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.

In the past decade, our understanding of galaxy evolution has been revolutionized by the discovery that luminous, dusty starburst galaxies were 1,000 times more abundant in the early Universe than at present. It has, however, been difficult to measure the complete redshift distribution of these objects, especially at the highest redshifts (z > 4). Here we report a redshift survey at a wavelength of three millimetres, targeting carbon monoxide line emission from the star-forming molecular gas in the direction of extraordinarily bright millimetre-wave-selected sources. High-resolution imaging demonstrates that these sources are strongly gravitationally lensed by foreground galaxies. We detect spectral lines in 23 out of 26 sources and multiple lines in 12 of those 23 sources, from which we obtain robust, unambiguous redshifts. At least 10 of the sources are found to lie at z > 4, indicating that the fraction of dusty starburst galaxies at high redshifts is greater than previously thought. Models of lens geometries in the sample indicate that the background objects are ultra-luminous infrared galaxies, powered by extreme bursts of star formation.

We have determined the relativistic light deflection of the quasar J0842+1835 as Jupiter passed within 3<7 on 2002 September 8, by measuring the time delay using the Very Long Baseline Array (VLBA) and Effelsberg radio telescopes at 8.4 GHz. At closest approach, general relativity (GR) predicts a radial (static) deflection of 1190 las and a tangential (retarded) deflection in the direction of Jupiter's motion of 51 las. Our experiment achieved an rms position error of 10 las and measured this retarded deflection to be 0:98 AE 0:19 (rms error) times that predicted by GR. The increased positional accuracy for this VLBI phase-referencing experiment was achieved by using two calibrator sources. Comments on the interpretation of this experiment are given.

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The motion and variability of the radio components in the low mass X-ray binary system Sco X-1 have been monitored with extensive VLBI imaging at 1.7 and 5.0 GHz over four years, including a 56-hour continuous VLBI observation in 1999 June. We detect one strong and one weak compact radio component, moving in opposite directions from the radio core. Their relative motion and flux densities are consistent with relativistic effects, from which we derive an average component speed of v/c=0.45±0.03 at an angle of 44 • ±6 • to the line of sight. This inclination of the binary orbit suggests a mass of the secondary star that is < 0.9 M ⊙ , assuming a neutron star mass of 1.4M ⊙ . We suggest that the two moving radio components consist of ultra-relativistic plasma that is produced at a working surface where the energy in dual-opposing beams disrupt. The radio lobe advance velocity is constant over many hours, but differs among lobe-pairs: 0.32c, 0.46c, 0.48c, and 0.57c. A lobe-pair lifetime is less than two days, with a new pair formed near the core within a day. The lobe flux has flux density that is variable over a time-scale of one hour, a measured minimum size of 1 mas (4 × 10 8 km), and is extended perpendicular to its motion. This time-scale and size are consistent with an electron radiative lifetime of < 1 hr. Such a short lifetime can be caused by synchrotron losses if the lobe magnetic field is 300 G or by adiabatic expansion of the electrons as soon as they are produced at the working surface. The lobes also show periods of slow expansion and a steepening radio spectrum. Two of the core flares are correlated with the lobe flares under the assumption that the flares are produced by a coherent energy burst traveling down the beams with a speed > 0.95c.The radio morphology for Sco X-1 differs from most other Galactic jet sources. Possible reasons for the morphology difference are: Sco X-1 is associated with a neutron star, it is a persistent X-ray source, the source is viewed significantly away from the angle of motion. However, the lobes in Sco X-1 are similar to the hot-spots found in many extragalactic radio double sources. Scaling the phenomena observed in Sco X-1 to extragalactic sources implies radio source hot-spot variability time-scales of 10 4 yr and hot-spot lifetimes of 10 5 yr.