Intergalactic Baryons in the Local Universe

TL;DR

This paper reviews observational and theoretical evidence for the distribution of baryons in the local universe, emphasizing the role of the warm-hot intergalactic medium (WHIM) as a significant baryon reservoir.

Contribution

It argues that most OVI absorbers at low redshift are shock-heated gas in the WHIM, providing a more accurate estimate of baryons in this phase.

Findings

Warm, photoionized phase accounts for ~30% of baryons at z~0

Enriched gas at T=10^5-10^6 K can account for ~10% of baryons at z<0.5

Most OVI absorbers are likely shock-heated gas, not photoionized

Abstract

Simulations predict that shocks from large-scale structure formation and galactic winds have reduced the fraction of baryons in the warm, photoionized phase (the Lya forest) from nearly 100% in the early universe to less than 50% today. Some of the remaining baryons are predicted to lie in the warm-hot ionized medium (WHIM) phase at T=10^5-10^7 K, but the quantity remains a highly tunable parameter of the models. Modern UV spectrographs have provided unprecedented access to both the Lya forest and potential WHIM tracers at z~0, and several independent groups have constructed large catalogs of far-UV IGM absorbers along ~30 AGN sight lines. There is general agreement between the surveys that the warm, photoionized phase makes up ~30% of the baryon budget at z~0. Another ~10% can be accounted for in collapsed structures (stars, galaxies, etc.). However, interpretation of the ~100 high-ion (OVI, etc) absorbers at z<0.5 is more controversial. These species are readily created in the shocks expected to exist in the IGM, but they can also be created by photoionization and thus not represent WHIM material. Given several pieces of observational evidence and theoretical expectations, I argue that most of the observed OVI absorbers represent shocked gas at T~300,000 K rather than photoionized gas at T<30,000 K, and they are consequently valid tracers of the WHIM phase. Under this assumption, enriched gas at T=10^5-10^6 K can account for ~10% of the baryon budget at z<0.5, but this value may increase when bias and incompleteness are taken into account and help close the gap on the 50% of the baryons still "missing".