Probing the Cosmic Baryon Distribution and the Impact of Active Galactic Nuclei Feedback with Fast Radio Bursts in CROCODILE Simulation

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Abstract We investigate the missing baryon problem using fast radio bursts (FRBs) to trace cosmic baryons. Our CROCODILE simulations, performed with the GADGET3/4-OSAKA smoothed particle hydrodynamics code, include star formation, supernova, and active galactic nuclei (AGN) feedback. We generate light cones from large-scale structure simulations to compute gas density profiles and dispersion measures (DMs) measurable by FRBs. Our results show that AGN feedback reduces central gas densities in halos, reshaping the boundary between the circumgalactic medium and intergalactic medium (IGM). Zoom-in simulations reveal that AGN feedback significantly modulates the DM contributions from foreground halos along different sight lines. Using the DM–redshift (DM– z ) relation and comparing it to the Macquart relation, we constrain the diffuse baryon mass fraction at z = 1 to f diff = 0.86 5 − 0.165 + 0.101 (fiducial) and f diff = 0.85 6 − 0.162 + 0.101 (NoBH), which include contributions from both IGM ( f IGM ) and halos ( f Halos ), serving as upper limits. We also quantify the redshift evolution of f diff and f IGM , and provide the fitting results. Our study provides a framework for understanding baryon distribution across cosmic structures, FRB host galaxies, and the role of AGN in shaping foreground DM contributions.

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