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  Current Research Highlight :: December 2004 all highlights

Observing reionization with a radio telescope

Researchers at the Max-Planck-Institute for Astrophysics have made predictions on observations of cosmic reionization with the next generation of radio telescopes.

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.

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.

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. linkPfeilExtern.gifLOFAR, linkPfeilExtern.gifPAST, linkPfeilExtern.gifSKA) 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 linkPfeil.gifMay 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:

linkPfeilExtern.gif Ciardi, Ferrara & White 2003

linkPfeilExtern.gif Ciardi & Madau 2003

linkPfeilExtern.gif Ciardi, Stoehr & White 2003

linkPfeilExtern.gif Di Matteo, Ciardi & Miniati 2004

linkPfeilExtern.gif Di Matteo et al. 2002

linkPfeilExtern.gif Fan et al. 2004

linkPfeilExtern.gif Kogut et al. 2003

linkPfeilExtern.gif Shaver et al. 1999

linkPfeilExtern.gif Spergel et al. 2003


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last modified: 2004-11-26