Sloan Digital Sky Survey IV: Mapping the Milky Way, Nearby Galaxies, and the Distant Universe

TL;DR

SDSS-IV unifies three core spectroscopic programs—APOGEE-2, MaNGA, and eBOSS—and two ancillary programs (SPIDERS, TDSS) to comprehensively map the Milky Way, nearby galaxies, and the distant universe. It combines dual-hemisphere facilities (APO and LCO) with advanced hardware (APOGEE-2S, MaNGA IFU bundles) and a robust data-management/public-data-release framework to deliver high-quality, homogeneous data: near-infrared high-resolution stellar spectra ($R\sim22{,}500$) for ~400,000 stars, optical integral-field maps for ~10,000 galaxies, and a large-volume cosmological redshift survey targeting $0.6<z<2.2$ with multi-tracer approaches. The project produces detailed Milky Way chemo-dynamical maps, spatially resolved galaxy evolution data, and powerful cosmological constraints (BAO, RSD, neutrino mass, and inflation signatures) from eBOSS, complemented by quasar, AGN, and variability science from TDSS/SPIDERS. Public data releases (e.g., DR13 onward) and an integrated data ecosystem enable broad community access, reproducibility, and cross-disciplinary impact across Galactic archaeology, galaxy formation, and cosmology.

Abstract

We describe the Sloan Digital Sky Survey IV (SDSS-IV), a project encompassing three major spectroscopic programs. The Apache Point Observatory Galactic Evolution Experiment 2 (APOGEE-2) is observing hundreds of thousands of Milky Way stars at high resolution and high signal-to-noise ratio in the near-infrared. The Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey is obtaining spatially-resolved spectroscopy for thousands of nearby galaxies (median redshift of z = 0.03). The extended Baryon Oscillation Spectroscopic Survey (eBOSS) is mapping the galaxy, quasar, and neutral gas distributions between redshifts z = 0.6 and 3.5 to constrain cosmology using baryon acoustic oscillations, redshift space distortions, and the shape of the power spectrum. Within eBOSS, we are conducting two major subprograms: the SPectroscopic IDentification of eROSITA Sources (SPIDERS), investigating X-ray AGN and galaxies in X-ray clusters, and the Time Domain Spectroscopic Survey (TDSS), obtaining spectra of variable sources. All programs use the 2.5-meter Sloan Foundation Telescope at Apache Point Observatory; observations there began in Summer 2014. APOGEE-2 also operates a second near-infrared spectrograph at the 2.5-meter du Pont Telescope at Las Campanas Observatory, with observations beginning in early 2017. Observations at both facilities are scheduled to continue through 2020. In keeping with previous SDSS policy, SDSS-IV provides regularly scheduled public data releases; the first one, Data Release 13, was made available in July 2016.

Figures (11)

• Figure 1: Model of the du Pont Telescope configuration during APOGEE-2S observations. The yellow transparent box indicates the bottom of the telescope, with the gray annulus indicating the location of the primary mirror. The scaling ring mechanism with a cartridge attached is just below the primary. A telescope fiber link connects the cartridge to a patch panel at the end of the boom. The instrument fibers travel down a movable boom to the wall of the dome, and are directed to the instrument room in the level below the telescope dome. The room on the dome level on the right side of the diagram is used for plate plugging and mapping during the night.
• Figure 2: APOGEE-1 and planned APOGEE-2 spectroscopic footprint in equatorial coordinates, centered at $\alpha_{\rm J2000}=270^\circ$, with East to the left. Black shows APOGEE-1 data, orange indicates APOGEE-2N, and red is APOGEE-2S. Blue shows projected MaNGA coverage for which APOGEE-2 can potentially have observations of stars (see also Figure \ref{['fig:manga-footprint']}). Because of logistical constraints and potential changes in the MaNGA plans, the final coverage of the halo may differ somewhat from this figure.
• Figure 3: Map of APOGEE-1 and APOGEE-2 distance limits at $b=0^\circ$ within the Galactic plane, compared to other Galactic plane spectroscopic surveys. These limits assume observations of stars at the tip of the red giant branch (for solar metallicity and 2 Gyr of age) using isochrones from bressan12a. To calculate the distance limit, we use the dust extinction prescription of bovy16a and limits of $H=12.2$ for APOGEE-2, $V=14$ for GALAH, and $V=19$ for Gaia-ESO (their faintest limit across all fields). Longer cohorts in APOGEE-2 extend correspondingly further.
• Figure 4: Top panel: Several subregions of the full APOGEE spectra for seven stars of a range of metallicities, as labeled on the right (plotted using the software described in bovy16b). The black lines are the data; the red lines are the best-fit ASPCAP model; the areas where the data are missing are masked due to sky contamination or other issues. Both data and model have been normalized to the pseudo-continuum $f_c(\lambda)$ (holtzman15a). Clean, strong lines identified by smith13a are labeled. Bottom panels: Elemental abundances relative to Fe for several of the species whose lines exist in the top panel, as a function of [Fe/H], for the APOGEE DR13 sample of 164,562 stars. APOGEE-2 can examine the major patterns as a function of Galactic location (e.g., nidever14a, hayden15a).
• Figure 5: Planned MaNGA spectroscopic footprint in equatorial coordinates, centered at $\alpha_{\rm J2000}=270^\circ$, with East to the left. Black shows the available MaNGA tiles; orange indicates example coverage for a simulated SDSS-IV MaNGA survey.