We used the Karl G. Jansky Very Large Array (VLA) to image one primary beam area at 3 GHz with 8 FWHM resolution and 1.0 µJy beam −1 rms noise near the pointing center. The P (D) distribution from the central 10 arcmin of this confusion-limited image constrains the count of discrete sources in the 1 < S(µJy) < 10 range. At this level the brightness-weighted differential count S 2 n(S) is converging rapidly, as predicted by evolutionary models in which the faintest radio sources are starforming galaxies; and ≈ 96% of the background originating in galaxies has been resolved into discrete sources. About 63% of the radio background is produced by AGNs, and the remaining 37% comes from star-forming galaxies that obey the far-infrared (FIR) / radio correlation and account for most of the FIR background at λ ≈ 160 µm. Our new data confirm that radio sources powered by AGNs and star formation evolve at about the same rate, a result consistent with AGN feedback and the rough The confusion at centimeter wavelengths is low enough that neither the planned SKA nor its pathfinder ASKAP EMU survey should be confusion limited, and the ultimate source detection limit imposed by "natural" confusion is ≤ 0.01 µJy at ν = 1.4 GHz. If discrete sources dominate the bright extragalactic background reported by ARCADE 2 at 3.3 GHz, they cannot be located in or near galaxies and most are ≤ 0.03 µJy at 1.4 GHz.

We use three years of data from the Swift/Burst Alert Telescope (BAT) survey to select a complete sample of X-ray blazars above 15 keV. This sample comprises 26 flat-spectrum radio quasars (FSRQs) and 12 BL Lacertae (BL Lac) objects detected over a redshift range of 0.03 < z < 4.0. We use this sample to determine, for the first time in the 15-55 keV band, the evolution of blazars. We find that, contrary to the Seyfert-like active galactic nuclei (AGNs) detected by BAT, the population of blazars shows strong positive evolution. This evolution is comparable to the evolution of luminous optical quasi-stellar objects (QSOs) and luminous X-rayselected AGNs. We also find evidence for an epoch dependence of the evolution as determined previously for radio-quiet AGNs. We interpret both these findings as a strong link between accretion and jet activity. In our sample, the FSRQs evolve strongly, while our best fit shows that BL Lac objects might not evolve at all. The blazar population accounts for 10%-20% (depending on the evolution of the BL Lac objects) of the cosmic Xray background (CXB) in the 15-55 keV band. We find that FSRQs can explain the entire CXB emission for energies above 500 keV solving the mystery of the generation of the MeV background. The evolution of luminous FSRQs shows a peak in redshift (z c = 4.3 ± 0.5) which is larger than the one observed in QSOs and X-rayselected AGNs. We argue that FSRQs can be used as tracers of massive elliptical galaxies in the early universe.

The future of cm and m-wave astronomy lies with the Square Kilometre Array (SKA), a telescope under development by a consortium of 17 countries. The SKA will be 50 times more sensitive than any existing radio facility. A majority of the key science for the SKA will be addressed through large-area imaging of the Universe at frequencies from 300 MHz to a few GHz. The Australian SKA Pathfinder (ASKAP) is aimed squarely in this frequency range, and achieves instantaneous wide-area imaging through the development and deployment of phase-array feed systems on parabolic reflectors. This large field-of-view makes ASKAP an unprecedented synoptic telescope poised J. Wall is the overall editor.

Abstract. We present the first results from the Complete Optical and Radio Absorption Line System (CORALS) survey. We have compiled a homogeneous sample of radio-selected QSOs from the Parkes Catalogue and searched for damped Lyman alpha systems (DLAs) towards every target, irrespective of its optical magnitude. This approach circumvents selection effects -particularly from intervening dust -which have long been suspected to affect DLA surveys in optically-selected, magnitude-limited QSO samples. The CORALS data set consists of 66 zem ≥ 2.2 QSOs in which 22 DLAs with absorption redshifts 1.8 ≤ z abs ≤ zem have been identified over a total redshift interval ∆z = 55.46. Three of the DLAs are classified as "associated" systems with z abs ∼ zem; of the 19 intervening DLAs, 17 are new discoveries. In this first paper of the CORALS series we describe the sample, present intermediate resolution spectroscopy and determine the population statistics of DLAs. We deduce a value of the neutral gas mass density traced by DLAs (expressed as a fraction of the closure density) log ΩDLAh = −2.59 +0.17 −0.24 , and a number density of DLAs per unit redshift n(z) = 0.31 +0.09 −0.08 , both at a mean redshift z = 2.37. Both values are only marginally higher than those measured in optically selected samples of QSOs. Taking into account the errors, we conclude that dust-induced bias in previous surveys may have led to an underestimate of these quantities by at most a factor of two. While n(z) is greater in fainter (B > 20) QSOs, the effect is only at the ∼1σ level and we have not uncovered a previously unrecognised population of high column density (N(H I) > 10 21 cm −2 ) DLAs in front of faint QSOs. These conclusions are tentative because of the limited size of our data set; in particular the distribution of column densities is poorly sampled at the high end where a much larger survey of radio-selected QSOs is required the improve the statistics.

We review the statistical properties of the main populations of radio sources, as emerging from radio and millimeter sky surveys. Recent determinations of local luminosity functions are presented and compared with earlier estimates still in widespread use. A number of unresolved issues are discussed. These include: the (possibly luminositydependent) decline of source space densities at high redshifts; the possible dichotomies between evolutionary properties of low-versus high-luminosity and of flat-versus steepspectrum AGN-powered radio sources; and the nature of sources accounting for the upturn of source counts at sub-mJy levels. It is shown that straightforward extrapolations of evolutionary models, accounting for both the far-IR counts and redshift distributions of star-forming galaxies, match the radio source counts at flux-density levels of tens of µJy remarkably well. We consider the statistical properties of rare but physically very interesting classes of sources, such as GHz Peak Spectrum and ADAF/ADIOS sources, and radio afterglows of γ-ray bursts. We also discuss the exploitation of largearea radio surveys to investigate large scale structure through studies of clustering and the Integrated Sachs-Wolfe effect. Finally we briefly describe the potential of the new and forthcoming generations of radio telescopes. A compendium of source counts at different frequencies is given in an appendix.

EMU is a wide-field radio continuum survey planned for the new Australian Square Kilometre Array Pathfinder (ASKAP) telescope. The primary goal of EMU is to make a deep (rms ,10 mJy/beam) radio continuum survey of the entire Southern sky at 1.3 GHz, extending as far North as þ308 declination, with a resolution of 10 arcsec. EMU is expected to detect and catalogue about 70 million galaxies, including typical star-forming galaxies up to z , 1, powerful starbursts to even greater redshifts, and active galactic nuclei to the edge of the visible Universe. It will undoubtedly discover new classes of object. This paper defines the science goals and parameters of the survey, and describes the development of techniques necessary to maximise the science return from EMU.

No abstract

We show that a dual‐population unification scheme provides a successful paradigm with which to describe the evolution and beaming of all bright extragalactic radio sources. The paradigm consists of two intrinsic radio‐source populations, based on the two distinct radio‐galaxy morphologies of Fanaroff‐‐Riley (FR) classes I and II. These represent the `unbeamed' or `side‐on' parent populations of steep radio spectra; the `beamed' source types, including flat‐spectrum quasars and BL Lac objects, arise through the random alignment of their radio axis to our line of sight where Doppler beaming of the relativistic radio jets produces highly anisotropic radio emission. We develop the model in two stages. In the first stage the source space density as a function of cosmic epoch is determined for the two parent populations, and for this we use low‐frequency source‐count and identification data to avoid biases due to Doppler‐enhanced radio emission. The second stage defines the beaming models for each population, using high‐frequency survey data and in particular the 5‐GHz source count in which at high flux densities the flat‐ and steep‐spectrum sources contribute in similar measures. We assume that the flat‐spectrum objects, quasars and BL Lac objects are `beamed' versions of FR I and FR II objects in which the close alignment of the radio axis with the line of sight has changed the radio appearance into a core‐dominated (flat‐spectrum) object. We adopt a simple parametrization of the beaming, orient the parent populations at random with a Monte Carlo process, and use a minimization process to determine beaming parameters that yield a best fit to the 5‐GHz source count. The best‐fitting parameters are found to be in good agreement with those measured observationally for individual radio sources. In this, the model accurately reproduces the change in source‐count form with frequency. Indeed the unified‐scheme paradigm has great predictive power, and we show how the model successfully describes several additional and independent data sets.