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From observations of the microwave background, we know that the Universe recombined at a redshift of about a 1000. However, we also know the intergalatic medium (IGM) to be highly ionized below a redshift of six. If it were neutral, then scattering by neutral hydrogen would absorb all the light below the Lyman-alpha transition in the spectra of quasars. Since we observe flux, the IGM must be very highly ionized, to a level of 1:10000 or so. Hence:the universe has been reionized.

At redshifts around 3, we know that quasars produce about the right number of photons to keep the IGM ionized. But what were the sources that reionized the IGM, and when did it happen? How can we detect the transition?

Suggestive evidence that HI reionization has been detected comes from the spectrum of SDSS QSOs (shown in the figure), and for HeII from the Hamburg/ESO bright QSO survey.

What do we expect reionization to look like? N Gnedin was one of the first to attempt to simulate the full problem of computing how ionizing photons emanate from star-forming galaxies, and ionize their surroundings.

But given all the uncertainties, may be semi-analytical models are good enough to give us an idea of what to look for, for example in the CMB, or in the Lyman-alpha forest?

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Radiative transfer, coupled with hydrodynamics, is a bit of a daunting task, given that we don't even know which sources produced the ionizing photons, or when. Presumably what is needed are simulation codes which are both fast and reliable. Fast, so that one can do many runs to make predictions of how one would distinguish between all these models. This RTN programme has in fact several groups that are working on this problem.

If the sources of ionizing photons are hot stars in galaxies, what fraction of the ionizing photons escapes? May be we can use observations to constrain this?

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Network collaborations on this topic

Does the structure of the IGM matter? For example, what is the importance of small haloes as photon sinks? Would that not be very different for a (luke) warm dark matter model? And depend on the amplitude of the power spectrum?

Neutral hydrogen can emit or absorb radiation in the 21-cm line. Once the IGM is reionized, this line disappears. Detecting this transition is one of the key science drivers behind LOFAR.

Can we confidently predict how this transition will look like? Do different models lead to different signatures? Can our models be used to guide the specifications required of LOFAR?

Network collaborations on this topic

At the moment (August 28, 2002), the most distant known collapsed object is a galaxy, at z=6.5. This object has a Lyman-alpha emission line.

Does this mean that the reionization redshift of HI is higher than 6.5? Should we not expect to see the damping-wing of the neutral IGM eating-away the emission line?

This object was found using gravitational lensing to amplify the signal. New instruments, such as Dazle, are being developed to look for high-z Lyman-alpha emitters. Suppose we find many of those ...would this constrain possible reionization models?

Figure caption.

Network collaborations on this topic

Reionization of Helium (HeII to He III) could be due to another type of sources, such as QSOs. These emit HeII ionizing photons, which are only produced by very hot stars.

Observations of the HeII forest of redshift 3 QSOs with HST show of strongly increasing and fluctuating optical depth - just what one would expect the transition from neutral to ionized to look like. Modeling of the spectrum suggests the need for local sources of softer photons.

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The temperature evolution of the IGM can also be used to constrain the reionization history.

The evolution of the hydrogen optical depth suggests that HeII reionization took place between redshifts 3.1 and 3.4. Can we use radiative transfer simulations to compute the expected temperature change?

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The evolution of the temperature before that suggests HI reionization took place after redshift 9.

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Network collaborations on this topic

The Jeans mass depends on temperature. Since reionization increases T, does it affect galaxy formation? Or is the influence of feedback dominant?

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Network collaboration on this topic

         Last Modified 28/08/2002

This page was written by Tom Theuns, comments to