The sensitivity of present ground-based detectors is quite close to
photon noise limits, typically an effective value
y(1
) ~ 10-4
hour-1/2
for a single bolometer. This is also typical for interferometers. All 3 kinds
of observing techniques are also currently limited by the range of
angular scales
that can be measured, whereas the angular distribution of clusters of galaxies
is widespread, from core radii to Abell radii, along with substructure scales.
We can see that the main goals of SZ observations in the next years are:
For point 1 and 2, interferometers are clearly very promising. Dedicated radio telescope arrays which are currently being built with modest size telescopes aim at covering a large range of angular scales (in particular the shortest baselines). By covering a large frequency bandwidth and by using smaller telescopes (!), improvements in detectivity could be better by as much as a factor of 1000 in the next 3 years (e.g. the AMI project: Jones, this conference). For point 3, bolometer arrays (with hundred to thousands of pixels) should bring a clear multiplex advantage over existing technology [16]. They will give unprecedented high angular resolution maps in the near future (20 arcseconds), that will be useful to study the detailed angular distribution of clusters (whether at low or high redshift) to unravel the cosmogony of large scale structures. In that respect, the structure of high redsihft clusters (e.g. Ebeling et al. [18]) which is far from smooth is interesting for cluster formation scenarios but may prove a show stopper for q0 and other second order effect measurements.
Future space missions will provide a different perspective. Planck will give an unbiased catalog of at least several thousands of SZ sources. MAP, although not sensitive to individual clusters, may still see some signal by correlation with large scale structures as seen in the optical (Refregier et al. [17]).
The interpretation of SZ data is depending on the quality of other data, and vice-versa. The arrival of Chandra and XMM-Newton is a strong incentive for improving SZ measurements. Comparison with visible and near-infrared data obtained by large telescopes (substructures and weak lensing) is also very valuable.
We have clearly moved from detection experiments towards a powerful tool for the study of clusters. A global approach, using SZ observations but also other wavelengths, is a must for the understanding of clusters of galaxies.
Acknowledgments
We wish to thank the organisers for such a pleasant and lively forum of discussions of which the SZ effect was one of the foci. We thank A. Refregier for allowing us to show the large scale SZ simulated map.