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Fig. 1:
Upper left panel: possible multi-armed spiral configuration for station layout.
Upper right panel: blow up of one "spot" of the spiral configuration shows a possible
fractal station layout. Lower panel: blow up of one "plus" of the fractal layout
shows antenna layout.
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Fig. 2:
The panels show maps of 21cm line emission (in terms of the so called
differential antenna temperature and in units of log K) at an observed
frequency of (from top to bottom and left to right) 98, 103, 110, 116, 123, 131,
139, 147 and 157 MHz.
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In the current cosmological framework, the diffuse gas (IGM),
initially in a highly ionization state, is expected to recombine, i.e. neutral atoms
are formed, ~450 thousand
years after the Big Bang (the estimated age of the Universe is ~13 billion years)
and remain neutral until the first sources of ionizing radiation form and reionize it.
Observations of distant quasars (e.g. Fan et al. 2004) provide information on the final
stages of the reionization process, while experiments on the cosmic microwave background
(CMB) radiation (e.g. Kogut et al. 2003; Spergel et al. 2003) give an estimate of the
abundance of electrons produced by it. But observations that map the temporal evolution
of reionization are not yet available.
It has long been known (e.g. Field 1959) that neutral hydrogen in the IGM may be directly
detectable at frequencies that fall in the radio band (in the range 70-170 MHz) and
measurements at different frequencies should allow us to probe accurately the
structure and the evolution of the reionizing gas. This experiment is particularly
attractive as a new generation of radio telescope (e.g.
 LOFAR,
PAST,
SKA) is under
construction. LOFAR, which is being built in the Netherlands, will use almost
40000 antennas grouped into roughly 100 "stations", distributed over an area
of about 400 kilometers across. The possible configuration for station layout
is shown in Fig. 1.
Researchers at the Max-Planck-Institute for Astrophysics have used
previously run simulations of the reionization process (see  May 2003 Research
Highlight; Ciardi, Stoehr & White 2003; Ciardi, Ferrara
& White 2003) to derive the 21cm line emission expected from
neutral IGM at radio frequencies (Ciardi & Madau 2003). In Fig. 2 maps
of the emission at different observed frequencies are shown. As
expected, at longer frequencies, which correspond to later times when
the IGM is more ionized, the emission is lower. Inhomogeneities in
the gas density and in its ionization state induce fluctuations in the
21cm line emission, with a maximum expected value of order of 10
mK. The next generation of low-frequency radio telescopes should be
sensitive enough to measure such fluctuations and to probe the
structure of the reionization process directly.
Nevertheless, observations of 21cm line emission from neutral gas in the IGM remains
a very challenging project due to contamination from sources which emit at radio
frequencies, such as our own Galaxy (e.g. Shaver et al. 1999), radio galaxies
(e.g. Di Matteo et al. 2002) or clusters of galaxies. Scientists at the
Max-Planck-Institute for Astrophysics have estimated the contribution to such
contamination from all possible sources of extra-galactic origin (Di Matteo,
Ciardi & Miniati 2004). They find that the emission from the extra-galactic
sources is stronger that the primary 21cm line emission, unless bright sources
are removed from the observed maps. In this case, on angular scales larger than
1 arcmin, the primary signal will be observable and free from contamination.
In conclusion, the next generation of low-frequency radio telescopes will be able
to map, for the first time, the temporal evolution of the reionization process
and to shed light on the nature of the sources that caused it.
B. Ciardi
Literature:
Ciardi, Ferrara & White 2003
Ciardi & Madau 2003
Ciardi, Stoehr & White 2003
Di Matteo, Ciardi & Miniati 2004
Di Matteo et al. 2002
Fan et al. 2004
Kogut et al. 2003
Shaver et al. 1999
Spergel et al. 2003
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