New optical surveys for gravitationally lensed quasars

Abstract

This thesis describes four new optical surveys for gravitationally lensed quasars. The goal of this work has been to (1) explore new strategies for conducting optical surveys, especially in the southern hemisphere, in the hopes of "fine-tuning" the survey techniques toward finding systems of particular scientific interest; and (2) to increase the number of optically-selected lenses, and by extension, to improve future statistical analyses of lens surveys by enlarging the sample size of quasars that have been surveyed for lensing. Optical surveys for strong lenses have the potential to discover an order of magnitude more systems than radio surveys, to produce optically-bright lenses that are well-suited for long-term monitoring and followup observations, and to yield statistical constraints on the overall geometry and dynamics of the Universe. Three of these surveys take the form of traditional "targeted" searches. These include: 1) a ground-based survey targeting 173 radio-loud quasars using the 2.4 m telescope at Michigan-Dartmouth-MIT Observatory (MDM), producing one certain lens and one binary quasar; 2) a ground-based survey targeting 377 quasars in the southern hemisphere using the 1.5 m telescope at Cerro-Tololo Interamerican Observatory (CTIO), producing two certain lenses and one binary quasar; 3) a space-based snapshot survey for close separation lenses targeting 320 quasars using the Hubble Space Telescope (HST), producing three certain lenses and maybe a fourth. The MDM survey has been designed to combine the high discovery efficiency of optical surveys with the rich image morphology often found in radio-loud lenses. The CTIO survey makes use of two new catalogs of bright quasars to provide a high discovery efficiency of 0.7%.

(cont.) The HST snapshot survey has searched for close separation lenses - spirals in particular - that are difficult to resolve from ground-based surveys. Of the eight systems discovered form these surveys, five are described in detail in this thesis. The first object, FBQ 1633+3134, is a 0"f7 double discovered from the MDM survey. Discrepant optical and radio flux ratios for this system means that it is most likely a physical binary quasar. The second, third and fourth objects -CTQ 414, HE 0230-2130, and CTQ 839 - were all discovered from the CTIO survey. The former two systems are confirmed gravitational lenses, while the latter is a binary quasar. CTQ 414 is a 12 double quasar that is well suited for future optical monitoring and a possible time-delay measurement, HE 0230-2130 is a complex four-image gravitational lens formed by two lensing galaxies, and CTQ 839 is almost certainly a binary quasar after repeated attempts to detect the hypothesized lensing galaxy have failed. The fifth system, the lensed quasar CTQ 327, is a 1"/2 double discovered from the HST survey which is also well suited for optical monitoring and a possible time-delay measurement. The superior angular resolution afforded by the HST snapshot survey is also used to constrain the matter density of any hypothetical population of dark, compact objects at high redshift to be less than 2.2% of the closure density for objects of mass 109 6M0, and rules out a closure density of any compact object in the mass range 1075 < M/M < 1011.5 at the 99.7% confidence level ...