Resonant Emission Line Halos as CGM Diagnostics across the AGORA Simulations

Abstract

Resonant emission lines such as Lyman-alpha and Mg II are among the brightest tracers to directly map the circumgalactic medium (CGM), probing its cool hydrogen and metal-enriched gas. Their resonant nature causes numerous scatterings that reshape spectral profiles and surface brightness distributions and complicate interpretation. We post-process the AGORA CosmoRun cosmological zoom-in simulations of a 10^12 Msun halo, run with multiple simulation codes sharing common initial conditions and calibrated physical models, using the GPU-accelerated THOR Monte Carlo radiative transfer code to produce synthetic IFU datacubes. THOR's speed and flexibility across different data structures enables systematic RT post-processing across all participating codes, allowing us to assess how differences in the simulated multiphase CGM propagate into observables. We find that predicted surface brightness profiles and spectral morphologies are sensitive to the underlying CGM thermodynamics, kinematics, and metal distribution. At the same time, significant code-to-code variation persists, reflecting genuine differences in how each code shapes the multiphase gas around the galaxy. Understanding these code and model variations is critical for robustly interpreting CGM emission from resonant lines in current and upcoming IFU surveys.