In recent years several attempts have been made to study the interstellar medium in high-redshift galaxies. Promising results have been obtained by millimetre and submillimetre observations, which sample the rest-frame far-IR emission of galaxies at (Andreani et al. 1993). The study of dust in distant galaxies is very important because it provides information on the physical state of the ISM, and its relation to other properties of the galaxies, such as activity and evolution. For example, if it is correct to assume that the rest-frame far-IR luminosity is a measure of the star-formation rate, then this could provide a way to estimate the evolutionary state of high-z galaxies. Establishing whether dusty galaxies are common at high redshifts is also important in order to evaluate the effects of dust obscuration on surveys of quasars and protogalaxies carried out in optical bands (Smail, Ivison & Blain 1997).
Several active galaxies with have been detected in submillimetre/millimetre bands, suggesting the presence of large amounts of dust in their host galaxies (M M ; see Hughes, Dunlop & Rawlings 1997 for a recent review; Omont et al. 1996b; Ivison et al. 1998). At high redshift, dust is also thought to be present in damped Ly absorption systems (Pettini et al. 1994; Pei, Fall & Bechtold 1991), in very red galaxies (Hu & Ridgway 1994), and in high-z radio galaxies (Cimatti 1996 and references therein). A substantial amount of dust is also expected in theoretical models of the evolution of galaxies at high-z (Mazzei & De Zotti 1996 and references therein). Finally, it is important to recall that molecular gas has been observed in a few distant active galaxies, allowing a direct estimate of the dust-to-gas mass ratio at large cosmological distances for the first time (see Omont et al. 1996a; Ohta et al. 1996).
We recall that particular caution is needed in the interpretation of the submillimetre/millimetre data of radio-loud objects. The existence of a thermal dust emission excess should be confirmed by checking whether the submillimetre/millimetre flux densities represent a real excess over the synchrotron spectrum. The radio galaxy B2 0902+343 () is an example of the problem, where the observed 1.3-mm flux density is not due to thermal emission from dust, but to the tail of the radio synchrotron spectrum (Downes et al. 1996 and references therein). Therefore, high-frequency ( GHz) radio observations are extremely important to derive the shape of the synchrotron spectrum and to estimate its contribution to the millimetre region.
In the present paper we present the results of millimetric observations of a small sample of radio galaxies and radio-loud quasars with , and additional high-frequency VLA observations of MG 1019+0535, a radio galaxy at . Throughout this paper we assume km s-1 Mpc-1, and define .
© European Southern Observatory (ESO) 1998
Online publication: December 8, 1997