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D. A. Coe, N. Benitez (JHU), T.J. Broadhurst (Racah Inst. Physics, Hebrew U.), K.C. Zekser (JHU), R.L. White (STScI), B. Frye (Princeton U.), H.C. Ford (JHU), G.D. Illingworth (UCO/Lick Obs.), ACS Science Team
Abell 1689 is the strongest lensing cluster in the sky, and HST equipped with ACS is our most powerful 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 these images, corresponding to 20 different multiply-imaged galaxies. This allows us to determine an extremely accurate and detailed massmap for A1689, but only after redshifts are obtained for the lensed systems. Despite extensive spectroscopic observations, redshifts have only been confirmed for 5 of these systems. 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 crowded field creates 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. Thus, we developed SExSeg, a new program designed to force SExtractor to use predefined detection and apertures. SExSeg also creates isophotal apertures adapted to the seeing of each image, yielding robust colors. The quality of our resulting bayesian photometric redshifts 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.
ACS was developed under NASA contract NAS 5-32865, and this research is supported by NASA grant NAG5-7697. We are grateful for an equipment grant from the Sun Microsystems, Inc.
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The author(s) of this abstract have provided an email address for comments about the abstract: coe@pha.jhu.edu
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Bulletin of the American Astronomical Society, 35#5
© 2003. The American Astronomical Soceity.