High-redshift radio galaxies and quasars at submillimetre wavelengths: assessing their evolutionary status

Abstract

We present new results of a study of the sub-millimetre continuum emission from a sample of 9 radio galaxies and 4 radio-quiet quasars at redshifts $z = 0.75 - 4.26$. The observations were made at 800$\mu m$, using the single-element bolometer UKT14 on the James Clerk Maxwell Telescope (JCMT), reaching a typical r.m.s. sensitivity of $\sigma_{rms} \sim 4$ mJy and represent some of the deepest submillimetre extragalactic measurements made to date. Three detections were achieved, of which two are secure (4C41.17, Dunlop et al. (1994) and H1413$+$117) and one (53W002) is tentative, whilst comparable upper-limits were obtained for 7 of the 10 remaining sources. We use these data as the motivation for a detailed discussion of the conversion from submillimetre and millimetre continuum fluxes to dust/gas masses and star formation rates at high-redshift, and determine these quantities from our own and other data on high-redshift radio-galaxies and quasars. In particular we have investigated the impact of the four main sources of uncertainty in deriving physical quantities from such data, namely i) potential contamination by galactic cirrus, ii) uncertainty in the value of the dust rest-frequency mass-absorption coefficient, iii) difficulty in constraining the dust temperature, and iv) estimation of the appropriate gas:dust ratio in high-redshift objects. Our discussion emphasises how important it will be to quantify and, where possible, minimize such uncertainties (via, for example, appropriate observational strategies) in order to fully capitalize on the ten-fold improvement in sensitivity offered by the imminent arrival of the next generation of bolometer arrays, such as SCUBA on the JCMT.