Sloan Galaxy Survey
Sloan Digital Sky Galaxy Survey
| Group Name | SloanGals |
| Reference | Sloan Digital Sky Survey |
| Prepared by | Eric Gawiser (Rutgers University) |
| Labels | No |
| Files | sdssgals.speck |
| Dependencies | none |
| Census | 696,417 galaxies |
The Sloan Digital Sky Survey (SDSS) is an ambitious project to map one-quarter of the sky. The survey will measure the position and brightness of more than 100 million objects and measure redshift (which yields a distance) to 1 million galaxies and quasars.
The telescope is located at Apache Point Observatory in south-central New Mexico (US) and began operating in June 1998. It is 2.5 meters (8.2 feet) in diameter and was designed specifically for this mapping project. The telescope takes images of the sky as well as spectra for individual objects. Imaging the sky is not too difficult compared with taking individual spectra, so the spectral catalog lags behind the imaging project.
The spectral range for the SDSS is 380-920 nm, stretching from the blue end of the visible spectrum to the red and barely into the infrared.
The data variables associated with the SDSS galaxies are the same as those in the 2dF galaxies, except for the data variable called release, which describes the data release (see below). The SDSS galaxies are also similar to the 2dF data in that they form triangular wedges, revealing those parts of the sky observed by the telescope. If the entire sky were covered, you would see a spherical distribution of galaxies surrounding the Milky Way (like the 2MASS galaxies). Right now, we see only a few select slices from that sphere.
These galaxies appear to extend beyond the 2dF survey to distances that exceed 5 billion light-years. However, the weblike structure of clusters, filaments, and voids seems to fade by about 2 billion light-years. Beyond this distance, the completeness of the survey drops so that only the intrinsically bright galaxies are visible. This is easily seen if you set a threshold on the distance.
Look at the nearby galaxies using the command
thresh distMly 0 2500showing galaxies out to 2.5 billion light-years. Now view the distant galaxies using the command
thresh distMly 3000 8000Now you're looking at galaxies between 3 billion and 8 billion light-years, and the lack of structure is easy to see.
Seeing Structure
In order to see any structure among the almost 700,000 galaxies, it helps to remove some of the galaxies from your view. To accomplish this, you can filter these data by their position in the sky (right ascension and declination), or by data release. For example, you can single out a “slice” of galaxies by selecting data between a narrow declination range, like:
thresh decdeg -5 5As always, type see all to return all the galaxies to your view.
You may also follow the evolution of the Sloan Digital Sky Survey by selecting galaxies based on their data release. The first data were released in 2003, followed by five more releases through 2007. Because data release 1 and 2 were released jointly, we include them both in data release 2. Try using the thresh command and see how the SDSS evolved over the past few years. For example: try typing thresh release 2 2 to see release 1 and 2. Or type thresh release 6 6 to see the latest release.
Is this the farthest out we can see? Continue to the next data group to find out.
© 2002-2005 American Museum of Natural History
Last Modified: 2007-12-19 by Brian Abbott