In the local Universe, most galaxies are dominated by stars, with less than ten per cent of their visible mass in the form of gas. Determining when most of these stars formed is one of the central issues of observational cosmology. Optical and ultraviolet observations of high-redshift galaxies (particularly those in the Hubble Deep Field) have been interpreted as indicating that the peak of star formation occurred between redshifts of 1 and 1.5. But it is known that star formation takes place in dense clouds, and is often hidden at optical wavelengths because of extinction by dust in the clouds. Here we report a deep submillimetre-wavelength survey of the Hubble Deep Field; these wavelengths trace directly the emission from dust that has been warmed by massive star-formation activity. The combined radiation of the five most significant detections accounts for 30-50 per cent of the previously unresolved background emission in this area. Four of these sources appear to be galaxies in the redshift range 2 < z < 4, which, assuming these objects have properties comparable to local dust-enshrouded starburst galaxies, implies a star-formation rate during that period about a factor of five higher than that inferred from the optical and ultraviolet observations. Recent years have seen the first meaningful attempts to determine the global star-formation history of the Universe, using the combined information provided by deep redshift surveys (for example, the Canada France Redshift Survey 1 ) reaching z Ϸ 1, and the statistics of Lyman-limit galaxies 2 at higher redshifts in, for example, the Hubble Deep Field (HDF) 3-5 . The results 6 imply that the starformation and metal-production rates were about 10 times greater at z Ϸ 1 than in the local Universe, that they peaked at a redshift in the range z Ϸ 1-1:5, and that they declined to values comparable to those observed at the present day at z Ϸ 4.These conclusions, derived from optical-ultraviolet data, may however be misleading, because the absorbing effects of dust within distant galaxies undergoing massive star-formation may have distorted our picture of the evolution of the high-redshift Universe in two ways. First, the star-formation rate (SFR) in known highredshift objects is inevitably underestimated unless some correction for dust obscuration is included in deriving the rest-frame ultraviolet luminosity. Second, it is possible that an entire population of heavily dust-enshrouded high-redshift objects, as expected in some models of elliptical galaxy formation 7 , have gone undetected in the optical-ultraviolet surveys. The extent of the former remains controversial 8-11 , while the possibility of the latter has until now been impossible to investigate. Submillimetre cosmologyAt high redshifts (z Ͼ 1), the strongly-peaked far-infrared radiation emitted by star-formation regions in distant galaxies is redshifted into the submillimetre waveband, and the steep spectral index of this emission on the long-wavelength side of the peak, at l Ϸ 100 m in the rest-frame, result...

No abstract

No abstract

No abstract

Hubble Space Telescope images of 3CR radio galaxies at redshifts 0.6 < z < 1.8 have shown a remarkable variety of structures, generally aligned along the radio axis, indicating that the radio source strongly influences the optical appearance of these galaxies. In this paper we investigate the host galaxies underlying this aligned emission, combining the HST data with ground‐based infrared images.  An investigation of the spectral energy distributions of the galaxies shows that the contribution of the aligned blue component to the K‐band light is generally small (∼ 10 per cent). The radial intensity profiles of the galaxies are well matched at radii ≲ 35 kpc by de Vaucouleurs’ law, demonstrating that the K‐band light is dominated by that of an elliptical galaxy. There is no evidence for a nuclear point source, in addition to the de Vaucouleurs profile, with a contribution ≳ 15 per cent of the total K‐band flux density, except in two cases, 3C 22 and 3C 41. We conclude that the K‐band emission of the distant 3CR galaxies is dominated by starlight. The magnitudes, colours and location of the distant 3CR galaxies on the projected fundamental plane indicate that their stellar populations formed at high redshift and have since been evolving passively.  Large characteristic radii are derived for the 3CR galaxies, indicating that they must be highly evolved dynamically, even at a redshift of 1. At radii larger than ∼ 35 kpc, a combined galaxy profile clearly shows an excess of emission as compared with de Vaucouleurs' law, indicating that at least some of the galaxies possess cD‐type haloes. This supports other independent evidence for the hypothesis that the distant 3CR galaxies lie in moderately rich (proto‐)clusters. Since the nearby FR II galaxies in the 3CR catalogue lie in more diffuse environments and do not possess cD haloes, the galactic environments of the 3CR galaxies must change with redshift. The K–z relation of the 3CR galaxies cannot, therefore, be interpreted using a standard ‘closed‐box, passively evolving stellar population’ model, whereby the galaxies that host distant 3CR sources will evolve into the galaxies that host nearby 3CR FR II sources.  At redshifts z ∼ 1, the absolute K magnitudes of the stellar populations of the 3CR galaxies are brighter than those of the lower radio power 6C galaxies, indicating that the 3CR galaxies contain a greater mass of stars; this is consistent with them lying towards the centres of clusters. Powerful high‐redshift radio galaxies possess radio beams the kinetic power of which is close to the Eddington limiting luminosity of a central supermassive black hole. Since the mass of the black hole is likely to scale in proportion to the mass of the host galaxy, the 3CR galaxies will contain more massive central engines than the 6C galaxies, which accounts for their more powerful radio emission. At redshifts z ≲ 0.6, the beam power of the radio sources is limited by the availability of fuel for the central engine rather than by the black hole mass, and so no correlation i...