# APOGEE

APOGEE performs a systematic survey of all Milky Way stellar components using high-resolution, multi-object, near-infrared spectroscopy.

## The APOGEE Survey

APOGEE is a large-scale, stellar spectroscopic survey conducted in the near-infrared (H-band) portion of the electromagnetic spectrum. APOGEE consists of two 300-fiber cryogenic spectrographs that operate onboard the 2.5-meter Sloan Foundation Telescope and the 1-meter NMSU Telescope at Apache Point Observatory (APO) in New Mexico, USA and the 2.5-meter Irénée du Pont Telescope of Las Campanas Observatory (LCO). APOGEE is unique among large spectroscopic surveys because it is able to acquire high-resolution (R~22,500) spectra across the entire Milky Way.

These pages provide a brief introduction to the APOGEE survey and its data products. See the APOGEE technical papers for detailed information on all aspects of the APOGEE survey and advice on how to cite these in your work.

The current data release is Data Release 16 (DR16). Users already familiar with APOGEE can visit the DR16 Synopsis page for a summary of how the data analysis and data products have changed.

## APOGEE Data Products

There are 5 major data products provided by the APOGEE survey:

• Spectra - Three different types of reduced spectra are available: visit spectra, combined spectra, and ASPCAP spectra. Descriptions for these three types of spectra and how to use them can be found here.
• Radial Velocities (RVs) - RVs can be found in the spectra files themselves, as well as in the allVisit catalog (data model) and allStar catalog (data model) for the individual visit RVs and combined RVs, respectively. For an introduction on how to use RVs, go here.
• Stellar Parameters - Stellar parameters, including effective temperature, surface gravity, metallicity are derived using the APOGEE Stellar Parameters and Chemical Abundance Pipeline (ASPCAP), and can be found in the individual aspcapStar spectra files as well as the allStar catalog. For instructions on how to use APOGEE stellar parameters, go here. Stellar Parameters can be found in the allStar summary file (data model).
• Chemical Abundances - ASPCAP also provides chemical abundances for 18 elements. However, the precision and accuracy of these abundances vary based on stellar parameters, so users should consult the Using Chemical Abundances page for how to properly use APOGEE chemical abundances. Chemical Abundances can be found in the allStar summary file (data model) in the "{X}_Fe” tags.
• Samples - APOGEE has carefully constructed its stellar samples to enable a user to translate between observations and underlying populations using its selection function. An overview of this information can be found here.

The RVs, stellar parameters, and chemical abundances can all be found in the allStar summary file (data model). Be sure to read and examine the Using pages for instructions from the APOGEE team on how these quantities should be used and analyzed.

For instructions on how to acquire and analyze data using the CAS, SAS, and/or SAW, go here.
For instructions on how to acquire intermediate data products, go here.

APOGEE observes a few hundred stars towards the Galactic bulge with the 2.5-meter Sloan Telescope during the full moon. The 1-meter NMSU telescope is in the foreground, with the lights of El Paso on the horizon.
Image Credit: S. R. Majewski

## Observing and Data Use Information

APOGEE Observing
gives an overview of how APOGEE spectroscopic data are taken.
APOGEE Data Access
describes all of the APOGEE data products that are available through the Science Archive Server.
Examples
provides examples of how to query the database or summary data files in order to retrieve a variety of information for different types of targets.
Using APOGEE Stellar Parameters
describes things you need to know if you plan to use the derived stellar atmospheric parameters.
Using APOGEE Chemical Abundances
describes things you need to know if you plan to use the derived elemental abundances.
Using APOGEE Spectra
describes features in APOGEE spectra about which anyone looking at spectra should be aware.
Using APOGEE Radial Velocities
describes things you need to know if you plan to use the APOGEE radial velocities.
Using APOGEE Targets/Samples
describes things you need to know if you plan to use the APOGEE targeting information.

## Software and Processing Information

APOGEE Target Information
describes the way in which APOGEE targets are chosen and how this is documented in the target flags.
APOGEE Visit Spectra Reduction
gives information about how individual visit spectra are observed, processed, and stored.
APOGEE Visit Spectra Combination
gives information about how the combined spectra for each star are created and stored.
APOGEE Radial Velocities
describes the derivation of radial velocities from both individual visit and final combined spectra.
APOGEE Stellar Parameter and Abundance Determination
describes how the stellar parameters and abundances available in APOGEE spectroscopic catalogs are derived.
APOGEE Caveats
gives a running list of known issues with this release.

## Additional References

• Bitmasks are fully described on the main bitmasks page.
• Glossary describes common terms.
• Technical papers and references describe how to cite the survey in your papers, lists all technical papers, and also provides references for the catalogs used in targeting.

## A Brief History of APOGEE

• As part of the planning for “After Sloan II” (AS2), it was felt that more mileage could come from the Sloan infrastructure investment by extending survey operations into bright time. An invitation to submit ideas for potential bright time experiments was extended to Michael Skrutskie (Univ. of Virginia) by the AS2 Steering Committee.
In August 2005, Skrutskie and John Wilson (Univ. of Virginia) submitted a white paper (Skrutskie & Wilson 2015) describing the potential of high throughput, multifiber, NIR spectroscopy on the Sloan 2.5-m telescope to the Astrophysical Research Corporation (ARC) Futures Committee.
• Around the same time, Steven Majewski (also Univ. of Virginia) was attending an Aspen Center workshop, where Katia Cunha (Steward Observatory and Observatorio Nacional) presented a multi-element analysis of Galactic radial metallicity gradients (from Cunha & Daflon 2005). This work combined nebular abundances of H II regions and photospheric abundances of young OB stars. These two tracers are bright and, thus, are accessible to spectroscopy at optical wavelengths, despite the heavy dust obscuration at low Galactic latitudes; these traces are, however, severely limited in number and biased towards young populations. Though state of the art at the time, this study was based on only several dozen sources inhomogeneously sampling the disk from 5-13 kpc from the Galactic center, and highly biased to very young sources. Moreover, the results of the study did not match the chemical evolution models of the Milky Way.
• At the same time, Majewski was conducting a large, one-star-at-a-time optical, low-resolution spectroscopic survey (the Grid Giant Star Survey). This survey identified bright but distant giant stars across the sky to serve as the astrometric grid for the (soon defunct) NASA Space Interferometry Mission. Building from that and inspired by Cunha’s presentation, Majewski suggested that an industrial-scale survey of the Milky Way with a specific focus on the previously much-neglected low-latitude Milky Way was timely, to address questions like those presented by Cunha in a comprehensive and statistically robust way. This aim necessitated working in the infrared to break through the thick veil of interstellar dust, moving to high resolution for accurate chemical abundance determinations, and focusing on giant stars, which are intrinsically bright and therefore accessible to large distances, and also represented in stellar populations of all ages.
• Back in Virginia, these various activities by Skrutskie, Wilson, and Majewski evolved the original multifiber NIR spectroscopy concept for SDSS-III. The program became a comprehensive and high spectral resolution survey of Galactic chemodynamics, targeting primarily (though not exclusively) giant stars spanning from the Galactic bulge, across the dust-obscured disk and out into the halo. The 2MASS database, whose creation was led by Skrutskie, was the perfect source catalog for such a survey. Another key aspect of the proposed survey, largely unique to Milky Way surveys, was the inclusion of multi-epoch observing. This strategy served two purposes: (i) to build high S/N spectra of fainter sources and (ii) to identify Doppler-variable stars for the identification of binary and even lower mass stellar companions.
• APOGEE was proposed as a project for SDSS-III in 2006 August and was officially approved by the ARC Board as one of the four SDSS-III projects in 2006 November. Selection of the APOGEE wavelength range and resolution was influenced by the available chemical species from known line transitions, the pioneering efforts by Verne Smith (NOIR Lab; e.g., Smith & Lambert 1985) and later work by Smith et al. 2002 and Cunha & Smith 2006, as well as extensive spectral modeling by Carlos Allende-Prieto (IAC). Because a number of the lines in the selected APOGEE region (1.5-1.7 microns) remained unidentified, the APOGEE team initiated collaborations with laboratory atomic physicists from Univ. of Wisconsin and Imperial College London to resolve this problem.
• The science goals for APOGEE required the creation of a unique, state-of-the-art spectrograph to undertake highly multiplexed fiber spectroscopy in the H-band. Four new technologies were innovated to make the first APOGEE spectrograph, for which the optical bench is held in a cryogenic and vacuum cryostat to approximately liquid nitrogen temperature. The first spectrograph was built on time and budget. Commissioning occurred at APO in Spring 2011. In 2017, APOGEE Instrument Scientist John Wilson and the APOGEE Team were given the Maria and Eric Muhlmann Award from the Astronomical Society of the Pacific for the innovative and successful design of the APOGEE spectrographs.
• Another innovation was the (necessary) automation of the processing and interpretation of the high volume of high-resolution spectra generated by the APOGEE survey. The core of the ASPCAP pipeline is Carlos Allende Prieto's optimization code, FERRE. Led by Jon Holtzman (NMSU) and David Nidever (initially Univ. of Virginia, now NOIR Lab and Montana State), a new code was developed to derive stellar parameters and stellar abundances efficiently and automatically using FERRE.
• Official APOGEE-1 survey data acquisition commenced in September 2011. It concluded three years later in July 2014, exceeding its original goal to survey 300,000 stars to $H = 12.2$ mag or fainter with $S/N \gt 100$. The first release of APOGEE-1 data occurred in DR10, followed by two subsequent releases, DR12 (July 2015) and DR13 (July 2016).
• As part of SDSS-IV, it was proposed to expand APOGEE-2 to the Southern Hemisphere, which provided a much better view of the Galactic bulge and inner disk and enabled expansion of APOGEE to the Magellanic Clouds and other Milky Way satellites. A second spectrograph was built for the du Pont telescope, which was delivered to Las Campanas Observatory in 2017.
• APOGEE-2 started Northern Hemisphere operations immediately after APOGEE-1 finished in 2014 and will conclude in July 2020. The Southern Hemisphere operations for APOGEE-2 began in early 2017 and will be conducted through September 2020. An additional change is that APOGEE-2N not only observes in bright time but co-observes with the SDSS MaNGA survey during the dark time.
• After the first few years of very efficient operations and fortuitously good weather, APOGEE-2 North observations were already a year ahead of schedule. The efficiency prompted the creation of the “Bright Time Extension (BTX)” program, to further expand the breadth of APOGEE-2 observations, spreading its Milky Way coverage as well as including special focus on the TESS Continuous Viewing Zones, Kepler and Kepler K-2 fields. At the present rate, it is projected that the final APOGEE + APOGEE-2 survey database may reach a million stars with several million total spectra.
• The first release of APOGEE-2 data took place in DR14 and included new reductions of the whole of SDSS-III/APOGEE data as well as a portion of the APOGEE-2 data from SDSS-IV. DR16 is the first data release to include data from APOGEE-2 South, which began full operations in April 2017. This release includes data from both hemispheres obtained through August 2018.