2dF Galaxy Survey
Two-degree Field Galaxy Survey
| Group Name | 2dFgals |
| Reference | 2dF Galaxy Redshift Survey (Colless+ 2003) |
| Prepared by | Eric Gawiser (Rutgers University) |
| Labels | No |
| Files | 2dfgals.speck |
| Dependencies | none |
| Census | 229,293 galaxies |
The Two-degree Field (2dF) Survey is a project designed to map portions of the extragalactic Universe. Mapping the Universe's structure provides astronomers with constraints on its formation and evolution.
The 2dF instrument is mounted on the 3.9-meter (12.8-foot) Anglo-Australian Telescope (AAT), located 450 km (280 miles) northwest of Sydney. The telescope has a 2-degree field of view on the sky, enabling large parts of the sky to be observed at one time. For each pointing of the telescope, the instrument can acquire up to 400 spectra simultaneously via optical fibers that feed into two spectrographs. The spectrographs see light that is between 350 nm and 800 nm, spanning the visible spectrum.
The survey was conducted from the extended APM Galaxy Survey. This catalog was derived from the Southern Sky Survey, taken by the UK Schmidt Telescope and scanned by the Automated Plate Measuring (APM) Machine. This extended catalog contains more than 5 million galaxies that span the north and south Galactic hemispheres.
The 2dF survey has three main components: the north Galactic pole strip, the south Galactic pole strip, and the random fields that surround the south Galactic pole strip. The galaxy survey is composed of about 230,000 galaxies with brightness and redshift measurements. For information on converting redshift to distance, please see “Redshift and Distance.”
Data Variables
The 2dF galaxy survey has the following data variables available (note that these are the same for the 2dF quasars and the Sloan galaxies and quasars):
| Data Variables for the 2dF and Sloan Surveys | |||
| Number | Name | Description | Units |
| 0 | Rmag | R magnitude | mag |
| 1 | redshift | Object's redshift | -- |
| 2 | Tlookback | The Lookback Time | Gyr |
| 3 | distMpc | Distance | Mpc |
| 4 | distMly | Distance | millions of light-years |
| 5 | radeg | Right Ascension | degrees |
| 6 | decdeg | Declination | degrees |
The zeroth data variable, Rmag, is the apparent R magnitude of the object. The R filter peaks at 658 nm, which is in the red end of the visible spectrum. In the 2dF galaxy survey, the magnitude has the range 11.69 ≤ Rmag ≤ 21.7 mags, so we're looking at dim objects indeed.
The redshift ranges from 0.0016 to 3.5, with a mean value of 0.118. The look-back time, how long ago an object emitted the light, should not exceed the age of the Universe, which is currently thought to be 13.7 billion years. The look-back time ranges from 0.02 billion to 11.7 billion years, meaning the oldest galaxies in these data date back 11.7 billion years. To see them, you must increase the brightness of the data set.
Following these, we have two distance variables, one in megaparsecs and the other in light-years. We also include their position on the sky, which is useful in setting a threshold for the data if you want to display only one part of the data set.
If you turn on the 20 billion-light-year grid, you will see that most galaxies extend to about 2.5 billion light-years before the data become sparse. As mentioned in “The Distance Scale,” the distance for each galaxy reflects the present location of that galaxy, not where its light left from. For example, a galaxy around 3 billion light-years in the Atlas has a look-back time of 2.7 billion years; the galaxy was 2.7 billion light-years away when the light we observe today left the galaxy. This discrepancy comes about from the expansion of the Universe; the farther an object is from us, the larger this discrepancy becomes.
Out to about 2 billion light-years, the 2dF galaxies show the large-scale structure of the Universe. Even in these thin slices, the pattern of clusters, connecting filaments, and voids is present in these data. Imagine if the 2dF were to look at the entire sky, we would be surrounded by galaxies that form this pattern. This is the goal of the Sloan Digital Sky Survey (SDSS), and we will discuss its latest results next.
© 2002-2005 American Museum of Natural History
Last Modified: 2007-12-19 by Brian Abbott