SDSS-III: Massive Spectroscopic Surveys of the Distant Universe, the Milky Way Galaxy, and Extra-Solar Planetary Systems

TL;DR

SDSS-III coordinates four spectroscopic surveys—BOSS, SEGUE-2, APOGEE, and MARVELS—to address pivotal questions in cosmology, Galactic evolution, and exoplanet demographics. By leveraging BAO in galaxy clustering and the Ly$\alpha$ forest, infrared high-resolution stellar spectroscopy, and precision radial velocities, the program targets percent-level distance measurements and detailed chemical and kinematic maps of the Milky Way, alongside a large, well-characterized exoplanet census. The project includes substantial instrument upgrades, cross-survey data-sharing strategies, and a commitment to public data releases (DR8 and beyond) to maximize scientific return. Together, these efforts promise transformative constraints on cosmic acceleration, the Galaxy’s assembly history, and giant planet formation and migration, with broad community access and impact.

Abstract

Building on the legacy of the Sloan Digital Sky Survey (SDSS-I and II), SDSS-III is a program of four spectroscopic surveys on three scientific themes: dark energy and cosmological parameters, the history and structure of the Milky Way, and the population of giant planets around other stars. In keeping with SDSS tradition, SDSS-III will provide regular public releases of all its data, beginning with SDSS DR8 (which occurred in Jan 2011). This paper presents an overview of the four SDSS-III surveys. BOSS will measure redshifts of 1.5 million massive galaxies and Lya forest spectra of 150,000 quasars, using the BAO feature of large scale structure to obtain percent-level determinations of the distance scale and Hubble expansion rate at z<0.7 and at z~2.5. SEGUE-2, which is now completed, measured medium-resolution (R=1800) optical spectra of 118,000 stars in a variety of target categories, probing chemical evolution, stellar kinematics and substructure, and the mass profile of the dark matter halo from the solar neighborhood to distances of 100 kpc. APOGEE will obtain high-resolution (R~30,000), high signal-to-noise (S/N>100 per resolution element), H-band (1.51-1.70 micron) spectra of 10^5 evolved, late-type stars, measuring separate abundances for ~15 elements per star and creating the first high-precision spectroscopic survey of all Galactic stellar populations (bulge, bar, disks, halo) with a uniform set of stellar tracers and spectral diagnostics. MARVELS will monitor radial velocities of more than 8000 FGK stars with the sensitivity and cadence (10-40 m/s, ~24 visits per star) needed to detect giant planets with periods up to two years, providing an unprecedented data set for understanding the formation and dynamical evolution of giant planet systems. (Abridged)

Figures (15)

• Figure 1: Schematic diagram of a BOSS spectrograph (one of two), with elements as labeled. The "slithead" is in fact a pseudo-slit containing 500 aligned fibers.
• Figure 2: The comoving space density of BOSS galaxies from data taken in Spring 2010. The separate contributions of the LOZ cut, CMASS cut, and previously observed SDSS-I/II galaxies are shown, together with the total. The dashed curve shows our "goal" of constant density to $z=0.6$ and tapering density beyond. There is a deficit near $z=0.45$ at the transition between the two cuts, where obtaining accurate photometric redshifts for target selection is difficult.
• Figure 3: Redshift distribution of objects targeted by the BOSS quasar survey and observed between December 2009 and July 2010 (red solid histogram). There are 12,867 quasars with $z \geq 2.20$, obtained from a total of 55,114 targets, of which 32,844 yielded reliable redshifts. The spike at $z=0$ represents stellar contaminants, which are 34% of the objects with reliable redshifts. For comparison, the black dotted histogram shows all quasars from the quasar catalog of SDSS DR7 schneider10, and the red dot-dashed histogram shows the previously known high-$z$ quasars in the area surveyed, which come mostly but not entirely from DR7 and were reobserved by BOSS.
• Figure 4: Examples of BOSS galaxy spectra (left) and quasar spectra (right), smoothed with a 3-pixel boxcar. In each panel, the black line is the spectrum and the red is the estimated error per pixel. The galaxy redshifts are 0.3182, 0.5315, and 0.7227 (top to bottom). The calcium H&K absorption features are near 5200, 6200, and 6800 Å (top to bottom). Other noticeable features are the Mgb absorption line and [OII] and H$\alpha$ emission lines. The quasar redshifts are 3.81, 2.16, and 2.49 (top to bottom). The Ly$\alpha$, CIV, and CIII] emission lines are identifiable features in these quasar spectra. The $2"$-fiber $i$-band magnitudes of the targets are listed above each panel.
• Figure 5: Planned footprint of the BOSS spectroscopic survey, showing both the NGC (left) and SGC (right) regions. Most of the imaging for SGC target selection was done as part of SDSS-III. Each circle marks the location of a spectroscopic plate. Blue circles represent plates that have been observed as of January 2011, while red circles represent plates that have been drilled but not yet observed. The small extension of the SGC region below the equator at RA$>30^\circ$ is intended to reach the "W1" field of the CFHT Legacy Survey.