Summary


The WiggleZ Dark Energy Survey is a survey of 240,000 emission-line galaxies measured with the AAOmega spectrograph of the 3.9-m Anglo-Australian Telescope (AAT). The aim of the survey is to measure the scale of baryon acoustic oscillations (BAO) imprinted on the spatial distribution of these galaxies. The survey will sample a volume of 1 Gpc3 over an area on the sky of 1000 square degrees with an average target density of 350 galaxies per square degree.


The target galaxies are selected using ultraviolet photometry from the GALEX satellite, with a flux limit of NUV < 22.8 mag. We also require that the targets are optically detected, with 20 < r < 22.5 mag. We use the Lyman break method applied to the ultraviolet colours and additional optical colour limits to select high-redshift galaxies. The galaxies have strong emission lines, permitting reliable redshift measurements in relatively short exposures on the AAT and the median redshift of the galaxies is zmed=0.6. The redshift range containing 90 per cent of the galaxies is 0.2 < z < 1.0.





Target Selection


   The WiggleZ Survey is obtaining redshifts for over 200,000 luminous blue star-forming galaxies with spectra dominated by patterns of strong atomic emission lines ( [OII] 3727Å , H-beta 4861Å and [OIII] 4959Å and 5007Å ). originating in the ionized nebular gas in star-forming regions. These emission lines provide a galaxy redshift in a relatively short exposure of 1 hour on the 3.9-m AAT, avoiding the need to detect the galaxy "continuum" light, which is about 4 magnitudes fainter than in the spectroscopic targets observed by 2dFGRS or SDSS.  The primary target database for the WiggleZ Survey is provided by the orbiting Galaxy Evolution Explorer (GALEX) satellite.  Star-forming galaxies produce prodigious amounts of UV emission from their population of hot, young stars.  In collaboration with WiggleZ, the GALEX satellite is delivering about 400 hours of new UV imaging data across the target fields.  In order to produce a precise angular position for follow-up spectroscopy, we match the UV catalogues with optical imaging from the Sloan Digital Sky Survey (SDSS) and the Red Cluster Sequence (RCS2) database obtained at the Canada-France-Hawaii Telescope.


Motivation


  The key design decision for a galaxy redshift survey is how to select the spectroscopic targets.  From a cosmological viewpoint, galaxies are simply tracer particles of the underlying spectrum of density fluctuations which encodes the baryon oscillation and growth information.  Different classes of galaxy vary in the details of how they trace the density fluctuations.  For example, red quiescent elliptical galaxies inhabit dense cluster environments more often than blue star-forming spiral galaxies.  However, on large cosmological scales these details of galaxy formation are unimportant and can be essentially reduced to a single number, a "galaxy bias factor" specific to each galaxy class.  The decision of galaxy target can then be simplified to observational considerations such as exposure time required to obtain a successful redshift.



The distribution of galaxies currently observed in the WiggleZ Survey fields. The observer is situated at the origin of the co-ordinate system.  The radial distance of each galaxy from the origin indicates the observed redshift, and the polar angle indicates the galaxy right ascension.  The faint patterns of galaxy clustering are visible in each field.




The Instrument


   We obtain WiggleZ redshifts using a multi-object fibre spectrograph at the AAT called AAOmega, which is one of the world's most complex and powerful pieces of astronomical instrumentation.  AAOmega provides the capacity to obtain up to 392 galaxy spectra simultaneously using optical fibres placed on the focal plane by a robot positioner.  The large field-of-view of the instrument, a 2-degree diameter circle on the sky, provides an unrivaled mapping speed.  The new dual-beam spectrographs were commissioned in February 2006 to replace the original spectrographs which had been used for the 2dFGRS.



Example spectra

(a) Galaxy with a high-quality redshift (z = 0.6628) determined by multiple emission lines; (b) Galaxy with a high-quality redshift (z = 0.3048) determined by multiple lines; (c) Galaxy with fewer confirming emission lines but a confident redshift (z = 0.8775) based on the [OII] doublet feature; (d) Galaxy with a redshift (z = 0.9173) based solely on the [OII] doublet; (e) Galactic star at z = 0; (f) Quasar with a high-quality redshift (z = 0.3048).




WiggleZ  and Future Projects


   The next decade is expected to be a golden age of cosmology as a series of impressive new surveys are executed.  In the landscape of galaxy redshift surveys, WiggleZ will be succeeded by the Baryon Oscillation Spectroscopic Survey (BOSS) which should be executed by the upgraded SDSS telescope between 2009 and 2014.  BOSS will target a vast volume of Luminous Red Galaxies over 10,000 sq deg out to redshift z = 0.7 and also plans to map higher-redshift structure along the lines-of-sight to high-redshift quasars.  At higher redshifts the Hobbly-Eberly Telescope Dark Energy Experiment (HETDEX) is planning to harvest spectra of Lyman-alpha emitting galaxies.  Future radio telescopes will begin to detect the faint glow of neutral hydrogen emission from high-redshift galaxies, enabling transformational radio redshift surveys.


    In parallel, a new suite of deep and wide optical imaging surveys will emerge.  The first PanStarrs telescope on Mauna Kea is already operating and another three are planned.  The Dark Energy Survey project will be mapping 5000 sq deg of southern sky from the 4-m Blanco telescope in Chile.  A dedicated 8-m wide-field Large Synoptic Survey Telescope (LSST) is on the horizon.  These deep images will allow cosmologists to detect millions of new supernovae and perform precise measurements of the faint signature of gravitational lensing.  New space missions have also been proposed, including the Joint Dark Energy Mission (JDEM) in the United States and the Euclid mission in Europe.


    This wealth of new data will pose an unprecedented challenge to the existing standard cosmological model.  If the unexpected need for dark energy is interpreted as a serious failure of the model, then we could be on the brink of a paradigm shift which introduces a new cosmological framework.


 

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