4.3. The future: the ISZO
Survey work requires the coverage of 100 deg2 or more of sky to a sensitivity of 30 µK or better, and for greatest effectiveness this should be possible in 1 year or less. Bolometer and radiometer arrays, or interferometers, are all potentially capable of making such surveys.
However, it will be important to defeat the confusion limit on such surveys imposed by primordial structure on MBR, and to separate the thermal and kinematic SZ effects. This requires a multi-wavelength survey, and one where the different wavelength bands are well matched in angular resolution and astrometric accuracy. The ideal facility for this type of work would probably be a bolometer array, sited on a single large telescope, with the signals separated by dichroics, and sitting on an excellent site. Experience of such facilities will come from BOLOCAM (Glenn 1998) and APEX (Schwan 2003).
To study the structures of clusters, it will be necessary to achieve sensitive sub-arcmin scale imaging over fields several arcmin in size. While this is a natural ability of interferometers, it might also be possible to achieve the necessary imaging quality using a bolometer array. Again, multi-wavelength operation will be necessary to separate the different SZ effects and to remove the contamination from radio sources and star-forming galaxies, so that well-matched bolometer arrays or scaled interferometer configurations will be needed. An aim should be access to the velocity channel with a sensitivity ~ 100 km s-1, which matches the noise imposed by confusion with primordial structures, and some polarization capability, to allow a first investigation of the polarization channel.
Many of these capabilities exist in the current generation of instruments, but it is notable that these instruments are designed at least roughly to match the sensitivity of the current generation of X-ray telescopes. The next generation of X-ray satellites (Con-X and XEUS) will have far higher sensitivity and better spectral capabilities, and so will provide far superior images of clusters of galaxies and of the thermal substructures within them. Now is therefore the time to begin considering an SZ effect telescope that could match these instruments in making detailed studies of cluster substructure and evolution, by achieving at least an order of magnitude improvement in sensitivity over currently-planned systems such as AMI.
One possible design would see the construction of scaled interferometers capable of operation at 30, 90, and 230 GHz (to operate in good atmospheric windows). The scaled design is adopted to permit similar areas of sky to be synthesized at each band: the use of a single telescope design capable of covering all three bands would lead to only small areas of the 230-GHz sky being accessible in a single synthesis. Baselines would be from about (300 - 104), providing ~ 20 arcsec resolution and sensitivity to angular scales as large as 10 arcmin. Individual antennas would be ~ 300 in size, to allow close packing at the smallest antenna-antenna separations. About 20% bandwidth at each band should be possible, but the bands would need to be subdivided to avoid excessive bandwidth smearing. Full polarization capability would allow the polarization channel to be explored (and the sensitivity of the array would allow some other polarization studies of large-scale structures). With ~ 20 antennas and non-uniform antenna separations, good instantaneous sensitivity could be obtained over the full range of angular scales, and with modern receivers the required sensitivity should be achievable.
An alternative design might be to use bolometer arrays at 110, 230, and 345 GHz on a telescope ~ 50 m in size (diffraction limited at the longest wavelength). About 1000 elements would be needed in each array, which is certainly feasible. This design could have high instantaneous sensitivity, making rapid surveys possible.
Either system would be a powerful follow-up to Planck measurements of the SZ effects of some thousands of clusters that we expect, and could provide the deep surveys that would reach to z > 1 in the SZ effects. However, both designs involve large-scale projects, and either would require an international team. This concept of an International Sunyaev-Zel'dovich effect Observatory (ISZO) requires considerable work to reach the level of a costed proposal, but it is clear that such an observatory will be a requirement within 10 years to take SZ effect studies to the next level of sophistication and accuracy.