The baryon content of the local intergalactic medium

Summary

In this review, we describe our surveys of low column density (Lyα) absorbers (N_HI = 10^12.5−16 cm⁻²), which show that the warm photoionized IGM contains ∼30% of all baryons at z ≤ 0.1. This fraction is consistent with cosmological hydrodynamical simulations, which also predict that an additional 20–40% of the baryons reside in much hotter 10⁵⁻⁷ K gas, the warm-hot IGM (WHIM). The observed line density of Lyα absorbers, dN/dz ≈ 170 for N_HI ≥ 10^12.8 cm⁻², is dominated by low-N_HI systems that exhibit slower redshift evolution than those with N_HI ≥ 10¹⁴ cm⁻². HST/FUSE surveys of OVI absorbers, together with recent detections of OVII with Chandra and XMM/Newton, suggest that 10–40% of all baryons could reside in the WHIM, depending on its assumed abundance (O/H ≈ 10% solar). We also review the relationship between the various types of Lyα absorbers and galaxies. At the highest column densities, N_HI ≥ 10^20.3 cm⁻², the damped Lyα (DLA) systems are often identified with gas-rich disks of galaxies over a large range in luminosities (0.03–1 L*) and morphologies. Lyman-limit systems (N_HI ≥ 10^17.3−20.3 cm⁻²) appear to be associated with bound bright (≥ 0.1–0.3 L*) galaxy halos. The Lyα absorbers with N_HI = 10¹³⁻¹⁷ cm⁻² are associated with filaments of largescale structure in the galaxy distribution, although some may arise in unbound winds from dwarf galaxies. Our discovery that ∼20% of low-z Lyα absorbers reside in galaxy voids suggests that a substantial fraction of baryons may be entirely unrelated to galaxies. In the future, HST can play a crucial role in a precise accounting of the local baryons and the distribution of heavy elements in the IGM. […]