2.10. Cluster formation studies

The SZ effects related to the formation of clusters should be those of lowest brightness and smallest angular size, since they are generated as cluster potential wells begin to accumulate gas, and that gas is heated by the impacts of infalling groups. The angular scales that are relevant are of order 10 arcsec, and the amplitudes of the effects are ~ 20 µK. Such structures are only detectable against the background of MBR confusion because they have a rising power spectrum while primordial MBR fluctuations have a falling spectrum at such small scales (Molnar and Birkinshaw 2000), and because it should be possible to use spectral techniques to separate the thermal SZ effect from primordial fluctuations.

Efforts to look at the first clusters are generally better in the SZ effects than in the X-ray: the SZ effects begin to increase in flux density beyond z ~ 1.6 because of the redshift-independence of their surface brightnesses and the behaviour of D_A(z), while it is already difficult to detect high-mass X-ray clusters beyond z = 1.

The prospects of detecting the SZ effects of the first clusters are not currently good, since even allowing for the efficiency with which SZ effect surveys can find distant clusters, the amplitude of the thermal signal is low. Indeed, for sufficiently fast infall and sufficiently cool accreting groups, the principal detectable SZ effect may be kinematic rather than thermal, and then the separation from primordial fluctuations in the MBR can only be done statistically (for example using deviations from the expected primordial power spectrum).

Crucial requirements for attempting this work are, therefore, the ability to attempt accurate spectral separation (and hence the requirement for good absolute calibration; Sec. 2.1.1), and the development of a suitable telescope for this work (Sec. 4.3).