But this map in itself is not the SDSS’s real accomplishment; its real success is the revolutionary new knowledge that has been gained as a result. This page summarizes some of the major discoveries that the SDSS has enabled.
The SDSS’s high-precision maps of cosmic expansion history using baryon acoustic oscillations (BAOs) have been especially influential in quantifying these results, yielding exquisite constraints on the geometry and energy content of the universe. BAOs were first detected in galaxy clustering by the SDSS-I and in the contemporaneous 2dF Galaxy Redshift Survey, and have since also been detected in intergalactic hydrogen gas using Lyman-alpha forest techniques.
These BAO measurements are beautifully complemented by the results of the SDSS-II Supernova Survey, which has provided the most precise measurements yet of cosmic expansion rates over the last four billion years. In addition, statistical measurements of galaxy motions and weak gravitational lensing provide some of the strongest evidence to date that Einstein’s General Relativity is an accurate description of gravity on cosmological scales.
With full quasar samples hundreds of times larger than those that existed before, the SDSS has given us the most accurate descriptions of the growth of black holes over cosmic history. SDSS spectra show that the properties of quasars have changed remarkably little from the early universe to the present day.
SDSS studies have probed the dark matter environments of quasars through clustering measurements, revealed populations of quasars whose central engines are hidden by obscuring dust, captured changes in quasar spectra that show clouds moving in the gravitational grip of the central black hole, and allowed a comprehensive census of the much fainter accreting black holes (active galactic nuclei, or AGN) in present-day galaxies.
The SDSS has transformed the field of systematic galaxy analysis with accurate measurements of hundreds of parameters for hundreds of thousands of galaxies across the full range of cosmic environments. SDSS studies have demonstrated a bimodal distribution of galaxy properties, with a clear separation between populations of star-forming galaxies like the Milky Way, and passive galaxies that have little or no ongoing star formation.
Using weak gravitational lensing and statistical analyses of galaxy clustering, the SDSS has mapped out the multi-faceted relationships between galaxies and their surrounding halos of dark matter, showing that passive galaxies are found mainly at the centers of massive halos or as satellites orbiting larger galaxies.
The comprehensive census of present-day galaxies from the SDSS provides an essential testing ground for theoretical models of galaxy formation, and a crucial end-point comparison for studies of galaxy evolution from Hubble Space Telescope and other observatories.
The Milky Way
The clumps and streams found in the first SDSS star maps showed that the outer Milky Way is full of complex substructure – a finding confirmed and quantified as the SDSS imaged larger areas and mapped the motions of hundreds of thousands of stars.
SDSS results support a theoretical picture of “hierarchical” galaxy formation, in which the Milky Way continues to grow by accreting and destroying smaller galaxies. The SEGUE and APOGEE surveys have provided our most comprehensive picture of the formation history of the Milky Way’s stellar disk by separately measuring the abundances of elements produced quickly by exploding massive stars (Type II supernovae) and more slowly by exploding white dwarfs (Type Ia supernovae).
SDSS measurements of the motions of stars in the disk and stellar halo have yielded the most precise determinations of the mass distribution of the Milky Way’s dark matter halo, implying a total halo mass of approximately one trillion solar masses, lower than many previous estimates.
With precise multi-color imaging of hundreds of millions of stars, the SDSS has enabled systematic characterization of stellar populations and the identification of large samples of rare or intrinsically faint objects. Hand-in-hand with the discovery of the most distant quasars came the discovery of numerous “brown dwarfs,” objects that form like stars but are not massive enough to ignite steady hydrogen fusion at their centers. SDSS catalogs have provided our most detailed understanding of the population of low mass stars, which are individually faint but collectively represent a large fraction of the Galaxy’s stellar mass. SDSS catalogs of white dwarfs, the Earth-sized embers left behind by sun-like stars at the ends of their lives, have been especially influential, revealing many subtleties of white dwarf physics and identifying metal-enriched systems that appear to be accreting material from surrounding belts of asteroids. The combination of APOGEE chemical abundance measurements with asteroseismological data from NASA’s Kepler satellite is opening a new era of stellar astrophysics that can probe the interior structure of thousands of stars.
Analyses of SDSS data have also led to the discovery and characterization of “hyper-velocity stars,” moving so quickly that they will escape from the Milky Way entirely, apparently as a result of interactions with the Galaxy’s central supermassive black hole.
The Local Group
SDSS imaging enabled the discovery of a new population of “ultra-faint” dwarf galaxies orbiting the Milky Way. To date, the majority of known Milky Way companions have been found by the SDSS, along with several new companions of the Andromeda galaxy.
With total light output as low as a thousand times the luminosity of the Sun, these tiny systems provide critical insights into the physics of galaxy formation and stringent tests of the properties of dark matter.
SDSS asteroid studies demonstrated a marked change in the size distribution of main belt asteroids at a diameter of about 5 km, implying fewer small asteroids than previously believed. They also showed that families of asteroids with distinct orbital properties also have distinctive colors, revealing the importance of “space weathering” that changes the surface appearance of asteroids over time. Dynamical families appear to be the result of collisions in the asteroid belt that produce cascades of smaller bodies, exposing fresh material that was previously the interior of a larger body.
Most SDSS asteroids are in the main belt between Mars and Jupiter, but repeat imaging has also enabled the SDSS to discover objects in the outer Solar System, near or beyond the orbit of Neptune. One of these, the remarkable object 2006 SQ 372, is on a highly eccentric orbit that takes it to a distance of 800 AU (800 times the Earth-Sun distance). Modeling suggests that it has been dynamically scattered from the inner zone of the Oort Cloud, a cloud of distant cometary bodies that is a remnant of the Solar System’s formation.
Other Science Results
The account above is only a broad sketch of some of the SDSS’s major contributions. More accounts of some of the numerous important discoveries from SDSS data can be found in our press releases and blog entries (see below) and in this 2007 video from the American Museum of Natural History, which provides an excellent introduction to the science themes of SDSS-I. More information is available from the museum’s Science Bulletin website.
The SDSS’s active program of public engagement includes regular communication with the media through a series of press releases and a part-time press officer. The SDSS press officer is Jordan Raddick (firstname.lastname@example.org).
The newest SDSS press releases are available through the SDSS Press Releases page of this website.
SDSS Science Blog
We also maintain a regular science blog of SDSS discoveries where you will find short descriptions of interesting scientific research and discoveries from the SDSS-III. We’d love to see your comments and questions about what you read here!
The Sloan Digital Sky Survey is one of the most cited surveys in the history of astronomy. Data from SDSS has been used in over 5,800 peer-reviewed publications in astronomy and other sciences; those papers in turn have been cited a total of 245,000 times. It is a testament to the public data commitment of SDSS that the overwhelming majority of those papers have been written by scientists outside the SDSS collaboration.
Publications produced within the SDSS collaboration are currently hosted on our SDSS-III website.