The 6dF Galaxy Survey (Six-degree Field Galaxy Survey), 6dF or 6dFGS is a

redshift survey In astronomy, a redshift survey is a survey of a section of the sky to measure the redshift of astronomical objects: usually galaxies, but sometimes other objects such as galaxy clusters or quasars. Using Hubble's law, the redshift can be used ...

conducted by the

Anglo-Australian Observatory The Australian Astronomical Observatory (AAO), formerly the Anglo-Australian Observatory, was an optical and near-infrared astronomy observatory with its headquarters in North Ryde in suburban Sydney, Australia. Originally funded jointly by the ...

(AAO) with the 1.2m

UK Schmidt Telescope The UK Schmidt Telescope (UKST) is a 1.24 metre Schmidt telescope operated by the Australian Astronomical Observatory (formerly the Anglo-Australian Observatory); it is located adjacent to the 3.9 metre Anglo-Australian Telescope at S ...

between 2001 and 2009. The data from this survey were made public on 31 March, 2009. The survey has mapped the nearby

universe The universe is all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development of the univers ...

over nearly half the sky. Its 136,304 spectra have yielded 110,256 new extragalactic redshifts and a new catalog of 125,071 galaxies. For a subsample of 6dF a

peculiar velocity Peculiar motion or peculiar velocity refers to the velocity of an object relative to a ''rest frame'' — usually a frame in which the average velocity of some objects is zero. Galactic astronomy In galactic astronomy, peculiar motion refers to ...

survey is measuring mass distribution and bulk motions of the local Universe. As of July 2009, it is the third largest redshift survey next to the

Sloan Digital Sky Survey The Sloan Digital Sky Survey or SDSS is a major multi-spectral imaging and spectroscopic redshift survey using a dedicated 2.5-m wide-angle optical telescope at Apache Point Observatory in New Mexico, United States. The project began in 2000 ...

(SDSS) and the

2dF Galaxy Redshift Survey In astronomy, the 2dF Galaxy Redshift Survey (Two-degree-Field Galaxy Redshift Survey), 2dF or 2dFGRS is a redshift survey conducted by the Australian Astronomical Observatory (AAO) with the 3.9m Anglo-Australian Telescope between 1997 and 11 Ap ...

(2dFGRS).

Description

The 6dF survey covers 17,000 deg² of the southern sky which is approximately ten times the area of the 2dFGRS and more than twice the spectroscopic areal coverage of the Sloan Digital Sky Survey. All redshifts and spectra are available through th
6dFGS Online Database
hosted at the Royal Observatory, Edinburgh. A
online 6dFGS atlas
is available through the University of Cape Town. The survey was carried out with the 1.2 meter UK Schmidt Telescope at

Siding Spring Observatory Siding Spring Observatory near Coonabarabran, New South Wales, Australia, part of the Research School of Astronomy & Astrophysics (RSAA) at the Australian National University (ANU), incorporates the Anglo-Australian Telescope along with a col ...

in New South Wales, Australia with the 6dF instrument permitting the observation of a field of 6 degrees per pointing. The instrument possesses a multi-object fiber spectrograph with 150

optical fibers An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparent fiber made by drawing glass ( silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a mea ...

per field plate. One feature of the 6dF Galaxy Survey compared to earlier redshift and peculiar velocity surveys is its near-infrared source selection. The main target catalogs are selected from the Two Micron All Sky Survey. There are several advantages of choosing galaxies in these bands. * The

near-infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from arou ...

spectral energy distributions of galaxies are dominated by the light of their oldest stellar populations, and hence, the bulk of their stellar mass. Traditionally, surveys have selected target galaxies in the optical where galaxies dominated by younger, bluer stars. * The effects of dust

extinction Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the Endling, last individual of the species, although the Functional ext ...

are smaller at longer wavelengths. For the target galaxies, this means that the total near-infrared luminosity is not dependent on galaxy orientation and so provides a reliable measure of galaxy mass. Furthermore, this means the 6dF can map the local Universe nearer to the plane of the Milky Way than would otherwise be possible through optical selection.

Baryon Acoustic Oscillation detection

The 6dF survey is one of the few surveys which are big enough to allow a baryon acoustic oscillation (BAO) signal to be detected. The low redshift of 6dF made it possible to derive the Hubble constant ''H''₀ with an uncertainty competitive with the distance ladder technique. The result is reported in Beutler et al. (2011) and is given by ''H''₀ = 67 ± 3.2 km/s/Mpc.

The peculiar velocity survey

The redshift of a galaxy includes the recessional velocity cause by the expansion of the Universe (see

Hubble's law Hubble's law, also known as the Hubble–Lemaître law, is the observation in physical cosmology that galaxies are moving away from Earth at speeds proportional to their distance. In other words, the farther they are, the faster they are moving a ...

) and the

of the galaxy itself. Therefore, a redshift survey alone does not provide an accurate three-dimensional distribution for the galaxies. However, by measuring both components separately, it is possible to obtain a survey of the (peculiar) velocities of the galaxies. In the 6dF survey distance estimators are based on the Fundamental plane of early-type galaxies.FP; Dressler et al. 1987, Djorgovski & Davis 1987 This requires measurements of the galaxies' internal velocity dispersions which is much more difficult than the determination of the redshift. This restricts the number of galaxies usable for the peculiar velocity survey to about 10% of the overall 6dF galaxies. Finally the peculiar velocities can be measured as the discrepancy between the redshift and the estimated distance.

References

External links

6dF homepage

6dFGS online database
{{DEFAULTSORT:6df Galaxy Survey Astronomical surveys Observational astronomy