PKS1830-211: The strongest lensed radio source

Abstract

The Ooty lens PKS1830-211 is a bright, variable radio source at a redshift of 2.507 consisting of a compact flat spectrum core, associated knot and jet, which are lensed by an intervening spiral galaxy at a redshift of 0.886. It forms two images separated by 000.98 with a ring passing through them. Multi epoch Very Large Array (VLA), Multi-Element Radio Linked Interferometer Network (MERLIN), Very Long Baseline Interferometer (VLBI) and Very Long Baseline Array (VLBA) observations have shown variability of the source by more than a factor of two, over time scales of less than a day to years, some of which mimic superluminality. Absorption lines of neutral hydrogen and many molecules have been observed in front of both the images. The molecular lines appear to dominate the image passing near the central part of the lens while the image farther away is dominated by HI 21 cm. Overall, the column density of various species appears to be similar to that found in the Milky Way. But isotopic ratios estimated from the line ratios of several isotopomers appear to indicate that the lens galaxy is less evolved than the Milky Way. The differential Faraday rotation between the images (upwards of 2000 rad m-2) is suggestive of a large-scale magnetic field of a few microgauss in the lens galaxy. The time delay between the images is estimated to be 26+4-5 days. Multi epoch VLB imaging as well as absorption line monitoring have provided rich information on the source property as well as tools for studying high-redshift objects