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It is clear that we shall learn a great deal from more extensive X-ray data on clusters at very high redshifts. Also, quasars surveys have now discovered enough objects to yield quantitative data on how quasars are clustered. There is tantalizing evidence that clustering exists, but does not depend steeply on redshift. The rate of evolution of clustering is a diagnostic for the cosmological density parameter, and also for the biasing factor. Clustering evolves more rapidly at later epochs, under the action of gravity, if Omega is high. On the other hand, if the objects being studied, galaxies or quasars, are strongly biased in their distribution relative to the mass, then the apparent clustering will develop less dramatically with redshift.

There is a possibility of using quasars to study incipient clustering right back to redshifts of order 5. I should like, however, briefly to mention a technique that offers the chance of probing largescale structure at even higher redshifts, perhaps even before the first galaxies and quasars 'switched on' and reheated the primordial plasma. This technique depends on studying the 21cm line expected from diffuse neutral hydrogen. In terms of brightness temperature, this line contributes much less than the 2.7° which we get from the microwave background. Its temperature is also much less than the non-thermal radio background due to synchrotron emission from extragalactic sources. It may nonetheless be possible to pick out the 21cm contribution, because of its characteristic angular structure, combined with fine structure in frequency space. See Figure 6 and its caption. The contribution to the radio background temperature at 1420 MHz due to uniformly distributed hydrogen at redshift z is easily calculated to be

Equation 3 (3)

The factor f is unity if the spin temperature is much higher than the radiation temperature. If there had been no heat input into the primordial gas before the relevant epoch, then the spin temperature would be lower than the radiation temperature and the intergalactic gas would show up in absorption. If a region lies along our line-of-sight which has a higher density than average, or has an expansion rate slower than the mean Hubble flow, the contribution from the 21cm line would be enhanced. For a linear fluctuation, the enhancement is five thirds the amplitude of the density perturbation, the extra two thirds coming from the reduced expansion rate in an overdense growing perturbation, which increases the hydrogen column density per unit redshift interval.

Figure 6

Figure 6. The dominant extragalactic backgrounds in the radio bands are the primordial 2.7 K black body radiation, and the non-thermal synchrotron background, whose brightness temperature goes as ~ v-2.7. Intergalactic HI emits and/or absorbs via the 21 cm transition, and in consequence changes the background temperature. Although this effect would be undetectably small if the HI were smoothly distributed, any 'clumping' of the gas would create spectral and angular structure in the background. By scanning in angle using a narrow bandwidth of frequencies, structures in the high-z neutral hydrogen could be detected. By comparing the angular structures seen in two 'maps' wade at slightly different frequencies, one could distinguish between effects due to discrete non-thermal sources (for which the two maps would correlate) and those due to HI (where the maps would not correlate), and thereby detect incipient large scale structure at redshifts z > 5.

If neutral hydrogen has largescale inhomogeneities, or its velocities are perturbed from the mean Hubble flow, there will therefore be spatial and spectral structure in the radio background we receive from it. Though small compared to the continuum radio background, this may be detectable by difference measurements, switching between nearby frequencies and directions. The expected fine structure in frequency space would allow the signal from non-uniform neutral hydrogen to be distinguished from patchiness in the non-thermal synchrotron background.

The most hopeful possibility for carrying out this kind of 'tomography' on protoclusters involves the proposed GMRT in India. This instrument is planned to comprise an array of 34 dishes, each 45m in diameter. The dishes will not be sufficiently well-surfaced to be effective at high frequencies. However, the array will be eight times more sensitive than the VLA at 327 MHz. It will also operate at lower frequencies in the range of 150-250 MHz, where the artificial background, particularly at its proposed location, is particularly low. This corresponds to redshifts between 6 and 8.5. The sensitivity is such that protoclusters would be detected, if they have the properties predicted by some theories. Furthermore, the precursors of specially large systems resembling the Great Attractor or the Shapley Concentration should reveal themselves in this way.

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