Cosmological hydrodynamic simulations predict that star-forming galaxies at z≈0.2z\approx0.2 continuously accrete cold gas at 2020–60 km s−160\ {\rm km\,s^{-1}}, at rates comparable to their star formation rates, yet direct observational evidence of inflow is scarce. Quasar sightline observations detect low-ionization-state gas (e.g., Mg II) corotating with galaxy disks near galaxy major axes, plausibly the observational signature of this accretion. The open problem, posed by Ho, Martin & Turner (2019) from their pilot study with the EAGLE suite of cosmological hydrodynamic simulations, is to close this simulation–observation loop: generate end-to-end synthetic low-ion absorption spectra (ionization balance, multi-component line profiles, instrumental convolution) from cosmological simulations of galaxies matching the observed population, obtain statistics on the probabilities of detecting corotating cold gas along sightlines from an enlarged simulated galaxy sample, and compare these predictions quantitatively with the z≈0.2z\approx0.2 quasar sightline measurements, so that the observed corotating cold gas can be interpreted in terms of simulated cold accretion. Later work has delivered pieces of this program — Mg II ion-fraction maps in EAGLE, mock Mg II spectra in TNG50 at z≃1z\simeq1, corotation fractions for Milky Way analogs in IllustrisTNG, and high-cadence tracer tracking — but no published study has completed the synthetic-spectrum confrontation for the z≈0.2z\approx0.2 low-ion corotation dataset.

Background

Galaxy disks would exhaust their gas reservoirs in far less than a Hubble time if star formation were not replenished by fresh gas, so the fueling of disks by accretion from the circumgalactic medium (CGM, the diffuse multiphase gas within a galaxy's dark matter halo, conventionally inside the virial radius rvirr_{\rm vir}) is a central question in galaxy evolution. Simulations distinguish cold-mode accretion — gas that remains near 10410^4 K while flowing in along filaments, dominant in halos below roughly 1012 M⊙10^{12}\,M_\odot — from hot-mode accretion through a virially shock-heated corona. EAGLE is a suite of cosmological smoothed-particle-hydrodynamics simulations with subgrid prescriptions for star formation and stellar feedback (the stochastic heating of gas particles representing supernova energy injection); because that feedback physics is implemented below the simulation's resolution scale, its detailed prescription can influence results in a way that is difficult to validate. Ho, Martin & Turner (2019) studied cold accretion onto EAGLE galaxies at z=0.27z=0.27 using two identification methods: a ballistic approximation, which integrates particle trajectories under gravity alone from a single simulation snapshot and predicts which particles will reach the inner galaxy within one disk rotation period (∼100\sim100 Myr), and hydrodynamic particle tracking, which follows the same particles through the simulation's full physics across stored timesteps. The two methods agree on mass inflow rates to within about a factor of two, with typical inflow speeds of 2020–60 km s−160\ {\rm km\,s^{-1}} and inflow rates comparable to the galaxies' star formation rates; the ballistic method tends to overestimate, and the discrepancy is dominated by feedback expelling or heating recently accreted gas. The authors warned that these inflow properties depend on the feedback implementation and may differ between simulation codes, that reconstructing accretion histories accurately requires simulation outputs at a cadence comparable to or shorter than the ∼100\sim100 Myr rotation period, and that a larger simulated volume with higher resolution is needed for statistically robust statements. On the observational side, quasar sightline absorption spectroscopy measures low-ionization-state ions such as Mg II (a tracer of ∼104\sim10^4 K gas) in the CGM of foreground galaxies; combining absorption-line velocity shifts with galaxy rotation curves shows cold gas corotating with galaxy disks near the galaxy major axes, out to impact parameters of roughly 100100 pkpc (proper kiloparsecs) (Martin et al. 2019, for 50 sightlines at z≈0.2z\approx0.2; French & Wakker 2020 for Lyα\alpha at z≤0.03z\le0.03). Ho, Martin & Turner (2019) therefore probed one simulated galaxy with mock quasar sightlines and found that sightlines near the major axis of an edge-on disk can detect corotating cold gas to ∼60\sim60 pkpc, suggesting the observations may be intersecting cold gas disks extended beyond the optical disks. However, their mock analysis selected cold gas by temperature (≤3×104\le3\times10^4 K) and used mean line-of-sight velocities: it computed no ionization balance (the equilibrium or non-equilibrium partition of an element among ionization states, needed to convert simulated gas density, temperature, metallicity, and the ultraviolet background into the ionic column densities actually observed) and generated no synthetic absorption spectra with realistic multi-component line profiles convolved with instrumental resolution, as required to compare with observed equivalent widths (the integrated strength of an absorption line) and velocity decompositions. Subsequent work built parts of the needed machinery: Ho, Martin & Schaye (2020) computed Mg II ion fractions in the high-resolution EAGLE run and mapped circumgalactic Mg II column density and rotation structure (axisymmetric rotating structures extending beyond 0.5 rvir0.5\,r_{\rm vir}, plus a quantified line-of-sight velocity-cut contamination problem); Ho, Martin & Schaye (2021) extended ion-fraction analysis to O VI (five-times-ionized oxygen, a tracer of warmer ∼105.5\sim10^{5.5} K gas); DeFelippis et al. (2021) generated mock Mg II spectra with photoionization post-processing from TNG50 (the highest-resolution run of the IllustrisTNG moving-mesh simulation suite) at z≃1z\simeq1 and compared them with the MEGAFLOW survey (a VLT/MUSE survey of Mg II gas flows around z≈1z\approx1 galaxies); Messere et al. (2026) built a mock absorption survey of Milky Way analogs in IllustrisTNG quantifying corotation fractions; and DeFelippis, Genel & Bryan (2026) demonstrated ∼2\sim2 Myr cadence Monte Carlo tracer tracking of CGM gas in a re-run subregion of the TNG100 volume (a 100 Mpc IllustrisTNG run). What no published work has delivered is the complete mapping for the low-redshift, low-ion corotation program: end-to-end synthetic spectra compared against the observed z≈0.2z\approx0.2 datasets.

Problem Statement

Establish a robust mapping between cold gas accretion predicted by cosmological hydrodynamic simulations and the low-ionization-state quasar absorption-line observations that probe it, in the regime of the existing z≈0.2z\approx0.2 star-forming galaxy samples. Concretely, the problem — completing the program initiated by Ho, Martin & Turner (2019) — requires two linked results. (1) End-to-end synthetic spectra: starting from simulated galaxies' cold gas density, velocity, and metallicity fields, compute ionization balance for the observed low ions (Mg II and neighboring low-ionization species, with the treatment of the ultraviolet background — the metagalactic ionizing radiation field — and of self-shielding, the attenuation of ionizing radiation inside dense gas clouds, stated), generate synthetic absorption spectra with realistic multi-component line profiles convolved with instrumental resolution, and analyze them with the same procedures observers apply (component decomposition, and Doppler shifts of the line components weighted by equivalent width, the integrated strength of an absorption line, relative to disk rotation). (2) Statistical confrontation: for a simulated galaxy sample matched to the observed population, predict the probability of detecting corotating cold gas, and the distributions of Doppler shifts, line widths, and inferred inflow-speed diagnostics, as functions of galaxy mass, disk inclination, azimuthal angle, and impact parameter, and compare them quantitatively with the observed z≈0.2z\approx0.2 sightline datasets. Two source-stated conditions govern the simulation-side inflow inference used in the comparison: reconstruction of inflow rates and speeds must track particles through simulation outputs at a time cadence of order ∼100\sim100 Myr or shorter (around or below the disk rotation period), since the single-snapshot ballistic approximation overestimates inflow by factors of ∼1.5\sim1.5–22 with the discrepancy dominated by feedback expelling or heating recently accreted gas; and the analysis must acknowledge that the deduced inflow properties depend on the subgrid stellar-feedback implementation and may be code-dependent. The unifying objective is the source paper's own: determine whether the observed corotating low-ion circumgalactic gas is consistent with simulated cold accretion — whether simulated inflow rates (comparable to star formation rates) and speeds (2020–60 km s−160\ {\rm km\,s^{-1}}) can be confronted with the absorption-line data. Delivering the ionization-modeled synthetic spectra, the enlarged-sample corotation-detection statistics, and their quantitative comparison with the z≈0.2z\approx0.2 quasar sightline measurements would allow the observed cold-gas kinematics to be interpreted in terms of simulated cold accretion.

References

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