Astron. Astrophys. 329, 399-408 (1998)
1. Introduction
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
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