Future probes (Reionisation)
From Eric's Wiki
For z>6 all is not lost when it comes to probes of the ionisation state. Star formation in galaxies produces a great deal of Lyα emission. While most of it is absorbed localy, particularly at shorter wavelengths than the source (Ly-break galaxies), velocity structure within the galaxy (or more precisely, in the HII bubble surrounding the source) can lead to the escape of some of the strong Lyα photons. There is a complication, while resonant scattering absorbs most of the photons near the line, if the absorbing column depth is high enough, significant absorbtion can occur far from the line centre in the damping wings. This can absorb much of the Lyα emission that would normally redshift itself to safety.
21cm emission & absorption before and after reionisation is observable (Sethi05). At ν=10 to 40 MHz, absorption (of the CMB?) can be observed. At 50-100 MHz, a strong and narrow absorption feature may be visible. At 60-100 MHz HI will be seen in emission. Every thing is contingent on the epoch of reionisation and the spectral shape.
Detectors: Mileura Widefield Array (MWA), LOw Frequency ARray (LOFAR), and PrimevAl Structure Telescope (PAST).
Methods: Global frequency step, direct imaging, HI forest, Stromgren sphere mapping, and statistical observations.
MBH05 give a description of the method for analyzing 21cm data. WH05 look specifically at the 21cm power spectum and how to tie it to models.
Surveys: Palomar-Quest (PQ) will provide high-z QSO targets (DBM05).
SKA will be able to map neutral gas via 21-cm emission and see a 21-cm forest in the spectra of the first objects (Carilli04).
Carilli05 is a good review of the use of the HI 21cm line as a probe of reionisation.
Peculiar motions of free electrons during reionisation lead to a Doppler anisotropy in the CMB. Patchy neutral gas can be mapped by its 21cm emission. The correlation (or anti-correlation) is strongest when the neutral fraction is about 50% (AKDS05).
MWA will not be able to constrain constrain cosmology, though it's potential successor MWA5000 will be able to constrain the slope and running spectral index of the power spectrum describing the primordial perturbations (BMH05).
The 21cm data will provide data over the sky at different frequencies corresponding to different redshifts. Hence, the correlations in tangential space (angular separation) and radial space (frequency separation) will be measurable. This will lead to a Multifrequency Angular Power Spectrum (MAPS). DCB06 examine the various effects that constrain the parameters of the future surveys such as the effects of bulk motion and the ability to separate forground sources, which will have high frequency separation correlation.
The intensity of the 21cm line is set by the spin temperature, Ts. Not considering reionisation, Ts is coupled to TCMB but not equivalent. A strong UV flux can decouple Ts. CAS06 show that the decoupling of Ts from TCMB can be used as a probe of the shape of the hard UV spectrum.
The spin temperature, Ts, is coupled to the gas temperature, Tg, at high redshift (z > 100), then drops slower than TCMB (which is always higher than Tg) until z∼30 at which point it approximates TCMB. Timescales become long enough for collisions to regain importance, dropping Ts to Tg by z∼12 (SC06, Fig 1.).
The 21cm angular power spectrum is set by the distribution of neutral hydrogen, the gas temperature, gas velocity, and topology of the ionisation field.
The limiting factor for 21cm observations is foreground subtraction (SC06).
There are a number of degeneracies among cosmological and astrophysical parameters in 21cm observations. Some of these degeneracies are breakable by combination with other data (eg. Planck) (SC06).
Though the Lyα forest is generated by neutral gas, there is a significant ionised fraction in the absorbing structures. This ionised gas inverse-Compton scatters CMB photons which is detectable by its S-Z effect (CBH05).
Prior to reionisation, the 21cm signal from collapsed minihalos dominates over emission from the diffuse shocked gas (Shapior05).
--Etittley 11:51, 25 June 2007 (BST)
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