The Epoch of Reionization
After recombination, early in the history of the universe,
baryonic matter was in the form of neutral hydrogen.
Imprinted on the hydrogen distribution were the primordial density
fluctuations that were detected and mapped by COBE and the subsequent
cosmic microwave background (CMB) experiments.
The surface of last scattering delineated by the CMB at a redshift
of about 1000 is the most distant structure in the universe that we
observe today. The next most distant structures we see
are galaxies and quasars at redshifts of 5 to 6.
Sometime between a redshift of 1000 and a redshift of 6, the neutral
hydrogen collapsed to form structures, the first stars and/or quasars
were formed, the galaxies assembled, and the intergalactic medium was
reionized. This important time
in the history of our universe is commonly referred
to as ``the epoch of reionization.''
Optical spectra of the the most distant quasars show Lyman-alpha
absorption troughs that suggest that universe was largely neutral
above a redshift of 6. However, the recent polarization results
from the WMAP mission suggest that reionization occurred at a much
higher redshift (about 20). Reconciling these different measurements
may involve a fairly complicated reionization history.
It should be possible to observe some of the processes at work during
the epoch of reionization.
observable signatures are provided by the neutral hydrogen through
observations of the 1.4 GHz line, redshifted to frequencies of tens to
hundreds of MHz.
The first of these is a possible
"reionization step" in the spectrum corresponding
to the fairly abrupt transition to full reionization that occured at
the end of the epoch of reionization. After the formation of the
first stars and/or quasars, the process of reionization probably
slowly as the Stromgren spheres began to grow. However, a sudden transition
to complete reionization might occur when the Stromgren spheres overlap and
the mean free path of photons increase by around two orders of magnitude.
provide a striking demonstration of this transition.
Another observable signature is the "reheating" of the neutral hydrogen
that occurs when the primordial density enhancements collapse under
the influence of gravity, the gas becomes hot, and it becomes visible
by emitting in the redshifted neutral hydrogen line. The figure below on
taken from Tozzi et al. (2000, ApJ, 528, 597), shows a simulation of
how these structures would appear to a radio telescope tuned to receive
signals at a frequency of 150 MHz, corresponding to a redshift of 8.5.
The figure below on the right shows the reheated structures at z=8 but
now in the form of a power spectrum. The superimposed light
grey area are the measurements errors we expect with the
Mileura Widefield Array
(see below); the dark grey area are those errors combined
with the cosmic variance.
Both these figures assume no reionization has taken place at z=8, which
is probably unlikely. However, it is useful to consider the un-reionized
case as a reference.
As the neutral hydrogen continues to collapse, yet another observable
signature occurs when the first stars and/or quasars form. Radiation
from these bright objects ionizes the nearby neutral hydrogen, causing
regions of ionized gas that appear as dark patches where the 21cm line
is no longer radiating. A simulation of this process is shown below
(Furlanetto, Sokasian, and Hernquist, 2004, MNRAS, 347, 187)
in the nine pictures that show the same slice of the universe at
redshifts 12.1, 11.1, 10.4, 9.8, 9.2, 8.7, 8.3, 7.9, and 7.6.
At early times (upper left corner), the hydrogen is still neutral, and
glows at radio wavelengths. At later times (lower right corner) the
hydrogen is almost completely ionized and appears dark.
In between there are interesting structures as the ionization fronts propagate
through the hydrogen.
The neutral hydrogen features shown above should be detectable with a large
radio telescope operating at low frequencies. The Kavli Institute's
radio astronomy group is part of a collaboration that is building a
low-frequency array at Mileura Station in Western Australia.
Last updated 7 August 2005