Baryon Fraction in Halos: Observational Constraints from HI, X-ray, and Stellar Components

          @misc{ pirsa_PIRSA:25070055,
            doi = {10.48660/25070055},
            url = {https://pirsa.org/25070055},
            author = {},
            keywords = {Cosmology},
            language = {en},
            title = {Baryon Fraction in Halos: Observational Constraints from HI, X-ray, and Stellar Components},
            publisher = {Perimeter Institute},
            year = {2025},
            month = {jul},
            note = {PIRSA:25070055 see, \url{https://pirsa.org}}
          }
          

Abstract

Feedback processes, particularly from active galactic nuclei (AGN), play a crucial role in redistributing baryons within halos. These mechanisms can displace gas to halo outskirts or eject it entirely, leading to baryon fractions below the cosmic mean. While simulations such as TNG, SIMBA, and EAGLE predict these effects across a wide halo mass range, observational constraints remain largely limited to high-mass groups and clusters. In this talk, I will present a systematic analysis of the observed baryon content of halos in the local universe across a mass range of $10^{10} - 10^{15} M_{\odot}$​, using a compilation of empirical measurements from the literature. We quantify the contributions of hot gas, stars, and cold gas to the total baryon budget, constructing baryonic mass-to-halo mass scaling relations. We also use the latest eROSITA and ASKAP data to provide the current constraints on the average hot gas and cold gas content in halos through stacking analysis. The baryonic scaling relations are constructed from group- and cluster-scale halos down to $10^{12} M_{\odot}$​ for all three components, while additional individual galaxy measurements allow us to extend HI and stellar mass scaling relations to $10^{10} M_{\odot}$​. By combining these relations with the halo mass function, we then determine the baryon density distribution as a function of halo mass and calculate the cosmic mass densities of stars, HI, and hot gas within halos in the local universe. Our results provide key observational constraints on the distribution of baryons in the local universe, offering insights into potential mechanisms, such as feedback, that regulate baryon retention and redistribution.