Abell 1689 is the strongest lensing cluster in the sky, and HST equipped with ACS is our strongest lensing telescope in the sky. Together, they have produced one of our deepest views of the visible universe to date, revealing lensed galaxies down to an intrinsic apparent magnitude of 30. We have identified an unprecedented number of arcs in our deep multicolor images of A1689, corresponding to 20 different multiply-imaged galaxies. This allows us to determine the massmap of A1689 very accurately and in much detail, but only after redshifts are obtained for the lensed systems. Despite extensive spectroscopic observations, only 5 redshifts have been confirmed. For the rest, we rely on Bayesian Photometric Redshifts (BPZ), obtained from the combination of our extensive UBVRIZJHK ground-based imaging and griz ACS data.
This, however, is not a trivial task, since A1689 is a highly crowded field, creating problems both for photometry and detection. Galaxy modeling and subtraction has done a great deal to alleviate these problems (Zekser et al., in preparation). Another challenge is posed by the broad range of quality of our images. Photometric redshifts require matched aperture photometry. And isophotal apertures have been shown to outperform circular apertures, especially in a crowded field and with irregularly shaped (lensed) galaxies.
Thus, we developed SExSeg. SExSeg forces SExtractor to run using predefined detection and apertures. SExSeg also creates isophotal apertures adapted to the seeing of each image. Our photometric method extracts the maximum amount of information and yields robust aperture-matched isophotal colors, thus improving the reliability of our bayesian photometric redshifts. This is demonstrated by the excellent agreement we find with the 87 spectroscopic redshifts available.
These and similar results from the wider ground-based field will be the foundation for our next endeavor: to merge the strong and weak lensing regimes in Abell 1689.
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