6dF Galaxy Survey: Mapping the Southern Sky
The 6dF Galaxy Survey (6dFGS) represents a massive leap in our ability to map the local Universe. By covering 17,000 square degrees of the southern sky, this survey provides a comprehensive view of galactic distribution, spanning an area approximately ten times larger than the 2dFGRS and more than twice the spectroscopic coverage of the Sloan Digital Sky Survey.
Key Facts
- Coverage: 17,000 square degrees of the southern sky.
- Instrument: 6dF multi-object fiber spectrograph.
- Telescope: 1.2 meter UK Schmidt Telescope.
- Selection Method: Near-infrared source selection via the Two Micron All Sky Survey (2MASS).
- Capacity: 150 optical fibers per field plate.
Technical Infrastructure and Observation
The survey was conducted at the Siding Spring Observatory in New South Wales, Australia. Utilizing the 1.2 meter UK Schmidt Telescope, the 6dF instrument allowed astronomers to observe a field of 6 degrees per pointing. To maximize efficiency, the instrument employed a multi-object fiber spectrograph capable of utilizing 150 optical fibers per field plate, enabling the simultaneous collection of data from numerous targets.
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The Advantage of Near-Infrared Selection
Unlike many previous redshift and peculiar velocity surveys that relied on optical light, the 6dFGS utilized near-infrared source selection. The primary target catalogs were derived from the Two Micron All Sky Survey (2MASS). This shift in wavelength provides several critical scientific advantages.
Accurate Stellar Mass Measurement
In the optical spectrum, observations are often dominated by younger, bluer stars. However, the near-infrared spectral energy distributions of galaxies are dominated by their oldest stellar populations. Because these older stars constitute the bulk of a galaxy's stellar mass, near-infrared data provides a more accurate representation of the total mass of the galaxy.
Overcoming Dust Extinction
Dust extinction—the absorption and scattering of light by interstellar dust—is significantly reduced at longer wavelengths. This means that near-infrared luminosity is not heavily dependent on the orientation of the galaxy, offering a more reliable measure of mass. Additionally, this capability allows the 6dFGS to map regions of the local Universe closer to the plane of the Milky Way, where optical light would otherwise be blocked by galactic dust.
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Survey Summary
| Feature | Details |
|---|---|
| Sky Coverage | 17,000 deg² |
| Comparison to 2dFGRS | ~10x the area |
| Comparison to SDSS | >2x spectroscopic areal coverage |
| Primary Data Source | Two Micron All Sky Survey (2MASS) |
| Observation Site | Siding Spring Observatory, Australia |
Frequently Asked Questions
Where can the 6dFGS data be accessed?
All redshifts and spectra are available through the 6dFGS Online Database hosted at the Royal Observatory, Edinburgh, while an online atlas is provided by the University of Cape Town.
Why is near-infrared light better for measuring galaxy mass?
Near-infrared light is dominated by older stellar populations, which make up the majority of a galaxy's stellar mass, whereas optical light is often skewed by a small number of young, bright blue stars.
How does the 6dFGS handle the interference of the Milky Way?
Because near-infrared wavelengths are less affected by dust extinction, the survey can see through the dust of the Milky Way's plane more effectively than optical surveys.
What instrument was used to collect the spectra?
The survey used the 6dF instrument, a multi-object fiber spectrograph with 150 optical fibers per field plate, attached to the 1.2 meter UK Schmidt Telescope.