, R.G.M. McMahon
, P. Cox
, E. Kreysa
,
J. Bergeron
Institut d'Astrophysique de Paris, C.N.R.S.
Institute of Astronomy, Cambridge
Observatoire de Marseille
Max-Planck-Institut für Radioastronomie, Bonn
ESO, Garching 
in
the ultraluminous infrared galaxy IRAS F10214+4724 and the
gravitationally lensed QSO H1413+117 (the Cloverleaf) have proved that
millimeter (and submillimeter) radioastronomy will become a major
cosmological tool to study primeval galaxies. Therefore, two years ago
we proposed a systematic search of 1.2mm continuum millimeter emission
in radio quiet QSOs with z>4. We then argued that their far IR
luminosity could be comparable to their huge UV luminosity, and that,
despite their large distance, they should be detectable since the
steep spectrum of dust emission in the millimeter range means that the
observed flux increases with red shift for constant luminosity.
Such a hypothesis was apparently confirmed by the first 1.2mm
detection in February 1993 of BR1202-07, the brightest object of the
sample and the third highest red shift known (
) (Mc Mahon et
al. 1994). Its subsequent detection at JCMT at submillimeter
wavelengths confirmed a very large spectral index (
3.5)
characteristic of dust (Isaak et al. 1994). However, during the last
two years, the continuation of our programme had been repeatedly
plagued by bad weather, until its decisive completion during our last
run in early February 1995 with the remarkable MPIfR 7-channel
bolometer array which allows a very efficient subtraction of the
remaining sky background. The results, together with those of Chini
and Krügel (1994), prove that the era of systematic millimeter
studies at high z is already opened with the unique capacities
provided by the 30m with bolometer arrays.
Five radio quiet QSOs with z>4 have been detected at a 5--sigma level
in addition to the initial detection of BR1202-07 reported in McMahon
et al. 1994. Their fluxes range from 2.5 to 10 mJy. All the
detections were independently detected during at least three different
nights. In addition 
10 other sources of the Cambridge APM
survey sample were searched for but not detected with rms levels
1.5 mJy i.e. with fluxes probably smaller than 3 mJy. We have
also observed the 
QSO, PC2132+0126 and find a flux of
1.0
0.8 mJy, whereas Andreani et al. (1994) reported a flux of
11.5
1.7 mJy.
Accordingly, we have shown that a large fraction of the color
identified z>4 radio quiet QSOs have a strong millimeter emission
detectable with the 30 meter. Although the following point remains to
be verified with the JCMT, it is very likely that the detected sources
have a large spectral index characteristic of dust emission, similar
to what is observed for BR1202-07. The very large amount of dust
implied, 
10
, is comparable to what is found
in the most luminous infrared starburst galaxies. It is quite possible
that a giant starburst is taking place or has recently taken place in
the host galaxies of these QSOs at a scale 
the central
kiloparsec. However, it is possible that most of the heating of the
observed dust is provided by the UV emitted by the central engine of
the quasar.
It is crucial to complement our results by submillimeter observations at the JCMT and by sensitive far--infrared photometry with ISO in order to characterize the dust temperature and derive the far--infrared luminosity. In addition, it is important to extend our work at z>4 to lower red shifts i.e. 1 to 3, in order to determine the frequency among them of detectable millimeter emission and investigate the relationship between this emission in QSOs with the dust emission in ultraluminous IRAS galaxies and radio galaxies.