ZEBRA

The Zurich Extragalactic Bayesian Redshift Analyzer

ZEBRA is a free and open-source software distributed under the GNU General Public License 3 and can be downloaded here.

Introduction

ZEBRA combines and extends several of the classical approaches to produce accurate photometric redshifts down to faint magnitudes. In particular, ZEBRA uses the template-fitting approach to produce Maximum Likelihood and Bayesian redshift estimates based on:

• An automatic iterative technique to correct the original set of galaxy templates to best represent the SEDs of real galaxies at different redshifts;
  
• A training set of spectroscopic redshifts for a small fraction of the photometric sample, to improve the robustness of the photometric redshift estimates; and
• An iterative technique for Bayesian redshift estimates, which extracts the full two-dimensional redshift and template likelihood function for each galaxy.

The workflow of ZEBRA. The input files are indicated on the top. The resulting output data on the bottom. A typical ZEBRA session would take the following route: 1) Check & correction of the photometric catalog; 2) Optimization of the spectral energy distributions (templates); 3) Maximum-Likelihood fit or Bayesian estimation of the probability distribution in template type - redshift space.

Input

The user can choose from, or has otherwise to provide, an initial set of templates and filter transmission curves. In addition a catalog is needed which provides ZEBRA with the magnitudes of the galaxies in question in the various filter bands.

Output

• In the photometry-check mode ZEBRA returns a catalog with calibrated photometry together with detailed information about the applied changes.
• In the template-optimization mode ZEBRA returns the corrected templates as wavelength --flux density (per unit wavelength) tables.
  
• In the Maximum-Likelihood mode ZEBRA returns the best fit redshift and template type together with their confidence limits estimated from constant chi^2 boundaries. ZEBRA also provides the likelihood functions for all galaxies in several output formats and the residuals between best fit template magnitude and measured magnitude for each galaxy in each filter band.
  
• In the Bayesian mode ZEBRA calculates the 2D-prior in redshift and template space in an iterative fashion. This prior (and, if specified, the interim prior of each iteration step) is returned. The final prior is used by ZEBRA to compute a posterior for each galaxy. The posterior can be saved as full 2D-table or in marginalized form. ZEBRA's output includes the most probable redshift and template type for each galaxy as defined by (i) the maximum of the posterior or (ii) after marginalizing over templates types or (iii) after marginalizing over redshifts, respectively.  The errors are calculated directly from the posterior.
  
• ZEBRA can also be employed to derive template-based k-corrections using the specified templates and filters.
  

Performance

In Feldmann et al. 2006 we have demonstrated ZEBRA's performance using the Subaru, CFHT and NOAO groundbased photometry (Capak et al. 2006; Taniguchi et al. 2006; ApJ COSMOS Special issue) on a sample of ~50,000 COSMOS galaxies and, as a training set, zCOSMOS spectroscopic redshifts that were available for 1.8% of the sample under study.

Download

In order to run ZEBRA on your machine you may try to download an appropriate, precompiled ZEBRA binary. Alternatively you can download the ZEBRA package and compile the code yourself. Please refer to the ZEBRA user manual on how to install ZEBRA properly. The ZEBRA package includes: the source code, examples on how to use ZEBRA, wrapper scripts and the user manual.

A note about binaries: We recommend the installation of ZEBRA from source as described in the user manual. Installation from source requires lapack and the GNU Scientific Library (gsl).

FAQ

Previous Versions

Change Log

Please report any bugs or problems you encounter with the code to: zebra@phys.ethz.ch.
If you are intending to extend / improve ZEBRA it would be kind to let us know as well.

It is kindly requested that the use of ZEBRA should be acknowledged with an explicit reference to Feldmann, Carollo, Porciani, Lilly et al., MNRAS 372, 564 (2006) in the bibliographic list of any resulting publication.

ZEBRA+

A new version of ZEBRA (ZEBRA+) is currently being built in order to:

• include extinction by dust
• estimate stellar masses, ages and metallicties
• adapt the treatment of AGN templates

Stay tuned!