Large-Scale Structure

The Astrophysics Department has a strong involvement in the 2dF Galaxy Redshift Survey (2dFGRS). This project, involving a team of 30 astronomers from Australia and the UK, is the largest ever redshift survey undertaken, and exploits the unique capabilities of the Two Degree Field (2dF) Spectrograph facility on the 3.9m Anglo-Australian Telescope. This instrument is able to obtain spectra for 400 objects simultaneously over a 2 degree field of view - a capability which at the moment is unrivalled by any other telescope in the world!

The goal of 2dFGRS is to secure high quality spectra and redshifts for 250,000 galaxies brighter than b(J)=19.5 over 2,000 square degrees ofsky. This sky coverage consists of two long strips, one in the southern galactic cap and the other in the northern galactic cap equatorial region, plus 100 randomly distributed 2 degree fields within the southern galactic cap region. These regions have been previously imaged as part of the APM Survey and it is this survey which provides the input catalogue for the 2dFGRS. The arrangement of the 2dFGRS survey fields on the sky is shown below:

With 2dFGRS obtaining an order of magnitude more redshifts than have ever been collected in any previous such survey, it will be able to address some of the most fundamental problems in large-scale sturcture, galaxy formation and cosmology. These are summarised below; areas of investigation being pursued at UNSW are highlighted:

  • Accurate measurement of the power spectrum of galaxy clustering on scales >30Mpc/h allowing, for the first time, a direct comparison with microwave background anisotropy measurements of fluctuations on the same spatial scales.
  • Measurements of the distortion of the clustering pattern in redshift space providing constraints on the cosmological density parameter, Omega, and the spatial distribution of dark matter.
  • A determination of variations in the spatial and velocity distributions of galaxies as a function of luminosity, type and
    star-formation history, providing important constraints on models of galaxy formation.
  • Investigations of the morphology of galaxy clustering and the statistical properties of the fluctuations, e.g. whether the initial fluctuations are Gaussian as predicted by inflationary models of the early universe.
  • A study of clusters and groups of galaxies in the redshift survey, in particular the space density of such systems, their distribution in velocity dispersion, and the spectral properties of their constituent members.
  • Application of novel techniques to classify the uniform sample of 250,000 spectra obtained in the survey, thereby obtaining a comprehensive inventory of galaxy types as a function of spatial position within the survey.

Survey progress and results

The survey reached the 100,000 redshift mark in May 2000, and at its current rate of progress should be completed by the end of 2001. The following plots highlight some of the results that have been obtained so far, including those to come from work being conducted here at UNSW:

The redshift cone plot based on the 112,867 galaxies observed as at the end of July 2000. The extent to which the data are revealing, in such delicate detail, the full range of large-scale structure that characterizes the galaxy distribution is clearly evident. When the survey is complete, it will provide such 3-dimensional maps of the galaxy distribution within a volume of 100,000,000 cubic Megaparsecs.

Examples of some the known rich Abell clusters that lie within the 2dFGRS and which are the subject of a detailed study here at UNSW. Such cone diagrams, which can be constructed from the survey data, allow us to precisely locate the clusters along our line of sight, to determine which galaxies are true members, and thus study bonda fide cluster populations free of contamination by galaxies and groups that are both in front and behind the cluster. This approach is being used to amass highly accurate redshift, velocity dispersion, and spectral line index information for a large (N=200-300) and complete sample of clusters and their galaxies, in an effort to understand stucture/galaxy formation and evolution within these very dense systems.

Another important area of investigation at UNSW is the search for `E+A' galaxies within the 2dFGRS and to determine in which environments they reside. These enigmatic galaxies, which are characterized by spectra with very strong Balmer absorption lines and no significant emission, have been commonly found in the cores of distant rich clusters. The enormous size of the 2dFGRS sample will allow us to determine whether, at lower redshifts, such objects are peculiar to the rich cluster environment or are more widespread. The figure above shows our means of detecting `E+A' galaxies in an objective way via the analysis of the equivalent widths of the [OII]3727 emission line and the mean of the Hß, H-gamma and H-delta absorption lines. Galaxies which simultaneously have EW([OII])<5A and EW(Hß+H-gamma+H-delta)>3A are defined as `E+A' galaxies; the 32 that we have identified from a sample of 90,000 spectra are clearly seen. We are in the process of determining what environments they are located.

Press release: Weighing the Universe with 100,000 galaxies

Fly-through movie: A trip through the universe!

For more information: The 2dFGRS Home Page

Staff and students involved in 2dFGRS:

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Jon Everett School of Physics - UNSW 2010