Revisiting the Missing Baryon Problem: Baryon Redistribution Across Halo Evolution

Abstract

The baryon fraction provides a fundamental diagnostic of baryonic matter distribution within dark matter halos and the baryon cycle. While its cosmic value is well constrained, the baryon content within halos on nearly all scales is significantly lower than the cosmic mean; this is called the ``missing baryon problem.'' Using the high spatial, mass, and temporal resolutions of the NewCluster simulation, we investigate the baryon fraction evolution by tracing halo assembly histories from merger trees and gas tracer particles. We identify three distinct evolutionary phases: an early growth phase driven by cold accretion in low-mass halos, a suppression phase likely dominated by supernova feedback, and a recovery phase in massive halos enabled by deeper potential wells and hot-mode accretion. We further find that the redshift dependence of baryon fraction, particularly in low-mass halos, is influenced by the thermal state of the intergalactic medium, with suppression emerging after the surrounding gas becomes persistently hotter than the halo virial temperature. In addition, the scatter in baryon fraction at fixed halo mass correlates with the large-scale environment, with baryon-rich halos preferentially residing closer to filamentary structures. Overall, our results highlight that baryon fraction evolution is governed by the interplay between feedback and environment, requiring a time-resolved perspective on the baryon cycle.