Identifying the Baryons in a Multiphase Intergalactic Medium

TL;DR

This paper reviews current observations of baryons in the low-redshift universe, highlighting the fractions in different cosmic structures and proposing future spectroscopic missions to locate the missing baryons crucial for galaxy evolution.

Contribution

It provides a comprehensive summary of baryon census at low redshift and suggests specific observational strategies to detect the remaining unaccounted baryons.

Findings

Approximately 60% of baryons are in the IGM.

About 5% of baryons are in the CGM.

Remaining 29% of baryons are unaccounted for.

Abstract

In this white paper, we summarize current observations of the baryon census at low redshift (Shull, Smith, & Danforth 2012). Measurements of Lya, O-VI, and broad Lya absorbers, together with more careful corrections for metallicity and ionization fraction, can now account for approximately 60% of the baryons in the intergalactic medium (IGM). An additional 5 +/- 3% may reside in the circumgalactic medium (CGM), 7 +/- 2% in galaxies, and 4 +/- 1.5% in clusters. This still leaves a substantial fraction, 29 +/- 13%, unaccounted for. We suggest improvements in measuring the baryons in major components of the IGM and CGM with future Ultraviolet and X-ray spectrographs. These missions could find and map the missing baryons, the fuel for the formation and chemical evolution of galaxies.