized over 0.3 < W2796 0 < 4 ˚A. The black solid and dashed curves respectively show the DESI distributions with and without a completeness correction, while the dotted red curve corresponds to the HIRES/UVES sample. The two PDFs are seen to be totally different. We ascribe this difference primarily to … view at source ↗

**Figure 9.** Figure 9: Mg II absorption properties for the identified 39 quasars common to both catalogs. Quasar names are given in J2000 format. Top panel: Redshifts of individual absorption components/systems identified in each catalog (black squares for HIRES/UVES, red triangles for DESI). Error bars for redshifts reported in the catalogs are omitted for clarity, as they are typically small (relative uncertainties ≲ 1%). Purp… view at source ↗

read the original abstract

Low-ionization metal absorption lines provide a primary probe of cool gas in and around galaxies. We confront observations of metal-line absorption in quasar spectra with predictions from the IllustrisTNG cosmological simulation in order to benchmark how well current galaxy formation models reproduce the observed circumgalactic medium (CGM) and intergalactic medium (IGM) absorption signatures. We implement two ionization prescriptions: a purely collisional model and a model including photo-ionization by a uniform ultraviolet background (UVB). Using a grid-based framework, we compute MgI, MgII and FeII column densities and construct column density probability distribution functions (PDFs) and equivalent width (EW) statistics for comparison with observations. The observational samples considered here are based on the High Resolution Echelle Spectrometer (HIRES), the Ultraviolet and Visual Echelle Spectrograph (UVES), the Sloan Digital Sky Survey (SDSS) and the Dark Energy Spectroscopic Instrument (DESI). The computed PDFs broadly reproduce the observed ones across the sampled column density range of $10^{11.4}\lesssim \text{N}\lesssim 10^{16}\ \rm{cm^{-2}}$, indicating that the simulation captures the dominant physical drivers of low-ionization absorption. We then compute the cosmic incidence of MgII systems, namely the evolution of their number with redshift $d\mathcal{N}/{dz}$. The model that includes UVB accurately produces $d\mathcal{N}/{dz}$ up to equivalent widths (EW) of $\rm W_0^{2796} < 0.6\ \mathring{A}$, consistent with low-density photo-ionized gas in the outer CGM. At high EWs of $\rm W_0^{2796} > 1\ \mathring{A}$ TNG underestimates $d\mathcal{N}/{dz}$ and fails to capture its rise toward $z\sim2$.

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

IllustrisTNG matches the low-ionization metal column density PDFs but underpredicts strong MgII incidence and its rise toward z~2.

read the letter

This paper shows that IllustrisTNG reproduces the column density PDFs for low-ionization metals like MgII but underpredicts the number of strong absorbers and fails to match the rise in their incidence toward z~2. What stands out as new is the specific quantitative mismatch for high equivalent width MgII systems. Previous work has compared simulations to absorption stats, but this focuses on 0.7 to 2 redshift range with TNG and shows a clear shortfall in strong lines. They do well by using both collisional ionization and a uniform UVB model, which lets them separate the contributions from dense gas versus photoionized outer regions. The PDFs matching across 10^11.4 to 10^16 cm^-2 is reassuring and suggests the simulation gets the broad distribution of cool gas right. Comparing to HIRES, UVES, SDSS, and DESI data adds credibility to the observational side. The soft spots are around the interpretation of the discrepancy. The stress-test concern about TNG resolution is valid here. At the quoted mass resolution of about 1.4 million solar masses and 0.7 kpc softening, the code may not resolve the sub-kiloparsec cold dense clumps that observations link to saturated strong MgII. The uniform UVB also skips local stellar or AGN radiation fields and self-shielding effects inside those structures. If those numerical limits drive the high-EW tail mismatch, then the paper's suggestion of shortcomings in galaxy formation physics needs more support. The abstract mentions the model accurately produces dN/dz up to 0.6 Angstrom but underestimates above 1 Angstrom, yet without seeing the exact methods for how they handle the simulation grid and ionization, it's tough to judge robustness. That said, the circularity is low since they use external catalogs, and the result is new. This paper is aimed at researchers who use cosmological simulations to model the CGM and IGM and those analyzing large spectroscopic surveys. A reader looking for benchmarks on how well TNG handles cool gas absorption would find it useful, even if the strong absorber part raises questions. It deserves a serious referee because the comparison is grounded in real data and simulation outputs, and the mismatch provides a concrete target for future model improvements. I recommend sending it to peer review rather than desk rejecting it.

Referee Report

2 major / 2 minor

Summary. The paper compares low-ionization metal absorption (Mg I, Mg II, Fe II) column densities and equivalent widths from the IllustrisTNG simulation against HIRES, UVES, SDSS, and DESI observations. Using a grid-based approach with collisional ionization and a uniform UVB photo-ionization model, it reports that the simulated column-density PDFs broadly match observations over 10^{11.4} ≲ N ≲ 10^{16} cm^{-2}, while the UVB model reproduces dN/dz only for W_0^{2796} < 0.6 Å and underestimates the incidence and redshift evolution of stronger systems (W_0^{2796} > 1 Å) toward z ∼ 2.

Significance. If the reported PDF agreement and targeted dN/dz discrepancies hold after addressing numerical caveats, the work provides a useful benchmark for the cool-gas content of the CGM/IGM in state-of-the-art cosmological simulations. The identification of where TNG succeeds (low-density photo-ionized gas) versus fails (dense, high-EW absorbers) supplies concrete guidance for refining sub-grid physics or resolution requirements in future galaxy-formation models.

major comments (2)

[§4] §4 (dN/dz and high-EW results): The claim that TNG underestimates dN/dz for W_0^{2796} > 1 Å and fails to capture the rise toward z ∼ 2 is load-bearing for the central conclusion. However, the TNG baryonic resolution (∼1.4 × 10^6 M_⊙, ∼0.7 kpc softening) is insufficient to resolve the sub-kpc, high-density cold clumps observationally linked to saturated Mg II. Without a resolution-convergence test or comparison to higher-resolution runs, the discrepancy cannot be cleanly attributed to galaxy-formation physics rather than numerical incompleteness in the cold-gas distribution.
[Methods] Methods section (grid-based column-density calculation): The paper does not provide sufficient detail on sightline selection, the precise UVB spectrum and normalization, self-shielding treatment, or how unresolved dense structures are handled. These choices directly affect whether the reported PDF agreement is robust or sensitive to the adopted prescriptions.

minor comments (2)

[Abstract and §4] Clarify in the abstract and §4 whether the dN/dz comparison uses the same redshift bins and EW thresholds as the observational catalogs.
[Simulation setup] Add a short paragraph or table summarizing the exact simulation parameters (box size, resolution, UVB model reference) for reproducibility.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their constructive and detailed comments, which have helped clarify several aspects of our analysis. We address each major comment below and describe the revisions we plan to implement.

read point-by-point responses

Referee: §4 (dN/dz and high-EW results): The claim that TNG underestimates dN/dz for W_0^{2796} > 1 Å and fails to capture the rise toward z ∼ 2 is load-bearing for the central conclusion. However, the TNG baryonic resolution (∼1.4 × 10^6 M_⊙, ∼0.7 kpc softening) is insufficient to resolve the sub-kpc, high-density cold clumps observationally linked to saturated Mg II. Without a resolution-convergence test or comparison to higher-resolution runs, the discrepancy cannot be cleanly attributed to galaxy-formation physics rather than numerical incompleteness in the cold-gas distribution.

Authors: We agree that the baryonic resolution of TNG limits the faithful representation of the smallest, densest cold clumps that dominate the strongest saturated Mg II systems. This is a substantive limitation of the current simulation suite. At the same time, the broad agreement we find in the column-density PDFs over 10^{11.4} ≲ N ≲ 10^{16} cm^{-2} indicates that the simulation captures the bulk of the photo-ionized cool gas responsible for weaker absorbers. The high-EW discrepancy is therefore likely a combination of numerical resolution and sub-grid physics. We will revise §4 to include an explicit discussion of these resolution caveats, reference prior work on CGM resolution requirements, and state that the reported underestimation of strong systems and their redshift evolution should be interpreted as a benchmark for future higher-resolution simulations rather than a definitive statement on galaxy-formation physics alone. revision: partial
Referee: Methods section (grid-based column-density calculation): The paper does not provide sufficient detail on sightline selection, the precise UVB spectrum and normalization, self-shielding treatment, or how unresolved dense structures are handled. These choices directly affect whether the reported PDF agreement is robust or sensitive to the adopted prescriptions.

Authors: We will expand the Methods section substantially. We will specify that sightlines are drawn randomly through the periodic simulation volume with a number chosen to ensure convergence of the PDFs and dN/dz statistics; that the UVB is the Haardt & Madau (2012) spectrum normalized at each redshift to the observed hydrogen photo-ionization rate; that self-shielding is implemented via a simple density-threshold prescription above which the UVB is attenuated; and that unresolved dense structures are handled by the native grid resolution of the simulation, with the caveat that sub-cell clumps are not captured. These additions will allow readers to evaluate the robustness of the reported PDF agreement. revision: yes

Circularity Check

0 steps flagged

No circularity: direct simulation-observation comparison using external data

full rationale

The paper extracts MgI, MgII and FeII column densities from the public IllustrisTNG simulation volume using two standard ionization prescriptions (collisional and UVB photo-ionization). It then constructs PDFs and dN/dz statistics and compares them to independent observational catalogs (HIRES, UVES, SDSS, DESI). No parameters are fitted to the target absorption statistics, no self-citation supplies a uniqueness theorem or ansatz that forces the result, and no reported quantity is defined in terms of itself. The noted under-prediction at high EW is an output of the comparison, not an input. This is a standard external-benchmark test and therefore self-contained.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The central claim rests on the accuracy of the IllustrisTNG sub-grid physics for gas distribution and on the validity of the two ionization prescriptions; both are standard but contain many untested assumptions about unresolved physics.

axioms (2)

domain assumption IllustrisTNG provides a sufficiently accurate three-dimensional distribution of gas density, temperature and metallicity in the CGM and IGM.
The ionization calculations are performed on top of the simulation output; any error in the gas fields propagates directly into the predicted column densities.
domain assumption A spatially uniform UVB is an adequate description of the photo-ionizing radiation field at these redshifts.
One of the two ionization models relies on this assumption; the paper does not explore local radiation sources.

pith-pipeline@v0.9.0 · 5661 in / 1525 out tokens · 29977 ms · 2026-05-10T15:53:15.240945+00:00 · methodology

discussion (0)

Reference graph

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