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Reionization and Structure Formation
A critical period in the evolution of the universe
is the end of the so-called "cosmic dark ages",
when the first collapsed structures
achieved sufficient temperatures and density
to ignite in nuclear fusion.
This transition
is marked by the photo-ionization
of the intergalactic medium --
the epoch of reionization.
Reionization is a complicated and extended process.
Measurements of hydrogen absorption spectra
against distant quasars
show no absorption out to redshift z ~ 6,
followed by an abrupt transition
to a fully blocked line
(the Gunn-Peterson effect).
Cosmologically significant amounts of neutral hydrogen
must be present at z > 6.
By contrast,
the
WMAP satellite
measurement of the polarization of the cosmic microwave background
shows significant amounts of ionized gas
at much larger redshift, z ~ 17.
The two measurements are not incompatible
(a few percent abundance of neutral hydrogen
will produce near-total absorption,
while still leaving enough ionized gas to scatter the CMB photons
and produce the polarization signal),
but clearly reionization did not occur as
a single instantaneous transition
from a fully neutral to a fully ionized universe.
ARCADE will provide a measurement of reionization
and subsequent structure formation.
Ionized gas in the universe
can be detected through its free-free emission
(thermal bremsstrahlung).
The signal is proportional to the path integral
of the electron density squared along the line of sight.
In temperature units,
the amplitude grows quadratically with wavelength.
Precise measurements of the CMB spectrum
at centimeter wavelengths
can detect the excess emission
from the integrated free-free emission at high redshift.
The free-free signal from the early universe can be calculated.
Haiman & Loeb (1997)
show that the ionized gas out to z ~ 6
must
produce a signal greater than a few tenths of a mK at 3 GHz.
Free-free emission from ionized gas is weighted
by the square of the electron density,
preferentially sampling denser regions.
Clumping of the IGM in halos enhances the high-redshift signal
(Oh 1999, Gnedin & Ostriker 1997).
The combined visibility is dominated by halo formation near reionization,
with predicted amplitude 2 mK < dT < 5 mK
at 15 cm wavelength (3 GHz frequency).
The
plot above
shows the predicted signal from reionization and structure formation
(purple curve),
along with the observing band
and 1 mK sensitivity expected for ARCADE.
The predicted signal
from the formation of observed structure
lies well below the current upper limit,
set by ground-based measurements at centimeter wavelengths.
Detecting this signal requires mK precision
at wavelengths longer than 10 cm,
well within the capabilities of ARCADE.
ARCADE will measure free-free emission to precision
Y_ff < 10^{-6}, over a factor of 10 below current upper limits.
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