FERENGI: Full and Efficient Redshifting of Ensembles of Nearby Galaxy Images

Artificial "redshifting" of galaxies: From SDSS to GEMS, STAGES and COSMOS

Barden, Jahnke & Häußler, 2008, ApJS, 175, 105


GEMS, STAGES, and COSMOS are different from previous large Galaxy evolution studies in the sense that homogeneous high-resolution morphological information is available for 40,000/1,000,000 galaxies out to redshifts beyond 1. The high spatial resolution allows an unprecedented statistical assessment of structural parameters as a function of redshift.

However, bandpass shifting and the (1+z)5 surface brightness dimming (for a fixed width filter) make standard tools for the extraction of structural parameters wavelength dependent. If only few (or one) observed high-res bands exist, this dependence has to be corrected to make unbiased statements on the evolution of structural parameters or on galaxy subsamples defined by morphology.


As a solution we saw the creation of calibration samples of low-redshift galaxy images that are artificially redshifted to different redshifts, by applying the correct cosmological corrections for size, surface brightness and bandpass shifting. We have created a code dubbed FERENGI (Full and Efficient Redshifting of Ensembles of Nearby Galaxy Images) that does this and selected a galaxy sample with existing 5 band data from the Sloan Digital Sky Survey. These ~100 galaxies are artificially redshifted to (up to) 15 distances out to z=1.1 and convolved to show the respective COSMOS, GEMS and STAGES point-spread-functions in the F814W (COSMOS) and F606W/F850LP (GEMS, STAGES) bands.

Data Products:

The redshifted galaxy images as well as the input image data can be downloaded from this page. We also provide a table with galaxy names, coordinates, RC3 classification, magnitudes, etc. to be used to select objects. The accompanying paper contains all relevant details about the redshifting procedure and sample selection.

Please note: This sample is in a sense hand picked to show a wide range of morphologies and pair constellations and is statistically in no way whatsoever complete or randomly selected!

Depth1 orbit1 orbit1 orbit
Pixel size0.05''0.03''0.03''
Version V814.51.1 V606.31.1 V850.31.1
Date15 Jul 200715 Jul 200715 Jul 2007
For selecting individual galaxies based on their morphological appearance click here to browse thumbnail images of all sample objects.


We also release the FERENGI redshifting code suite with its different (currently two) components:
  • The FERENGI redshifting code itself, that converts an input set of images of the same galaxy observed in different wavelengths to an output redshift and instrument properties. FERENGI can in principle process any input set of galaxies, provided they fulfill the specific set of formats and conditions laid out in the documentation. The user will have to provide images in the right formats, maybe with his or her own preparatory wrapper script.
When using the FERENGI code or data products we would like you to please acknowledge the paper - if you feel that the FERENGI-contribution justifies it we would also not decline to act as co-authors in your project.

If you have question or comments, please contact us at or .

Knud Jahnke, Marco Barden, 19 May 2009