We have performed a sensitive (σ ~ 1.5mJy) systematic study of the 1.25 mm emission of ~22 radio-quiet QSOs at z ≳ 4, with the IRAM 30m telescope equipped with bolometer arrays. Five radio-quiet QSOs at z > 4 have been detected at a 5-σ level in addition to the initial detection of the z = 4.7 QSO BR1202-0725 reported in McMahon et al. (1994). The detected fluxes range from 2.5 to 10 mJy. All the reported detections were independently confirmed at the 3-σ level on at least three different nights. In addition 10 other QSOs from the Cambridge APM survey sample and 6 others QSOs from the literature were searched for millimetre emission but not detected with 2-σ upper limits of 3-4 mJy.
From this systematic study of about half of the known optically selected z > 4 QSOs, some general trends of their millimetre emission can be inferred. All the QSOs we have detected pertain to the APM sample and are among those which have the largest UV rest-frame luminosities. The detection rate within the APM sample is 6 out of 16 observed, compared with zero in the remaining 6. Two of the four APM broad absorption line QSOs observed were detected and four of the seven weak lined APM QSOs were detected, whereas none of the five strong lined APM QSOs were detected. Thus there is evidence for enhanced millimetre emission from luminous QSOs with weak broad emission lines or broad absorption lines.
There is one clear case known of strong lensing amongst the six millimetre detected objects with z > 4. In light of the fact that both previously known objects with confirmed strong millimetre emission at z>2 are gravitationally lensed, i.e. H1413+ 117 and IRAS F10214+4724, sensitive high resolution observations of these z > 4 QSOs are required to determine whether gravitational lensing effects need to be taken into account.
Assuming that the millimetre wave continuum emission is due to dust emission, the very large amount of dust implied, ~ 10^8 h^(-2) M_⊙ , means that the host galaxies of these QSOs have undergone a substantial phase of star formation. If the gas-to-dust ratio in these galaxies is similar to that in lower redshift objects, the total gas mass would be ~ 10^(11) M_⊙.
We have begun to explore the 1.25 mm emission of bright radio-quiet QSOs in the redshift range 1.5 to 3.5, using criteria which seem to favor millimetre detections, established from our z > 4 detections. One source was detected at z = 2.70. We have also observed three QSOs with z > 3 that were previously studied at 1.25mm by Andreani et al. (1993) who reported detections at a level higher than 3σ. We have been unable to confirm any of these reported detections. In particular we have a 3σ upper limit of 3.2 mJy for the z = 3.19 QSO PC2132+0126 for which Andreani et al. reported a flux of 11.5 ± 1. 7 mJy. Either this source has substantially varied during the period between the two sets of observations or the single channel bolometer observations were affected by systematic errors
