Malcolm Longair

High-redshift star formation in the Hubble Deep Field revealed by a submillimetre-wavelength survey

The advent of sensitive sub-mm array cameras now allows a proper census of dust-enshrouded massive star-formation in very distant galaxies, previously hidden activity to which even the faintest optical images are insensitive. We present the deepest sub-mm survey of the sky to date, taken with the SCUBA camera on the James Clerk Maxwell Telescope and centred on the Hubble Deep Field. The high source density found in this image implies that the survey is confusion-limited below a flux density of 2 mJy. However, within the central 80 arcsec radius independent analyses yield 5 reproducible sources with S(850um) > 2 mJy which simulations indicate can be ascribed to individual galaxies. We give positions and flux densities for these, and furthermore show using multi-frequency photometric data that the brightest sources in our map lie at redshifts z~3. These results lead to integral source counts which are completely inconsistent with a no-evolution model, and imply that massive star-formation activity continues at redshifts > 2. The combined brightness of the 5 most secure sources in our map is sufficient to account for 30 - 50% of the previously unresolved sub-mm background, and we estimate statistically that the entire background is resolved at about the 0.3 mJy level. Finally we discuss possible optical identifications and redshift estimates for the brightest sources. One source appears to be associated with an extreme starburst galaxy at z~1, whilst the remaining four appear to lie in the redshift range 2 < z < 4. This implies a star-formation density over this redshift range that is at least five times higher than that inferred from the ultraviolet output of HDF galaxies.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.

The SCUBA 8-mJy survey — I. Submillimetre maps, sources and number counts

We present maps, source lists and derived number counts from the largest, unbiased, extragalactic submillimetre (submm) survey so far undertaken with the SCUBA camera on the James Clerk Maxwell Telescope (JCMT). Our maps are located in two regions of sky (ELAIS N2 and Lockman-Hole E) and cover 260arcmin2 , to a typical rms noise level of sigma 850 ~=2.5mJybeam-1 . We have reduced the data using both the standard JCMT surf procedures, and our own IDL -based pipeline which produces zero-footprint maps and noise images. The uncorrelated noise maps produced by the latter approach have enabled us to apply a maximum likelihood method to measure the statistical significance of each peak in our maps, leading to properly quantified errors on the flux density of all potential sources. We detect 19 sources with signal-to-noise ratios (S/N)>4, and 38 with S/N>3.5. To assess both the completeness of this survey and the impact of source confusion as a function of flux density, we have applied our source-extraction algorithm to a series of simulated images. The result is a new estimate of the submm source counts over the flux-density range S 850 ~=5-15mJy, which we compare with estimates derived by other workers, and with the predictions of a number of models. Our best estimate of the cumulative source count at S 850 >8mJy is per square degree. Assuming that the majority of sources lie at z >1.5, this result implies that the comoving number density of high-redshift galaxies forming stars at a rate in excess of 1000Msolar yr-1 is ~=10-5 Mpc-3 , with only a weak dependence on the precise redshift distribution. This number density corresponds to the number density of massive ellipticals with L >3-4L * in the present-day Universe , and is also the same as the comoving number density of comparably massive, passively evolving objects in the redshift band 1<z <2 inferred from recent surveys of extremely red objects. Thus the bright submm sources uncovered by this survey can plausibly account for the formation of all present-day massive ellipticals. Improved redshift constraints, and ultimately an improved measure of submm source clustering can refine or refute this picture.

Dust‐obscured star formation and AGN fuelling in hierarchical models of galaxy evolution

A large fraction of the luminous distant submillimetre-wave galaxies recently detected using the Submillimetre Common-User Bolometer Array (SCUBA) camera on the James Clerk Maxwell Telescope appear to be associated with interacting optical counterparts. We investigate the nature of these systems using a simple hierarchical clustering model of galaxy evolution, in which the large luminosity of the SCUBA galaxies is assumed to be generated at the epoch of galaxy mergers in a burst of either star formation activity or the fuelling of an active galactic nucleus (AGN). The models are well constrained by the observed spectrum of the far-infrared/submillimetre-wave background radiation and the 60-µm counts of low-redshift IRAS galaxies. The ratio between the total amount of energy released during mergers and the mass of dark matter involved must increase sharply with redshift z at z < 1, and then decrease at greater redshifts. This result is independent of the fraction of the luminosity of mergers that is produced by starbursts and AGN. One additional parameter – the reciprocal of the product of the duration of the enhanced luminosity produced by the merger and the fraction of mergers that induce an enhanced luminosity, which we call the activity parameter – is introduced, to allow the relationship between merging dark matter haloes and the observed counts of distant dusty galaxies to be investigated. The observed counts can only be reproduced if the activity parameter is greater by a factor of about 5 and 100 at redshifts of 1 and 3 respectively, as compared with the present epoch. Hence, if merging galaxies account for the population of SCUBA galaxies, then the merger process must have been much more violent at high redshifts. We discuss the counts of galaxies and the intensity of background radiation in the optical/nearinfrared wavebands in the context of these hierarchical models, and thus investigate the relationship between the populations of submillimetre-selected and Lyman-break galaxies.

HST, radio and infrared observations of 28 3CR radio galaxies at redshift z ∼ 1 — I. The observations

Hubble Space Telescope images are presented of a sample of 28 3CR radio galaxies with redshifts in the range 0.6 < z < 1.8, together with maps at comparable angular resolution of their radio structure, taken using the Very Large Array. Infrared images of the fields, taken with the United Kingdom InfraRed Telescope, are also presented. The optical images display a spectacular range of structures. Many of the galaxies show highly elongated optical emission aligned along the directions of the radio axes, but this is not a universal effect; a small number of sources are either symmetrical or misaligned. Amongst those sources which do show an alignment effect, the morphology of the optical emission varies greatly, from a single bright elongated emission region to strings of optical knots stretching from one radio hotspot to the other. The infrared images display much less complexity. Although their significantly lower angular resolution would wash out some of the smaller structures seen in the HST images, it is clear that these galaxies are less aligned at infrared wavelengths than in the optical. In this paper, we discuss the galaxies individually, but defer a statistical analysis of the multi-waveband properties of the complete sample of sources to later papers in this series.

Starburst galaxies and structure in the submillimetre background towards the Hubble Deep Field

We use an 850-μm SCUBA map of the Hubble Deep Field (HDF) to study the dust properties of optically-selected starburst galaxies at high redshift. The optical/infrared (IR) data in the HDF allow a photometric redshift to be estimated for each galaxy, together with an estimate of the visible star-formation rate. The 850-μm flux density of each source provides the complementary information: the amount of hidden, dust-enshrouded star formation activity. Although the 850-μm map does not allow detection of the majority of individual sources, we show that the galaxies with the highest UV star-formation rates are detected statistically, with a flux density of about S 850 =0.2 mJy for an apparent UV star-formation rate of 1 h −2 M ⊙ yr −1 . This level of submillimetre output indicates that the total star-forming activity is on average a factor of approximately 6 times larger than the rate inferred from the UV output of these galaxies. The general population of optical starbursts is then predicted to contribute at least 25 per cent of the 850-μm background. We carry out a power-spectrum analysis of the map, which yields some evidence for angular clustering of the background source population, but at a level lower than that seen in Lyman-break galaxies. Together with other lines of argument, particularly from the NICMOS HDF data, this suggests that the 850-μm background originates over an extremely wide range of redshifts — perhaps 1≲ z ≲6.

Submillimetre observations of the Hubble Deep Field and Flanking Fields

We present an extended analysis of the Submillimetre Common User Bolometer Array (SCUBA) observations of the Hubble Deep Field (HDF), expanding the areal coverage of the Hughes et al. study by a factor of ~1.8 and containing at least three further sources in addition to the five in that study. We also announce the public release of the reduced data products. The map is the deepest ever made in the submillimetre (submm), obtained in excellent conditions (median 850-mum optical depth of 0.16). Two independent reductions were made, one with SURF and the other with a wholly algorithmic IDL analysis which we present in detail here. Of the three new sources, all appear to be at z>~ 0.9 and one is provisionally associated with an extremely red object (I-K > 5). There appears to be no significant cross-correlation signal between the 850-mum fluctuations and sources detected by ISOCAM, the Very Large Array (VLA) or Chandra, nor with very red objects (I-K > 4), nor quasars and quasar candidates in the HDF (notwithstanding a small number of individual weak candidate detections). This is consistent with interpretations where the 850-mum selected galaxies are at higher redshifts than those currently probed by ISOCAM/VLA, and predominantly not Compton-thin active galactic nuclei (AGN). There are only one or two compelling cases for the radio source being the submm source. Nevertheless, most SCUBA-HDF point sources have a nearby radio source apparently well-separated from the submm centroid.

An asymmetric relativistic model for classical double radio sources

There is substantial observational evidence against the symmetric relativistic model of FR II radio sources. An asymmetric relativistic model is proposed which takes account of both relativistic effects and intrinsic/environmental asymmetries to explain the structural asymmetries of their radio lobes. A key parameter of the model is the jet-side of the double sources, which is estimated for 80 per cent of the FR II sources in the 3CRR complete sample. Statistical analyses of the properties of these sources show that the asymmetric model is in agreement with a wide range of observational data, and that the relativistic and intrinsic asymmetry effects are of comparable importance. Intrinsic/environmental asymmetry effects are more important at high radio luminosities and small physical scales. The mean translational speed of the lobes is found to be consistent with the speeds found from spectral ageing arguments. According to a Gaussian model, the standard deviation of the distribution of vlobe is σvl=0.04c. The results are in agreement with an orientation-based unification scheme in which the critical angle separating the radio galaxies from quasars is about 45°.

Deep spectroscopy of z∼ 1 6C radio galaxies — I. The effects of radio power and size on the properties of the emission-line gas

The results of deep long-slit optical spectroscopy for a sample of eight 6C radio galaxies at redshift z∼ 1 are presented. Emission-line ratios are derived for many emission lines with rest-frame wavelengths of 1500–4500 A and the kinematic properties of the emission-line gas are derived from an analysis of the two-dimensional structure of the [O ii] 3727-A emission line at ≈5 A spectral resolution. In general, the 6C spectra display many characteristics similar to those of more powerful 3CR sources at the same redshifts. The emission-line region gas kinematics are more extreme for the smaller radio sources in the sample, which often display distorted velocity profiles. The ionization state of the emission-line region also varies with radio size: the spectra of large radio sources (>120 kpc) are consistent with photoionization by an obscured active galactic nucleus (AGN), whilst smaller (<120 kpc) sources typically exist in a lower ionization state and have spectra that are better explained by additional ionization due to shocks associated with the expanding radio source. The kinematic and ionization properties of the 6C radio galaxies are clearly linked. As for the 3CR sources, smaller radio sources also typically possess more extensive emission-line regions, with enhanced emission-line luminosities. A high-velocity emission-line gas component is observed in 6C 1019+39, similar to that seen in 3C 265. It is clear that the best interpretation of the spectra of radio sources requires a combination of ionization mechanisms. A simple model is developed, combining AGN photoionization with photoionization from the luminous shock associated with the expanding radio source. The relative contributions of ionizing photons from shocks and the central AGN to an emission-line gas cloud vary with radio source size and the position of the cloud. This model provides a good explanation for both the ionization properties of the emission-line regions and the radio size evolution of the emission-line region extents and luminosities.

Deep spectroscopy of distant 3CR radio galaxies: the data

ABSTRA C T Deep long-slit spectroscopic data are presented for a sample of 14 3CR radio galaxies at redshift z , 1, previously studied in detail using the Hubble Space Telescope, the Very Large Array, and the UK Infrared Telescope (UKIRT). Analysis of the [O ii] 3727 emission-line structures at ,5 A spectral resolution is carried out to derive the kinematic properties of the emission-line gas. In line with previous lower resolution studies, a wide variety of kinematics are seen, from gas consistent with a mean rotational motion through to complex structures with velocity dispersions exceeding 1000 km s 21 . The data confirm the presence of a high-velocity gas component in 3C 265 and detached emission-line systems in 3C 356 and 3C 441, and show for the first time that the emission-line gas in the central regions of 3C 324 is composed of two kinematically distinct components. Emission-line fluxes and the colour of the continuum emission are determined down to unprecedentedly low observed wavelengths, l , 3500 A, sufficiently short that any contribution of an evolved stellar population is negligible. An accompanying paper investigates the variation in the emissionline ratios and velocity structures within the sample, and draws conclusions as to the origin of the ionization and kinematics of these galaxies.

A jet-cloud interaction in 3C 34 at redshift z=0.69

We report the detection of a strong jet-cloud interaction at a distance of 120 kpc from the nucleus of the radio galaxy 3C34, which has redshift z=0.69. Hubble Space Telescope images of the radio galaxy show a long narrow region of blue emission orientated along the radio axis and directed towards a radio hotspot. The William Herschel Telescope has been used to provide long-slit spectroscopic data of this object, and infrared observations made with the United Kingdom InfraRed Telescope have enabled its spectral energy distribution to be modelled. We propose that the aligned emission is associated with a region of massive star-formation, induced by the passage of the radio jet through a galaxy within the cluster surrounding 3C34. A star-formation rate of about 100 solar masses per year is required, similar to the values necessary to produce the alignment effect in high-redshift radio galaxies. The consequences of this result for models of star formation in distant radio galaxies are discussed.