Measuring the Propert ies of the Gravitational Lens P K S 1 8 3 0 -

PKS 1830-211 is the strongest known radio gravitational lens by almost an order of magnitude and has the potential to provide a measurement of Ho, provided the lensing system can be parameterized. Attempts to identify optical counterparts, to measure redshifts, have so far proved unsuccessful and this has lead to radio and millimetre spectral line observations. We present our discovery of an absorption system at z = 0.19. A brief description is also made of our ATCA observations to measure the lensing time delay for this source. The lensed image of PKS 1830—211 possesses a symmetric morphology, comprising two compact, flat-spectrum components of similar brightness located on opposite sides of a 1 arcsec ring. This suggests a close alignment of the lensed source behind the lensing mass. Moreover, there is evidence of unusually high rotation measures in some parts of the source which argues that the lensing galaxy is probably a gas-rich spiral (Nair, Narasimha, k Rao 1993) and suggests the possibility of detecting molecular absorption. We undertook a survey in June 1995 for redshifted Hi and OH absorption with the Parkes 64 m radio telescope, as part of a cooperative observation program with the Project Phoenix group (Tarter 1996). We detected only a single absorption feature with two sub-components of similar amplitude, centered at 1191.1 MHz with an overall line width of approximately 50 km/s. VLBI observations were made in September 1995 with four telescopes of the Australian Long Baseline Array (LBA) (Jauncey et al. 1994); Hobart, Coonabarabran, Parkes and five antennas of the Australia Telescope Compact Array (ATCA) acting as a phased array. S2 recorders (Wietfeldt et al. 1991) were used, operating in dual polarization (LCP and RCP) with 4 MHz bandwidth centered on 1191.0 MHz. The VLBI data were correlated at the ATNF VLBI correlator. Our VLBI image (Figure 1) shows the compact components of PKS 1830211, but little of the low brightness ring which is heavily resolved at this resolution. Figure 1 also shows the absorption spectrum of each component, clearly demonstrating that the two velocity components of the absorption system obscure different parts of the source. The low velocity component is heavily resolved and therefore must be obscuring only the extended ring while the high Now at Institute of Space and Astronautical Science, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229, Japan. E-mail: [email protected]