The Observed Cosmic Baryon Budget: Searching for All the Baryons in the Universe

Abstract

We present an updated estimate of the observable baryon density in the local universe. When compared to the cosmic baryon fraction f\(^{cosmic}\)\(_{b}\) = 0.016 ± 0.008, previous estimates based on observations have found a staggering fraction of "missing" baryons (≥ 90%) when adding the contributions of stellar mass, the hot gas in groups and clusters, and the cold gas in galaxies. When looking at individual virialized structures a similar issue has emerged: the baryonic content in galaxies, groups, and clusters falls short of the cosmic fraction, with increasing deficit from clusters to galaxies. We use the most up-to-date data to determine the baryon census for individual virialized structures. Using direct X-ray observations of the hot intracluster medium and extrapolations of the observed gas density profile, we determine the gas content in groups and clusters out to the virial radius, showing that the baryon fraction within the virial radius is consistent with the cosmic fraction. Using recently observed absorption measurements of the circumgalactic medium (CGM) around galaxies, which adds a major new reservoir of baryons not previously observed, we determine the gas budget in galaxies, showing that it too approaches the cosmic fraction. With this data, we proceed to use the halo mass function to determine the integrated gas mass density across all virialized structures in the local universe with M ≥ 10\(^{10}\) M§: f\(_{gas}\) = 0.106 ± 0.021. We determine the universal stellar fraction by using the observed luminosity function coupled with the stellar mass-to-light ratios for spiral and elliptical galaxies; we find f\(_{stars}\) = 0.008 ± 0.001. Combining the gas and stellar contributions we find the observed baryon budget for the low-redshift universe: f\(_{b}\) = 0.114 ± 0.026. By comparing it with the cosmic baryon fraction, we are observing at least 71% of the cosmic baryons. This is a lower bound given that we only consider structures with M ≥ 10\(^{10}\) M§. By including virialized structures with masses as low as 10\(^{8}\) M§ the baryon fraction can reach to ~ 94% of the cosmic baryons.