Digital Universe Guide - Hayden Planetarium
Digital Universe Guide - Hayden Planetarium
Digital Universe Guide - Hayden Planetarium
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196 4. THE EXTRAGALACTIC ATLAS<br />
4.3.14 Sloan <strong>Digital</strong> Sky Galaxy Survey<br />
Group Name SloanGals<br />
Reference Sloan <strong>Digital</strong> Sky Survey<br />
Prepared by Eric Gawiser (Rutgers University)<br />
Labels No<br />
Files sdssgals.speck<br />
Dependencies none<br />
Census 696,417 galaxies<br />
The Sloan <strong>Digital</strong> Sky Survey (SDSS) is an ambitious project to map one-quarter of the sky. The<br />
survey will measure the position and brightness of more than 100 million objects and measure redshift<br />
(which yields a distance) to 1 million galaxies and quasars.<br />
The telescope is located at Apache Point Observatory in south-central New Mexico (US) and began<br />
operating in June 1998. It is 2.5 meters (8.2 feet) in diameter and was designed specifically for this<br />
mapping project. The telescope takes images of the sky as well as spectra for individual objects.<br />
Imaging the sky is not too difficult compared with taking individual spectra, so the spectral catalog lags<br />
behind the imaging project.<br />
The spectral range for the SDSS is 380 nm − 920 nm, stretching from the blue end of the visible<br />
spectrum to the red and barely into the infrared.<br />
The data variables (see p. 194) associated with the SDSS galaxies are the same as those in the 2dF<br />
galaxies, except for the data variable called release, which describes the data release (see below).<br />
The SDSS galaxies are also similar to the 2dF data in that they form triangular wedges, revealing those<br />
parts of the sky observed by the telescope. If the entire sky were covered, you would see a spherical<br />
distribution of galaxies surrounding the Milky Way (like the 2MASS galaxies). Right now, we see only a<br />
few select slices from that sphere.<br />
These galaxies appear to extend beyond the 2dF survey to distances that exceed 5 billion<br />
light-years. However, the weblike structure of clusters, filaments, and voids seems to fade by about<br />
2 billion light-years. Beyond this distance, the completeness of the survey drops so that only the<br />
intrinsically bright galaxies are visible. This is easily seen if you set a threshold on the distance.<br />
Look at the nearby galaxies using the command<br />
thresh distMly 0 2500<br />
showing galaxies out to 2.5 billion light-years. Now view the distant galaxies using the command