B. F. Burke

A VLA Gravitational Lens Survey

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 1012 M ⊙ compact objects, luminous or dark, must be substantially less than the critical density.

The radio galaxy RC J1148+0455

Based on data from the MIT-GB-VLA 4850-MHz survey, we investigate the radio structure of RC J1148+0455 with a steep radio spectrum (α=−1.04) from the COLD catalog. The radio source consists of two components, each having a complex structure. We analyze an optical identification of the source by using 6-m telescope images. The centroid of the radio source falls on a group of eight galaxies at a 24m level in R.

Calibration Issues in the Search for Large-Scale Anisotropies in the 87 Green Bank and Parkes-Mit-Nrao Surveys

Until recently, there were no large scale radio surveys which were sensitive enough to have a sufficient number of sources to detect a dipole anisotropy. The 87 Green Bank (87GB) (Gregory and Condon, 1991) and Parkes-MIT-NRAO (PMN) (Griffith, 1993; Griffith et al., 1994, 1995; Wright et al., 1994) radio surveys are the first such efforts and are the most complete radio surveys to date. We would like to analyze the source counts in these surveys in order to test the isotropy of the sky distribution of extragalactic sources at cosmological redshifts. Of particular interest is a dipole anisotropy, which could arise from local motion with respect to the rest frame of extragalactic radio sources. Any detected anisotropy will give information about the source sky distribution at a redshift of z ∼ 1 (Condon, 1988), an epoch between the COBE observations at z ∼ 1000 and the local universe.

The Angular Size Distribution of 4741 Radio Sources in the MIT-VLA Snapshot Archive

The MIT all-sky and VLA snapshot surveys are described, and the scientific results to date are summarized. The largest angular size distribution is given for 4741 MIT-VLA sources, and a Monte-Carlo algorithm is proposed to model the cosmological evolution of jets in radio-loud AGN. Further progress requires compiling redshifts for a complete sub-sample.

The VLA Light Curves of 0957+561, 1979–1994

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.

Preliminary VLA Snapshots of Southern Radio Sources from the Parkes-MIT-NRAO (PMN) Survey

Selection criteria for 1800 MIT-VLA snapshots of PMN radio sources are described, and 6 new MG & PMN lens candidates are presented.

Resolution of galaxy and third image of gravitational lens 2016+112

The components of 2016+112 are variable. It is probable that all three compact objects are variable, but the absolute flux density scale is difficult to determine. The strong variation of C is contrary to expectations, because VLBI observations show C is extended and must vary more slowly than a compact object. If the C flux density decrease is due to variation of a third image, this image must have very large percentage flux density changes. Any third image, C′, must be very near C, < 0.19 arcseconds away for images with A/C′ flux density ratios < 2 magnitudes. Component C has been resolved, but measuring the contribution of the quasar image to the radio flux density requires further observations. The Narasimha et al. model that is not excluded by these observations suggests that a massive galaxy is located at the radio component C.