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".Comment: Invited review to appear in "Future Directions in Ultraviolet
Spectroscopy", Oct 20-22, 2008, Annapolis, MD, M. E. Van Steenberg, ed.
(April 2009). 8 pages, five figure
