A ground-based observing campaign was devoted to the search for the Sunyaev-
Zeldovich (SZ) effect towards X-ray ROSAT Clusters in the millimetric spectral
domain. A double-channel photometer, explicitly built to search for this effect
simultaneously at two wavelengths around the peak of the Cosmic Background
Radiation spectrum (1.2 and 2 mm), was adapted at the focus of the SEST antenna
and observed a few objects selected from the ROSAT Survey.
Both the positive (1.2 mm) and the negative signal (2 mm) were detected towards
the Cluster RXJ0658-5557. A signal at 1.2mm was also found for A2744. For the
FIRST time there is a strong evidence for this important cosmological effect
because both the enhancement and the decrement were detected on the SAME source.
In order to assess the reliability of our findings further investigations of
the nature of the detected signals mainly in the FIR and radio domains are
mandatory. From optical pictures of the clusters it appears that the position
of the X-ray emission, which was pointed with our instrument, does not
correspond to any optical source. Are there other kinds of sources ?
ISO offers the 'unique' opportunity to check this question because the only way
to disentangle a source signal from that due to the SZ itself is represented
by observations in the FIR domain where the SZ signal is negligible. In fact at
all other wavelengths of interest (i.e. those available from the ground, 850um,
750um and at lambda>3mm) the SZ signal is either stronger or negative.
A complementary aim of this work is to address issues to the question of the
presence of cold dust in the intracluster medium. At present this point is not
settled from observations and also theoretically it is not yet clear how dust
can survive in extreme hot environments such as those of cluster centers.
We therefore propose to map with ISO-PHOT three clusters A2744, RXJ0658-5557
and RXJ1347-1145, which are very good candidates from our mm observations and
very strong ROSAT and ASCA sources.
Note that even a null result will be extremely important because it will
confirm the FIRST detection of the 'positive' signal of the SZ effect and will
have important consequences on the presence of dust in intracluster media.