  Recent ultraviolet and X-ray observations of intergalactic OVI and OVII
absorption systems along lines of sight to bright quasars have opened a new
window onto the ``warm-hot intergalactic medium'' (WHIM). These systems appear
to provide a significant reservoir for baryons in the local universe, and
comparison to cosmological simulations suggests that their abundance roughly
matches theoretical predictions. Here we use analytic arguments to elucidate
the physical properties of the absorbers and their role in structure formation.
We first show that if the absorbers result from structure-formation shocks, the
observed column densities naturally follow from postshock-cooling models, if we
include fast-cooling shocks as well as those that cannot cool within a Hubble
time. In this case, the known OVI absorbers should show stronger OVII
absorption than expected from collisional-ionization equilibrium (and much more
than expected for photoionized systems). We then argue that higher-temperature
shocks will be spatially associated with more massive virialized objects even
well outside the virial radius. Thus the different oxygen ions will trace
different structures; OVII absorbers are the most common because that ion
dominates over a wide temperature range (corresponding to a large range in halo
mass). If each dark-matter halo is surrounded by a network of shocks with total
cross section a few times the size of the virialized systems, then we can
reproduce the observed number densities of absorbers with plausible parameters.
A simple comparison with simulations shows that these assumptions are
reasonable, although the actual distribution of shocked gas is too complex for
analytic models to describe fully. Our models suggest that these absorbers
cannot be explained as a single-temperature phase.
