Pippa Goldschmidt

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.

Observations of the Hubble Deep Field with the Infrared Space Observatory V. Spectral energy distributions starburst models and star formation history

We have modelled the spectral energy distributions of the 13 HDF galaxies reliably detected by ISO. For 2 galaxies the emission detected by ISO is consistent with being starlight or the infrared ’cirrus’ in the galaxies. For the remaining 11 galaxies there is a clear mid-infrared excess, which we interpret as emission from dust associated with a strong starburst. 10 of these galaxies are spirals or interacting pairs, while the remaining one is an elliptical with a prominent nucleus and broad emission lines. We give a new discussion of how the star formation rate can be deduced from the far infrared luminosity and derive star formation rates for these galaxies of 8-1000 φM⊙ per yr, where φ takes account of the uncertainty in the initial mass function. The HDF galaxies detected by ISO are clearly forming stars at a prodigious rate compared with nearby normal galaxies. We discuss the implications of our detections for the history of star and heavy element formation in the universe. Although uncertainties in the calibration, reliability of source detection, associations, and starburst models remain, it is clear that dust plays an important role in star formation out to redshift 1 at least.

A Comparison of the Optical Properties of Radio-loud and Radio-quiet Quasars

We have made radio observations of 87 optically selected quasars at 5 GHz with the VLA in order to measure the radio power for these objects and hence determine how the fraction of radio-loud quasars varies with redshift and optical luminosity. The sample has been selected from the recently completed Edinburgh Quasar Survey and covers a redshift range of 0.3≤z≤1.5 and an optical absolute magnitude range of -26.5≤MB≤-23.5 (h=1/2, q0=1/2). We have also matched other existing surveys with the Faint Images of the Radio Sky at Twenty Centimeters and NRAO VLA Sky Survey radio catalogs and combined these data so that the optical luminosity-redshift plane is now far better sampled than before. We have fitted a model to the probability of a quasar being radio-loud as a function of absolute magnitude and redshift, and from this model we infer the radio-loud and radio-quiet optical luminosity functions. The radio-loud optical luminosity function is featureless and flatter than the radio-quiet one. It evolves at a marginally slower rate if quasars evolve by density evolution, but the difference in the rate of evolutions of the two different classes is much less than was previously thought. We show, using Monte Carlo simulations, that the observed difference in the shape of the optical luminosity functions can be partly accounted for by Doppler boosting of the optical continuum of the radio-loud quasars, and we explain how this can be tested in the future.

Observations of the Hubble Deep Field with the Infrared Space Observatory - IV. Association of sources with Hubble Deep Field galaxies

We discuss the identification of sources detected by ISO at 6.7 and 15μm in the Hubble Deep Field (HDF) region. We conservatively associate ISO sources with objects in existing optical and near-infrared HDF catalogues using the likelihood ratio method, confirming these results (and, in one case, clarifying them) with independent visual searches. We find fifteen ISO sources to be reliably associated with bright [I814(AB) < 23] galaxies in the HDF, and one with an I814(AB) = 19.9 star, while a further eleven are associated with objects in the Hubble Flanking Fields (ten galaxies and one star). Amongst optically bright HDF galaxies, ISO tends to detect luminous, star-forming galaxies at fairly high redshift and with disturbed morphologies, in preference to nearby ellipticals.

The UVX quasar optical luminosity function and its evolution

The recently finished Edinburgh UVX quasar survey at B<18 is used together with other complete samples to estimate the shape and evolution of the optical luminosity function in the redshift range 0.3<z<2.2. There is a significantly higher space density of quasars at high luminosity and low redshift than previously found in the PG sample of Schmidt & Green, with the result that the shape of the luminosity function at low redshifts (z<1) is seen to be consistent with a single power law. At higher redshifts the slope of the power law at high luminosities appears to steepen significantly. There does not appear to be any consistent break feature which could be used as a tracer of luminosity evolution in the population.

Observations of the Hubble Deep Field with the Infrared Space Observatory - I. Data reduction, maps and sky coverage

We present deep imaging at 6.7 micron and 15 micron from the CAM instrument on the Infrared Space Observatory (ISO), centred on the Hubble Deep Field (HDF). These are the deepest integrations published to date at these wavelengths in any region of sky. We discuss the observation strategy and the data reduction. The observed source density appears to approach the CAM confusion limit at 15 micron, and fluctuations in the 6.7 micron sky background may be identifiable with similar spatial fluctuations in the HDF galaxy counts. ISO appears to be detecting comparable field galaxy populations to the HDF, and our data yields strong evidence that future IR missions (such as SIRTF, FIRST and WIRE) as well as SCUBA and millimetre arrays will easily detect field galaxies out to comparably high redshifts.

The dusty environment of quasars: far-infrared properties of optical quasars

We present Infrared Space Observatory (ISO) far-IR photometry of a complete subsample of optically selected bright quasars belonging to two complete surveys selected through multicolor (U, B, V, R, I) techniques. The ISOPHOT camera on board the ISO satellite was used to target these quasars at wavelengths of 7.3, 11.5, 60, 100, and 160 μm. Almost two-thirds of the objects were detected at least in one ISOPHOT band. The detection rate is independent of the source redshift, very likely because of the negative K-correction of the far-IR thermal emission. More than a half of the optically selected QSOs show significant emission between 4 and 100 μm in the quasar rest frame. These fluxes have a very likely thermal origin, although in a few objects an additional contribution from a nonthermal component is plausible in the long-wavelength bands. In a color-color diagram these objects span a wide range of properties, from AGN-dominated to ULIRG-like. The far-IR composite spectrum of the quasar population presents a broad far-IR bump between 10 and 30 μm and a sharp drop at λ > 100 μm in the quasar rest frame. The amount of energy emitted in the far-IR is on average a few times larger than that emitted in the blue, and the ratio LFIR/LB increases with the bolometric luminosity. Objects with fainter blue magnitudes have larger ratios between the far-IR (λ > 60 μm) fluxes and the blue-band flux, which is attributed to extinction by dust around the central source. No relation between the blue absolute magnitude and the dust color temperature is seen, suggesting that the dominant source of FIR energy could be linked to a concurrent starburst rather than to gravitational energy produced by the central engine.