History and Archives

Collaborators on the Sloan Digital Sky Survey think they can pull off a "three-peat"--duplicating the success of two earlier sky sweeps.

The survey started in 1998 and made its way into the astronomical journals in 2000. Every year since then, at least one paper about the SDSS has made it in the list of the top 10 astronomy papers of the year.

SDSS-I and SDSS-II have spawned the spin offs Galaxy Zoo and Galaxy Zoo 2, which allowed anyone to classify galaxies from a home computer. More than 200,000 have done just that so far.

The Sloan Web site has gotten more than 467 million hits since 2001.

The astrophysics experiment has a following worthy of a championship sports team, and collaborators hope to keep the excitement rolling with the survey's third phase started last year.

The most anticipated results from the six-year Sloan Digital Sky Survey-III will be from the Baryon Oscillation Spectroscopic Survey, one of three scientific themes researched by the project's four surveys, says SDSS Director and University of Chicago Professor Richard Kron.

BOSS will map the 3-D distribution of 1.5 million luminous red galaxies, to measure the scale of density fluctuations in the universe, using spectra--an ordered array of the components of radio-frequency waves--collected from fall 2009 to spring 2014.

Pressure waves in the early expanding universe oscillated up to the time when the pressure fell to zero as the universe created conditions for ions and electrons to recombine and form atoms. The dark matter was pulled in by the density enhancements in the normal atomic matter, called baryonic matter. This resulted in a frozen pattern with a recognizable scale. Scientists can use the scale of these patterns in the density of matter to map the universe's outward acceleration, caused by "dark energy."

Kron, a scientist with the Fermilab Experimental Astrophysics group, highlighted this focus of the upcoming third survey in a talk to peers at a retreat of Fermilab's Center for Particle Astrophysics in April, held at Fermilab. Kron, the project director for the first two SDSS runs, also discussed the discoveries of the original SDSS, which took place from 2000 to 2005, and SDSS-II which took place from 2005 to 2008.

Fermilab had a number of collaborators on the first two SDSS runs and continues to analyze data from those. Fermilab scientists Jim Annis, Huan Lin, Brian Yanny, Chris Stoughton, John Peoples, and Kron are collaborators for SDSS-III.

"The survey [BOSS] will also help us calibrate photometric redshifts from the Dark Energy Survey," Kron says.

To measure distances, astrophysicists need to look for objects in the universe that have known values of luminosity or physical size.

BOSS will precisely measure the apparent size of a ruler using galaxy clustering (the baryon oscillation scale). The size of the ruler depends on only basic physical quantities: the distance sound traveled up to the time when particles recombined to form atoms and when the pressure waves stalled.

The SDSS surveys use a 2.5 meter telescope at the Apache Point Observatory in New Mexico. The telescope has a large-format digital camera and fiber-fed spectrographs.

The original goal of the first SDSS was to map large-scale structures using galaxies and quasars as tracers, both in imaging and spectroscopy. In five years, the scientists measured one million galaxy redshifts and 100,000 quasar redshifts. Kron says that the data gained from the first survey was useful across many fields of astronomy.

The SDSS-II survey used the same instrumentation and same selection of galaxies and quasars as the first. Improvements were made to the processing software. One of the discoveries of SDSS-II was a number of hyper-velocity stars, which move so fast they are hypothesized to have been accelerated with a slingshot-like effect as they passed near the massive black hole at the center of the Milky Way.

"The project has been a big success and has already made substantial contributions to the field of dark energy research," Kron says.