====================================================================
==> In this proposal, more time is being requested for GSERRA.SZCLUST
====================================================================
The spectral distorsion of the microwave cosmic background radiation by inverse
Compton scattering in the hot intracluster electron gas (the so-called Sunyaev
Zel'dovitch effect, SZ thereafter) is potentially a powerfull tool of observa-
tional cosmology. Measurements  of this distorsion in the submillimeter to
centimeter range, used in conjunction with X ray data, allows the determination
of important parameters such as the total cluster gas mass, its peculiar
velocity and the Hubble constant. Reliable measurements of the negative part of
the effect become now available at millimeter and centimeter wavelengths.
However, the knowledge of the positive part at submillimeter wavelengths is also
required in order to derive the physical parameters. Such measurements are now
within the reach of the PRONAOS/SPM balloon borne submillimeter instrument. The
flight of the instrument sept. 22/23 1996 has revealed an excellent sensitivity.
Data have been accumulated on four clusters : A 478, A 2142, A 2163 and A 2218.
They need a refined analysis for a possible detection of the positive SZ effect.
In the submilliteter range, emission from the galaxies in the cluster and fore-
ground dust in our Galaxy are expected to be comparable or larger than the
cosmological signal, and will polute the SZ measurement. Individual galaxies
contributing significantly must therefore be identified, and their submillimeter
flux be measured with enough accuracy to allow a precise subtraction. Due to its
wavelength coverage ISO will offer a unique opportunity to make these
measurements toward clusters for which detection of the positive part is
foreseen in the near future. We therefore propose to observe several clusters
with PHOT (90 and 180 um) and CAM (15 um).
The clusters selected for this program are all massive clusters, for which a
good measurement of the negative SZ effect is available. The proposed
observations will form a unique and homonegeous data set on the best SZ clusters
for the next coming years.