6dF Galaxy Survey
Encyclopedia
The 6dF Galaxy Survey 6dF or 6dFGS is a redshift survey
conducted by the Anglo-Australian Observatory
(AAO) with the 1.2m UK Schmidt Telescope between 2001 and 2009. The data from this survey were made public on 31 March 2009. The survey has mapped the nearby universe
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 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
(SDSS) and the 2dF Galaxy Redshift Survey
(2dFGRS).
The survey was carried out with the 1.2 meter UK Schmidt Telescope at Siding Springs Observatory 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 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.
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. 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.
Redshift survey
In astronomy, a redshift survey, or galaxy survey, is a survey of a section of the sky to measure the redshift of astronomical objects. Using Hubble's law, the redshift can be used to calculate the distance of an object from Earth. By combining redshift with angular position data, a redshift...
conducted by the Anglo-Australian Observatory
Anglo-Australian Observatory
The Australian Astronomical Observatory , formerly the Anglo-Australian Observatory, is an optical/near-infrared astronomy observatory with its headquarters in suburban Sydney, Australia...
(AAO) with the 1.2m UK Schmidt Telescope between 2001 and 2009. The data from this survey were made public on 31 March 2009. The survey has mapped the nearby universe
Universe
The Universe is commonly defined as the totality of everything that exists, including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos, the world and nature...
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 survey is measuring mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
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
Sloan Digital Sky Survey
The Sloan Digital Sky Survey or SDSS is a major multi-filter 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 was named after the Alfred P...
(SDSS) and the 2dF Galaxy Redshift Survey
2dF Galaxy Redshift Survey
In astronomy, the 2dF Galaxy Redshift Survey , 2dF or 2dFGRS is a redshift survey conducted by the Anglo-Australian Observatory with the 3.9m Anglo-Australian Telescope between 1997 and 11 April 2002. The data from this survey were made public on 30 June 2003...
(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 the 6dFGS Online Database, hosted at the Royal Observatory, Edinburgh. An 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 Springs Observatory 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 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 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 extinctionExtinctionIn biology and ecology, extinction is the end of an organism or of a group of organisms , normally a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and recover may have been lost before this point...
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 6dFGS 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 6dFGS survey is one of the few surveys which are big enough to allowe a Baryon Acoustic Oscillation (BAO) signal to be detected. The low redshift of 6dFGS made it possible to derive the Hubble constant H0 with an uncertainty competitive with the distance ladder technique. The result is reported in Beutler et al. (2011) and is given by H0 = 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 law) and the peculiar velocityPeculiar velocity
Peculiar motion or peculiar velocity refers to the true velocity of an object, relative to a rest frame.-Galactic astronomy:In galactic astronomy, the term peculiar motion refers to the motion of an object through space.Local objects are usually related in terms of proper motion and radial...
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
Fundamental plane
The fundamental plane may refer to:* Fundamental plane , which divides a spherical coordinate system* Fundamental plane , which shows an empirical relationship among mean surface brightness, velocity dispersion and effective radius of an elliptical galaxy...
of early-type galaxies. 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.