Halo

The Milky Way’s metal-poor stellar halo is mostly built up from the ancient accretion of dwarf galaxies. Studying the chemical abundances of these stars provides insights to the Milky Way’s accretion history, the formation and evolution of dwarf galaxies, first galaxies during the epoch of reionization, and early nucleosynthesis. Mapping the kinematics of these stars can recover the signature of individual accretion events and constrain the shape of the gravitational potential of the Milky Way, including the dark matter halo and the interaction with the Large Magellanic Cloud.

Instead of a random sample of halo stars, SDSS-V is targeting three extreme regions of the halo parameter space:

1. the distant halo, targeting tracers that reach to 30 kpc and beyond,
2. the metal-poor halo, selecting red giant stars likely to have [Fe/H] < -2, and
3. the local halo, selecting stars within 2 kpc and with tangential
   velocities > 150 km/s, which mostly consist of low-mass (K/M)
   main-sequence stars from the Milky Way inner halo.

Starting in DR20 (targetdb version 1.0 and later), the halo program was converted from a spare fiber program to a “shell” carton: it does not control targeting, but halo targets are observed with higher priority than before. DR20 thus represents a substantial increase in the number of halo targets. Details about the distant and metal-poor programs are described in Chandra et al. (2026) . A description of the Local Halo is in the DR20 paper.

Because the halo stars are chosen to be rare outliers, it is highly recommended to use the Value Added Catalogs to study halo stars. We recommend using the MINESweeper VAC for the distant and metal-poor halo sub-programs. Filtering the BOSS spAll file for Gaia sources with the kinematic cut or the appropriate cartons with sdss_semaphore is the best way to get the local halo sample. There are no great spectroscopic stellar parameter catalogs for the local halo stars (the dominant population is metal-poor K and M dwarfs), but the BOSS-CLAM VAC provides a reasonable start.

Cartons

Cadences

During plate operations, mwm_halo_bb and mwm_halo_sm targets could be observed with BOSS at low priority on any plate except those with the RM cadence.

In the FPS era, mwm_halo_bb_boss and mwm_halo_sm_boss targets were taken with the low-resolution optical BOSS spectrograph with the bright_1x1 cadence.

For all other cartons, each halo carton was provided three different times at different priorities for one (*_boss_single), two (*_boss), and three (*_boss_triple) exposures (and the same for APOGEE) for maximum flexibility. Three exposures corresponds to the same target being visited 3 times for a single SPIDERS visit, which is why we stopped there. The targets are identical for each cadence, and it is recommended to select on the *_single cartons.