In this review , we describe our surveys of low column density ( Ly \alpha ) absorbers ( N _ { HI } = 10 ^ { 12.5 - 16 } cm ^ { -2 } ) , which show that the warm photoionized IGM contains \sim 30 % of all baryons at z \leq 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 10 ^ { 5 - 7 } K gas , the warm-hot IGM ( WHIM ) .
The observed line density of Ly \alpha absorbers , d { \cal N } / dz \approx 170 for N _ { HI } \geq 10 ^ { 12.8 } cm ^ { -2 } , is dominated by low- N _ { HI } systems that exhibit slower redshift evolution than those with N _ { HI } \geq 10 ^ { 14 } cm ^ { -2 } .
HST/FUSE surveys of O VI absorbers , together with recent detections of O VII with Chandra and XMM/Newton , suggest that anywhere from 20–70 % ( with large errors ) of the baryons could reside in the WHIM , for an assumed abundance O/H \approx 10 % solar .
We also review the relationship between the various types of Ly \alpha absorbers and galaxies .
At the highest column densities , N _ { HI } \geq 10 ^ { 20.3 } cm ^ { -2 } , the damped Ly \alpha ( 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 ( \geq 0.1–0.3 L ^ { * } ) galaxy halos .
The Ly \alpha absorbers with N _ { HI } = 10 ^ { 13 - 17 } cm ^ { -2 } are associated with filaments of large-scale structure in the galaxy distribution , although some may arise in unbound winds from dwarf galaxies .
Our discovery that \sim 20 % of low- z Ly \alpha 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 .
These studies will be especially effective if NASA finds a way to install the Cosmic Origins Spectrograph ( COS ) on Hubble , allowing an order-of-magnitude improvement in throughput and a comparable increase in our ability to study the IGM .