Overview of the DESI Milky Way Survey

TL;DR

DESI MWS outlines a large-scale, inclusive spectroscopic survey targeting about seven million Milky Way stars to map the outer disk and halo and to probe dark matter via kinematics and chemistry. The program combines three main target classes (main-blue, main-red, main-broad) with high-priority subsamples (white dwarfs, nearby stars, RR Lyrae, and BHBs) and a backup program, all observed with DESI’s multi-arm spectrographs. A dedicated data pipeline (RVS, SP, WD) extracts radial velocities, atmospheric parameters, and abundances, with validation from the DESI survey validation (SV) campaigns showing radial velocities ~1 km/s and [Fe/H] precision ~0.2 dex in representative targets. The work demonstrates DESI’s capability to deliver a transformative, forward-modelable dataset for Galactic archaeology, structure, and stellar/planetary evolution, enabling robust comparisons with Gaia and external surveys and informing models of the Milky Way’s assembly and dark matter distribution.

Abstract

We describe the Milky Way Survey (MWS) that will be undertaken with the Dark Energy Spectroscopic Instrument (DESI) on the Mayall 4m telescope at the Kitt Peak National Observatory. Over the next 5 yr DESI MWS will observe approximately seven million stars at Galactic latitudes |b|>20 degrees, with an inclusive target selection scheme focused on the thick disk and stellar halo. MWS will also include several high-completeness samples of rare stellar types, including white dwarfs, low-mass stars within 100pc of the Sun, and horizontal branch stars. We summarize the potential of DESI to advance understanding of Galactic structure and stellar evolution. We introduce the final definitions of the main MWS target classes and estimate the number of stars in each class that will be observed. We describe our pipelines for deriving radial velocities, atmospheric parameters, and chemical abundances. We use ~500,000 spectra of unique stellar targets from the DESI Survey Validation program (SV) to demonstrate that our pipelines can measure radial velocities to ~1 km/s and [Fe/H] accurate to ~0.2 dex for typical stars in our main sample. We find the stellar parameter distributions from ~100 sq. deg of SV observations with >90% completeness on our main sample are in good agreement with expectations from mock catalogs and previous surveys.

Figures (25)

• Figure 1: The DESI MWS footprint. Gray lines indicate the approximate Galactic latitude limit of the survey, $b\pm20\hbox{$^\circ$}$. The density of MWS targets is shown in gray scale. Colored symbols indicate known Milky Way satellites (stars) and globular clusters (triangles). Points and tracks (colors given in the legend) show the four most prominent streams, as reported in the galstreams compilation galstreams_software: Sgr, represented by the Law_Majewski_2010 model; Palomar 5 PriceWhelan2019_pal5; GD 1 PriceWhelanBonaca2018_gd1 and Orphan Koposov2019. We also show the approximate extent of the Hercules--Aquila cloud as reported in galstreamsgrillmair_carlin2016. Many other less prominent streams and stellar overdensities are known in the MWS footprint mateu2022.
• Figure 2: Various types of white dwarf systems observed by DESI. Most white dwarfs have atmospheres dominated by hydrogen or helium, and their spectra contain only Balmer (DA) or He I (DB) lines. As the white dwarfs cool, they eventually cease to produce these lines, resulting in featureless spectra (DC). However, $\simeq20\%$ of white dwarfs exhibit spectroscopic peculiarities. Accretion of planetary material results in photospheric contamination by metals (DAZ, DBZ, and DZ). The presence of carbon in the atmospheres of white dwarfs can indicate the dredge up of material from the white dwarf core, or, in higher-mass white dwarfs, is an indication of a merger, possibly descending from R Corona Borealis stars (DQ). Up to 10% show magnetic fields (up to $10^{9}\,\mathrm{MG}$) via Zeeman splitting across all atmospheric compositions (e.g. DAH), and serve as laboratories for atomic physics under extreme conditions. Finally, white dwarf binaries span a wide range of evolutionary channels. One common type of white dwarf binary is a cataclysmic variable (CV), where a main-sequence companion accretes onto a white dwarf via an accretion disk that can be identified from double-peaked line profiles.
• Figure 3: Top panels: Density of MWS main sample targets at high Galactic latitude ($b > 70\,\deg$) in the space of $g - r$ color and proper-motion, in our broken-power-law halo Galaxia mock catalog (left) and the real MWS target catalog (right). The vertical line marks the color separation between the main-blue and main-red/main-broad samples. The horizontal lines show the separation in proper-motion between main-red and main-broad at $r=16$ (upper dotted line) and $r=19$ (lower dashed line). The main-sequence turnoff and giant branch of the globular cluster M15 are visible as density peaks at $\log_{10}\,| |/\mathrm{mas\,yr^{-1}}\sim0.4$. Bottom panels: The contribution of thick disk (left) and halo stars (right) to the density distribution in the Galaxia mock catalog.
• Figure 4: The color and proper-motion distribution of all MWS main sample targets, as shown in the top right panel of Fig. \ref{['fig:pm_color_4panel']}, here for the full MWS footprint (left). The middle and right panels show the separation of targets into main-blue/main-red (separated by $g-r=0.7$, vertical dashed line) and main-broad (separated according to the parallax and magnitude-dependent proper-motion criteria described in the text; the proper-motion cuts at the bright and faint limits of the sample are shown by horizontal lines).
• Figure 5: Number of MWS main sample target stars beyond a given heliocentric distance in our broken power-law variant of the Galaxia model (see text), for stars selected in the MWS footprint according to the main-blue, main-red and main-broad criteria. In the left panel, mock MWS targets are separated by Galaxia structural component (thin disk, thick disk and halo), and in the right panel they are separated by MWS target category. Profiles for the original single-power-law halo model sharma2011 are shown by dotted lines of the same color.