The Apache Point Observatory Galactic Evolution Experiment (APOGEE)

TL;DR

APOGEE presents a high-resolution, near-infrared spectroscopic survey of the Milky Way designed to uniformly sample all major Galactic components, including heavily dust-obscured regions. The instrument combines 300 fibers with a 1.5–1.7 μm spectral window at $R\sim22{,}500$, delivering multi-element abundances for ~10^5 stars and precise radial velocities to map chemodynamical structure across the Galaxy. The paper details motivations, technical requirements, instrument design, survey strategy, data-processing pipelines, and demonstrated capabilities, highlighting the survey’s success in producing a uniform public data set (DR12) and enabling broad applications from time-domain spectroscopy to mapping Galactic extinction and ISM features. The APOGEE framework lays the foundation for APOGEE-2 in SDSS-IV, expanding to the southern hemisphere and enhancing all-sky Galactic archaeology with parallel improvements in software and lineage of data products.

Abstract

The Apache Point Observatory Galactic Evolution Experiment (APOGEE), one of the programs in the Sloan Digital Sky Survey III (SDSS-III), has now completed its systematic, homogeneous spectroscopic survey sampling all major populations of the Milky Way. After a three year observing campaign on the Sloan 2.5-m Telescope, APOGEE has collected a half million high resolution (R~22,500), high S/N (>100), infrared (1.51-1.70 microns) spectra for 146,000 stars, with time series information via repeat visits to most of these stars. This paper describes the motivations for the survey and its overall design---hardware, field placement, target selection, operations---and gives an overview of these aspects as well as the data reduction, analysis and products. An index is also given to the complement of technical papers that describe various critical survey components in detail. Finally, we discuss the achieved survey performance and illustrate the variety of potential uses of the data products by way of a number of science demonstrations, which span from time series analysis of stellar spectral variations and radial velocity variations from stellar companions, to spatial maps of kinematics, metallicity and abundance patterns across the Galaxy and as a function of age, to new views of the interstellar medium, the chemistry of star clusters, and the discovery of rare stellar species. As part of SDSS-III Data Release 12, all of the APOGEE data products are now publicly available.

Figures (38)

• Figure 1: APOGEE in the context of other Galactic archaeology surveys, past, present and future. The top panel shows the number of Milky Way stars, observed or anticipated, as a function of survey resolution. For those surveys with at least a resolution of $R = 10,000$, the bottom panel shows the expected nominal depth of the survey for a star with $M_{\rm V}=-1$ in the case of no extinction ( right end of arrows) and in the case of $A_V = 10$ ( left end of arrows). In both panels, already completed surveys are shown in black, ongoing surveys in dark gray, and planned surveys in light gray. For surveys with multiple resolution modes, data in the top panel are plotted separately for high resolution (HR), medium resolution (MR) and/or low resolution (LR). For the Gaia/ESO survey, data for "Inner Galaxy" and "Halo" subsamples are shown separately as well. "Gaia-RV" includes Gaia high resolution spectra of enough $S/N$ to deliver radial velocities, whereas "Gaia" indicates only those with $S/N$ high enough for abundance work. For Gaia we adopted $A_G/A_V$ from Jordi2010, assuming $(V-I_C)_0=1.7$; sample numbers were taken from http://www.cosmos.esa.int/web/gaia/science-performance.
• Figure 2: In three overlapping wavelength regions, the distribution of telluric absorption ( top spectra in each panel), airglow ( middle spectra), and atomic lines in the spectrum of the star Arcturus ( bottom spectra). Some prominent atomic lines in the Arcturus spectrum that guided the ultimate selection of the APOGEE wavelength region are identified and color-coded as high priority ( red), medium priority ( blue) and lower priority ( black). Also indicated are the extremes in the potential shift in the lines from extremes in radial velocity variation for potential (e.g., halo) Milky Way stars (adopted as $\pm700$ km s$^{-1}$ in the lines).
• Figure 3: Summary of the $S/N$ experiments described in Appendix B for each of 15 chemical elements. For each, the minimum required $S/N$ to measure 0.1 dex precision abundances is plotted for a variety of resolutions from $R=15,000$ to 30,000, and for three metallicities, [Fe/H] $=-2$, $-1$, and 0. For Al, Si, and Mg the data points for all three modeled metallicities fall on top of one another.
• Figure 4: Layout of the APOGEE spectrograph optical bench within the cryostat. The fiber train coming from the telescope enters the cryostat on the left.
• Figure 5: Schematic figure showing the arrangement of fibers and wavelengths across the three APOGEE detectors. The wavelengths indicate the edges of the arrays as well as fiducial wavelength positions (indicated by grey dots) corresponding to the "mid-chip" properties given in Table \ref{['tab:Inst_Char']}. The location of the Littrow ghost (curved line) and the super-persistent region (grey area) are also indicated. The dashed line at 1.68 $\mu$m shows the red limit of the wavelength coverage for which the technical performance of the instrument was specified by the science requirements, but the instrument performance is still good redward of this.