5.1. Ly
Forest
The Ly forest evolves away
dramatically from high to
low redshift, as is strikingly clear from the spectra of
z ~ 3 and z ~ 1 quasars in Figure 9.
The evolution of the Ly lines with
Wr(Ly) > 0.3
Å can be characterized by a
double power law with 
~ 2 for
1.8 < z < 4.5 and ~ 0.2
for z <1.8.
Help in understanding the physical picture has come from
sophisticated N-body/hydrodynamic simulations that
incorporate the gas physics and consider cosmological
expansion of the simulation box.
The dynamical evolution of the HI gas can
be described as outflow from the centers of
voids to their surrounding shells, and flows along
these sheets toward their intersections where
the densest structures form. This picture is consistent with observational
determinations of the ``sizes'' of Ly
structures. It is difficult to obtain direct measurements of
sizes except in some special cases to use
``double lines of sight'', close quasar pairs, either
physical or apparent due to gravitational lensing.
If the spectra of the two quasars both have a
Ly
absorption line at the same wavelength that implies a
``structure'' which covers both lines of sight.
From these studies, it is found that
``structures'' are at least hundreds of kpc in extent.
|
Figure 9. Illustration of structure
evolution of intergalactic gas from high to low redshift.
The upper spectrum of a z = 3.6 quasar is a Keck/HIRES observation,
while the lower spectrum is a FOS/HST observations of a z = 1.3 quasar.
Higher redshift quasars show a much thicker forest of
Ly |
At redshifts z = 5 to z = 2 dN/dz for
Ly forest
absorption is quite large, but it is declining
very rapidly over that range. This dramatic evolution in the
number of forest clouds is mostly due to the
expansion of the universe, with a modest contribution
from structure growth.
At z < 2, the extragalactic background radiation field
is falling, and Ly structures
are becoming more neutral.
Therefore, the more numerous, smaller N(H) structures are
observed at a larger N(HI) and this will
counteract the effect of expansion, thus slowing
the decline of the forest.
The high redshift Ly forest was
once thought to be primordial material, but in fact it is observed
to have a metallicity of 0.1% solar, even at z = 3.
For N(HI) < 1014 cm-2, the expected
N(C IV) would
be below the detection thresholds of current observations,
so truly pristine material still eludes us.
Perhaps it does not exist.
To spread metals all through the intergalactic medium
may have required a ``pre-galactic'' population of stars
at z > 10 that polluted all of intergalactic space.