THERMAL DUST EMISSION FROM QUASARS .1. SUBMILLIMETER SPECTRAL INDEXES OF RADIO-QUIET QUASARS

Submillimetre and millimetre continuum observations of 10 IRAS-selected, radio-quiet quasars (RQQs) are presented. The first detections of three RQQs, PG 0050 + 124 (IZw 1), PG 01 57 + 001 (Mrk 1014) and Mrk 376 have been made at 800 and 450 mum using the He-3 bolometer UKT 14 on the 15-m James Clerk Maxwell Telescope. These submillimetre data, together with existing 1.3-mm observations, demonstrate that the measured submillimetre spectral indices, (alpha(sm)) = 3.75 +/- 0.48, significantly exceed the critical theoretical limit of alpha(sm) = 2.5 predicted for the self-absorption of synchrotron emission. This result is independent of any contributions to the 100-mum IRAS fluxes from cirrus emission in the host galaxies, extended circumnuclear star formation and FIR emission from companion or confusing sources. All current non-thermal models are rejected in favour of the alternative explanation that the FIR luminosity is dominated by thermal emission from warm (45-60 K) dust grains. The submillimetre optical depth and source-size for the thermal emission cannot yet be constrained by these data and, as a result, no discrimination can be made between dust heated by an extended (> 1 kpc) starburst region or a central compact luminosity source. The high gas masses (> 10(10) M.) in RQQs inferred from the submillimetre continuum observations are in agreement with the H-2 masses determined from CO measurements. Alternatively, the results show that the M(H-2)/M(dust) ratio measured in RQQs (approximately 3 70 +/- 150) is consistent with that measured in spiral galaxies and ultraluminous IRAS galaxies.