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National Aeronautics and Space Administration

Goddard Space Flight Center

Astrophysics Science Division | Sciences and Exploration

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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.

Schematic: Free-free emission along line of sight
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.
Diffuse vs halo emission
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).
CMB Data vs expected signals
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.