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In approximately half the systems currently recognized as strongly gravitationally lensed, the background object is an extragalactic radio source. Radio observations have played an important role in the identification of lensed systems, and the properties of radio sources allow some of the astrophysical applications of lensing to be realized. High redshift galaxies can be studied through lens modeling and by observing more than one ray path through the lens. The morphological, spectral, and polarization information of high resolution radio images provides strong constraints on the mass distribution in the lensing galaxies. On cosmological scales, radio variability has been applied to the time delay measurement of angular diameter distance.

The Murchison Widefield Array (MWA) is one of three Square Kilometre Array Precursor telescopes and is located at the Murchison Radio-astronomy Observatory in the Murchison Shire of the mid-west of Western Australia, a location chosen for its extremely low levels of radio frequency interference. The MWA operates at low radio frequencies, 80–300 MHz, with a processed bandwidth of 30.72 MHz for both linear polarisations, and consists of 128 aperture arrays (known as tiles) distributed over a ~3-km diameter area. Novel hybrid hardware/software correlation and a real-time imaging and calibration systems comprise the MWA signal processing backend. In this paper, the as-built MWA is described both at a system and sub-system level, the expected performance of the array is presented, and the science goals of the instrument are summarised., National Science Foundation (U.S.) (Grant AST CAREER-0847753), National Science Foundation (U.S.) (Grant AST-0457585), National Science Foundation (U.S.) (Grant AST-0908884), National Science Foundation (U.S.) (Grant PHY-0835713), United States. Air Force Office of Scientific Research (Grant FA9550-0510247), Smithsonian Astrophysical Observatory, MIT School of Science

The gravitational lens 0957+561 has been monitored by our group for 16 years, using the VLA at 5 GHz. Since our last report in 1992, both images have returned to their quiescent flux levels, and the A image has brightened again. We apply various analysis methods to the light curves, and obtain a preliminary estimate of τ = 455 ± 40 days for the time delay.

We present multifrequency radio images of the Einstein ring gravitational lens system, MG 1131+0456, acquired with the Very Large Array telescope. Continuum data at 5, 8, 15, and 22 GHz are presented, ranging in resolution from 0.09″ to 0.39″. Linearly polarized emission, detected in the 5 and 8 GHz images, reveals complex structure that will provide strong constraints on lens models. We find evidence for flat spectrum components at the positions of the doubly imaged compact components, suggesting that the compact objects are radio cores and that the possibility of variability should not be excluded. At 8 GHz, a new unresolved component near the center of the ring is detected; it is likely to be the third and fifth images of parts of the background source. Varying degrees of Faraday rotation, with large rotation measure, are detected across the source

Gravitational lens surveys are of cosmological interest because they provide a way to measure the gravitational field of both luminous and dark matter. Many of the other methods used to detect the presence of dark matter, such as studies of galaxy rotation curves and cluster dynamics, require that there be luminous objects in the gravitational field that act as tracers of the mass. This may introduce a selection effect. In constrast, in studies of gravitational lenses, the beacon we observe can be far (at distances of order one thousand Mpc) from the gravitational field. In this paper we describe a VLA survey designed to detect gravitational lensing on sub-arc second and arc second scales. We also present a preliminary result of the radio data: we find that the density of matter in the form of a uniform, comoving number density of 1011 to 1012M⊙ compact objects, luminous or dark, must be substantially less than the critical density.

I wish to make a correction. In my article ( Science , April 17, 1891), instead of the word rati kowaněñ , on p. 219, second column, at the end of the first paragraph, read rati kowaněñ's . This error was perhaps due to an oversight of the copyist in transcribing with a typewriter from my script notes, and overlooked in revision.

On June 1, 1984 we conducted a seven station 18-cm VLBI observation of the 2016+112 gravitational lens system. Preliminary brightness distributions for A and B have been obtained via model fitting. Weak correlated flux density was detected in the C component region.

Fifty-two of the large angular diameter (a> or =2.'5) galaxies included in the 21-cm survey of isolated galaxies being undertaken at Arecibo by Haynes and Giovanelli have been mapped along their optical major axes using the 305-m telescope and the low sidelobe flat feed system. The observed fluxes have then been compared with a numerical convolution of an assumed H I distribution sigma/sub H/(r) and the beam pattern G(theta) to produce an estimate of the characteristic size. The derived H I scale length R/sub 70/ is that radius interior to which 70% of the model's H I mass is contained. With still a significant scatter, the H I diameter D/sub 70/ = 2R/sub 70/ is on the average about 1.2 times the UGC blue diameter a, and shows no dependence on morphological type for types later than Sa. Application of the observed relationship between H I size and optical diameter and of an assumed double Gaussian (centrally depressed) H I surface density distribution produces proper corrections for source to beam size. Corrected fluxes derived from Arecibo data do not differ systematically from those measured at the 91-m telescope, proving the applicability of the calibration and correction procedure. The H Imore » mass M/sub H/ is shown to be nearly proportional to the square of the linear H I diameter, implying that the H I surface density averaged over the disk is almost constant.« less

In the course of a radio-optical search for gravitational lenses, a multiple radio source, 0023 + 171, was discovered, which has two optical counterparts at z = 0.946 separated by 5 arcsec. Their redshifts are the same within the measurement error, and their optical spectra show the same emission lines with the same relative intensities. One interpretation of these measurements is that the images of 0023 + 171 are gravitationally lensed. However, because the radio morphology is complex and the optical objects are faint, the existing data cannot exclude another interpretation, i.e., that the components of 0023 + 171 are two physically associated radio galaxies or two components of a single galaxy. The only possible evidence in favor of a lensing object is extremely faint optical emission located approximately 1 arcsec from one of the images. If this multiple source is indeed gravitationally lensed, the combination of available data imply a mass-to-light ratio for the lensing matter equal to about 1000.