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Bayesian photometric red shifts: Public software and application to Cosmology


Despite the spectacular technical progress of telescopes and spectrographs in the last years, it is unfeasible to systematically measure red shifts for galaxies fainter than I~24.5 a limit reached in the early 90' with the Keck telescope, and this situation is not expected to change in the foreseable future. On the other hand, deep observations with the Hubble Space Telescope and the best ground based telescopes routinely detect and measure the colours of thousands of galaxies fainter than the spectroscopic limit.

This faint galaxy population hides the answer to many of the most important questions in observational cosmology, and the only method to extract red shift information from them is the so-called "photometric red shift" technique, which models the expected colours of the galaxies as a function of red shift and compares them with the observations, reaching reasonable accuracy. Here I propose to improve and extend the bayesian approach described in Benitez 2000, implemented in the BPZ software package. This approach has proved itself to be one of the most reliable and precise, and BPZ has been adopted by large projects and surveys as the Advanced Camera for Survey pipeline, the Keck Team Survey and the GOODS survey at STScI.

I plan to focus on two areas: the template library and the empirical red shift/type and red shift/size distributions used as priors by the bayesian approach. The result of this work will be a public software package, which based on previous experience with BPZ will be widely used by the European astronomical community.

I will apply this software package to two major astronomical projects:
a) the Advanced Camera for Surveys Lensing Cluster Survey. This program is observing 5 of the most massive galaxy clusters in the sky to depths close to the Hubble Deep Field and has already obtained unprecedented constraints on the shape of the cluster dark matter haloes.
b) the ALHAMBRA Survey.

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