Dust , Gas and the Evolutionary Status of the Radio Galaxy , 8 C

We present the results of new rest-frame far-IR observations of the z = 4.25 radio galaxy, 8C 1435+635, which not only confirm that it contains an enormous quantity of dust (as first inferred from its mm-wave detection by Ivison 1995) but also allow the first meaningful constraints to be placed on the mass of this dust and associated gas. The new measurements consist of: (i) clear detections of sub-mm continuum emission at λobs = 450 and 850μm obtained with the new sub-mm bolometer array, SCUBA, on the James Clerk Maxwell Telescope (JCMT), (ii) continuum upper limits at λobs = 350, 750 and 175μm obtained with SCUBA and the PHT far-IR camera aboard the Infrared Space Observatory (ISO), and (iii) a sensitive upper limit on the CO(4−3) line flux obtained with the IRAM 30-m MRT. The resulting restframe 33—238-μm continuum coverage allows us to deduce that 2× 10 M of dust at a temperature of 40±5 k is responsible for the observed mm/sub-mm emission. Using our CO upper limit, which constrains MH2/Md to < 950, we go on to calculate robust limits on the total gas reserves (H2 + H i) which are thereby constrained to between 4× 10 and 1.2× 10 M . The sub-mm properties of 8C 1435+635 are thus strikingly similar to those of the z = 3.80 radio galaxy, 4C 41.17, the only other high-redshift galaxy detected to date at sub-mm wavelengths whose properties appear not to be exaggerated by gravitational lensing (Dunlop et al. 1994; Hughes, Dunlop & Rawlings 1997). The inferred gas masses of both objects are sufficiently large to suggest that the formative starbursts of massive elliptical galaxies are still in progress at z ' 4. Observations of complete samples of radio galaxies spanning a range of redshifts and radio luminosities will be required to determine if the spectacular far-IR properties of 8C 1435+635 and 4C 41.17 are primarily due to their extreme redshifts or their extreme radio luminosities. Subject headings: cosmology: observations – cosmology: early universe – galaxies: evolution – galaxies: formation – galaxies: ISM – galaxies: individual: 8C 1435+635