Thermal dust emission from quasars - I. Submillimetre spectral indices of radio-quiet quasars

Abstract

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 0157 + 001 (Mrk 1014) and Mrk 376 have been made at 800 and 450 μm using the 3He bolometer UKT14 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, 〈αsm〉 = 3.75 ± 0.48, significantly exceed the critical theoretical limit of αsm = 2.5 predicted for the self-absorption of synchrotron emission. This result is independent of any contributions to the 100-μm 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 ( >1010 M⊙) in RQQs inferred from the submillimetre continuum observations are in agreement with the H2 masses determined from CO measurements. Alternatively, the results show that the |${M}_{\text H_2}/{M}_\text {dust}$| ratio measured in RQQs |$( \sim 370 \pm 150)$| is consistent with that measured in spiral galaxies and ultraluminous IRAS galaxies.