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Spectroscopic Confirmation of Dark Energy Survey Strong Lensing Systems and Spectra for Photometric Redshift Calibration


Principal Investigator: Elizabeth Buckley-Geer, Fermi National Accelerator Laboratory


Program Summary:

Over five observing seasons, which started in August 2013, the Dark Energy Survey (DES) will carry out a wide field survey of 5000 deg2 over the Southern Galactic Cap in the 5 filters grizY and a 30 deg2 supernova survey spread over 10 fields. Among the millions of astronomical objects that will be imaged by DES are rare instances of "strong lensing" systems, where the effects of general relativity, Einstein's theory of gravity, are demonstrated in a visually striking fashion. When a foreground "lens" object is by chance very closely aligned, along the line of sight, on the sky with a much more distant background "source" object, the light from the source may be significantly deflected as it passes by the lens, due to the gravitational field produced by the lensing object's mass. When the distant source is fuzzy single distant background galaxy the this strong lensing effect leads to big distortions in it's appearance: it may be transformed into a long bright arc, maybe into multiple blue knots or, in the rarest cases, into a so-called Einstein ring. On the other hand if the source is a quasar (a quasar is a compact region in the center of a massive galaxy, that surrounds its central supermassive black hole) then it will be transformed into multiple stellar-like objects.

This wide variety in appearance of strongly lensed images is a consequence of the complexities of the lensing mass, which can range from an individual galaxy to a rich cluster of many galaxies, together with the much more massive dark matter halos in which the (luminous) galaxies reside. Studies of strong lensing systems can thus reveal to us properties of the distribution of dark matter that accompanies galaxies and galaxy clusters. Moreover strong lensing can provide yet another way to study dark energy. For example, cosmological parameters, including dark energy, will affect the abundance and frequency of strong lensing systems and hence influence how many such systems we'll find in DES. The lensed quasars are particularly useful for determining the Hubble constant which governs the rate of expansion of the universe. We expect to find many new lenses quasars in DES.

In order to use these strong lenses for constraining dark matter and dark energy we need to know how far away they are and we do this by measuring their redshifts. We observe the object and obtain a spectrum of it's light and look for features such as absorption lines and emission lines. These features can be compared with known features in the spectrum of various chemical compounds found on earth. If the same features are seen in an observed spectrum from a distant source but occurring at shifted wavelengths then the redshift can be calculated. Each lensing system typically contains multiple images of the source object so we will use the GMOS instrument in its multi-object mode to efficiently obtain many spectra simultaneously.

In addition to the primary lensing targets, we are also piggybacking observations of about another 50 galaxy targets per field, for a total of about 3000 galaxy redshifts over the course of our full program. This sample will be used to improve the measurement of the redshift distribution of faint DES galaxies, by providing a galaxy sample distributed over many fields spread across the DES footprint, as opposed to our present situation of relying on a small number of fields for this purpose. Better knowledge of the galaxy redshift distribution will in particular be used to help improve the determination of dark energy cosmological parameters by DES.

The team consists of 29 scientists from the USA, Australia, Brazil, the United Kingdom, Switzerland, Japan and Taiwan.


  • Filipe Adballa, University College London
  • Adriano Agnello, UC Santa Barbara
  • Sahar Allam, Space Telescope Science Institute
  • Robert Brunner, University of Illinois at Urbana-Champaign
  • Mattias Carrasco, University of Illinois at Urbana-Champaign
  • Francisco Castander, Institut de Ciencies de l'Espai
  • Carlos Cunha, Stanford University
  • Luiz da Costa, Observatorio Nacional (ON/MCT)
  • Clecio De Bom, Centro Brasileiro de Pesquisas Físicas
  • H. Thomas Diehl, Fermi National Accelerator Laboratory
  • Christopher Fassnacht, UC Davis
  • David Gerdes, University of Michigan
  • Karl Glazebrook, Centre for Astrophysics & Supercomputing, Swinburne University
  • Julia Gschwend, Observatorio Nacional (ON/MCT)
  • William Hartley, ETH, Zurich
  • David Lagattuta, ARC Centre of Excellence for All-sky Astrophysics, Swinburne University
  • Geraint Lewis, University of Sydney
  • Huan Lin, Fermi National Accelerator Laboratory
  • Marcio Maia, Observatorio Nacional (ON/MCT)
  • Martin Makler, Centro Brasileiro de Pesquisas Físicas,
  • Felipe Menanteau, University of Illinois at Urbana-Champaign
  • Anupreeta More, Kavli IPMU
  • Anna Niernberg, UC Santa Barbara
  • Brian Nord, Fermi National Accelerator Laboratory
  • Sherry Suyu, Academia Sinica Institute of Astronomy and Astrophysics
  • Tommaso Treu, UC Santa Barbara
  • Douglas Tucker, Fermi National Accelerator Laboratory