[Paper Review] Searching for the Imprints of AGN Feedback on the Lyman Alpha Forest Around Luminous Red Galaxies

Vikram Khaire, Teng Hu+4 more|arXiv (Cornell University)|Nov 14, 2023

Galaxies: Formation, Evolution, Phenomena4 citations

TL;DR

This study uses Lyman-alpha forest absorption around luminous red galaxies (LRGs) in the Illustris and IllustrisTNG simulations to test how AGN feedback models affect circumgalactic gas. After calibrating the UV background to match observed line densities, it finds that IllustrisTNG predicts 1.5–2× higher Lyα line density (dN/dz) than Illustris at impact parameters up to 10 pMpc, a difference detectable with HST/COS data at >12σ significance, making dN/dz(r⊥) a powerful observational benchmark for AGN feedback models.

ABSTRACT

We explore the potential of using the low-redshift Lyman-$α$ (Ly$α$) forest surrounding luminous red galaxies (LRGs) as a tool to constrain active galactic nuclei (AGN) feedback models. Our analysis is based on snapshots from the Illustris and IllustrisTNG simulations at a redshift of $z=0.1$. These simulations offer an ideal platform for studying the influence of AGN feedback on the gas surrounding galaxies, as they share the same initial conditions and underlying code but incorporate different feedback prescriptions. Both simulations show significant impacts of feedback on the temperature and density of the gas around massive halos. Following our previous work, we adjusted the UV background in both simulations to align with the observed number density of Ly$α$ lines ($ m dN/dz$) in the intergalactic medium and study the Ly$α$ forest around massive halos hosting LRGs, at impact parameters ($r_{\perp}$) ranging from 0.1 to 100 pMpc. Our findings reveal that $ m dN/dz$, as a function of $r_{\perp}$, is approximately 1.5 to 2 times higher in IllustrisTNG compared to Illustris up to $r_{\perp}$ of $\sim 10$ pMpc. To further assess whether existing data can effectively discern these differences, we search for archival data containing spectra of background quasars probing foreground LRGs. Through a feasibility analysis based on this data, we demonstrate that ${ m dN/dz} (r_{\perp})$ measurements can distinguish between feedback models of IllustrisTNG and Illustris with a precision exceeding 12$σ$. This underscores the potential of ${ m dN/dz} (r_{\perp})$ measurements around LRGs as a valuable benchmark observation for discriminating between different feedback models.

Motivation & Objective

• To assess whether the Lyman-alpha forest around luminous red galaxies (LRGs) can serve as a probe of AGN feedback in cosmological simulations.
• To compare the impact of different AGN feedback prescriptions—strong thermal feedback in Illustris versus milder kinetic feedback in IllustrisTNG—on the distribution of cold gas in the circumgalactic medium (CGM) and intergalactic medium (IGM).
• To calibrate the UV background in both simulations so that their global Lyα forest line density (dN/dz) matches observations, enabling a fair comparison of feedback effects.
• To evaluate whether existing HST/COS quasar spectra probing LRGs can statistically distinguish between the two feedback models using the dN/dz(r⊥) profile.

Proposed method

• Simulations from the Illustris and IllustrisTNG projects at z = 0.1 are used, which share identical initial conditions and cosmological parameters but differ in AGN feedback implementation.
• The UV background optical depth (Γ_HII) is adjusted in both simulations to match the observed dN/dz of the intergalactic medium, ensuring consistency with observed Lyα forest statistics.
• Halo catalogs are used to identify massive dark matter halos hosting LRGs; mock high-resolution Lyα forest spectra are generated at varying impact parameters (r⊥) from 0.1 to 100 pMpc.
• Spectra are forward-modeled with realistic signal-to-noise (S/N) and resolution matching HST/COS observations (S/N ≈ 7 per pixel, 6 km/s resolution), and fitted using an automated Voigt profile fitting code.
• The Lyα line density profile dN/dz(r⊥) is computed for both simulations and compared across impact parameters to quantify differences in gas distribution due to feedback.
• A feasibility analysis uses archival HST/COS quasar spectra (94 quasars, 3193 LRGs) to estimate the statistical precision of dN/dz(r⊥) measurements, projecting significance levels for distinguishing the two feedback models.

Experimental results

Research questions

• RQ1 Can the Lyα forest around LRGs distinguish between strong thermal AGN feedback (Illustris) and milder kinetic feedback (IllustrisTNG)?
• RQ2 How does AGN feedback alter the distribution of cold, dense gas responsible for Lyα absorption in the circumgalactic medium at different impact parameters?
• RQ3 To what extent can existing HST/COS data resolve differences in dN/dz(r⊥) between extreme feedback models?
• RQ4 Does the dN/dz(r⊥) profile serve as a robust observational benchmark for calibrating feedback prescriptions in cosmological simulations?

Key findings

• After UV background calibration, the dN/dz(r⊥) profile in IllustrisTNG is 1.5 to 2 times higher than in Illustris at impact parameters up to 10 pMpc.
• The difference in dN/dz(r⊥) arises from the higher fraction of cool baryons (38.5% in IllustrisTNG vs. 23.2% in Illustris) due to milder AGN feedback, which preserves more cold gas in the CGM.
• The 2D b–N_HI distribution shapes converge to the IGM at large r⊥, but the normalization (i.e., line density) remains significantly higher in IllustrisTNG across all impact parameters.
• Feasibility analysis shows that with 94 HST/COS quasar spectra probing 3193 LRGs, dN/dz(r⊥) measurements can distinguish between Illustris and IllustrisTNG models at a significance exceeding 12σ.
• The dN/dz(r⊥) profile is thus a highly sensitive observable for constraining AGN feedback models, even with current data.
• This result positions dN/dz(r⊥) around LRGs as a critical benchmark for testing and calibrating feedback prescriptions in future cosmological simulations.