HST and Keck Snapshot Surveys for Lensed Quasars

We are using the Hubble Space Telescope Imaging Spectrograph (STIS) to obtain images of bright quasars to look for new examples of gravitational lensing. The search is tuned to look for systems which would be difficult to discover from the ground, those with small separations between the lensed components. Most theories of quasar lensing predict more close separation lenses than are observed from the ground; deciding whether this deficit is real or an observational selection effect is important for using lenses to constrain the intervening galaxy mass distribution or cosmological models. After about 250 snapshots during Cycles 8 and 9, the program has found three lensed quasars, one with a separation of only 0.64'' (Gregg et al. 2000), one with 1.2'' separation, and a third with a surprisingly large separation of 3.4''.

The left panel is a STIS image of the closest separation lens that we have found so far (0.64"). Removal of the two point sources (images of the quasar) at first suggested possible detection of the lensing galaxy (white arrow in right panel), but subsequent imaging with HST has not confirmed this. Click on the image for a larger version.

We are also using the Near InfraRed Camera (NIRC) at Keck Observatory to obtain short exposure images of many quasars which are not in the HST target list but are also good lensing candidates. This program has not yet turned up any new lenses, but NIRC images of two of the new HST lenses has been able to detect and characterize the lensing galaxy.

The upper left panel at left shows the NIRC image of a lensed quasar found in the STIS survey. The separation between components is 1.2". The upper right panel shows the large residuals left after removing two point sources; the excess light suggests that the lensing galaxy is detected. The lower left panel shows an elliptical galaxy model (plus residuals) which, when added to the mix, nicely accounts for the excess light, as demonstrated by the greatly reduced residuals in the lower right panel. Click on the image to see a bigger version.

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UCD Physics Dept.
Cosmology Research Group
Institute of Geophysics and Planetary Physics at the Lawrence Livermore National Laboratory

Modified February, 2003
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