LAMOST Experiment for Galactic Understanding and Exploration (LEGUE) The survey science plan

Abstract: We describe the current plans for a spectroscopic survey of millions of stars in the Milky Way galaxy using the Guo Shou Jing Telescope (GSJT, formerly the Large Area Multi-Object Spectroscopic Telescope - LAMOST). The survey will obtain spectra for 2.5 million stars brighter than $r<19$ during dark/grey time, and 5 million stars brighter than $r<17$ or $J<16$ on nights that are moonlit or have low transparency. The survey will begin in fall of 2012, and will run for at least four years. The telescope design constrains the optimal declination range for observations to $10^\circ<\delta<50^\circ$, and site conditions lead to an emphasis on stars in the direction of the Galactic anticenter. The survey is divided into three parts with different target selection strategies: disk, anticenter, and spheroid. The resulting dataset will be used to study the merger history of the Milky Way, the substructure and evolution of the disks, the nature of the first generation of stars through identification of the lowest metallicity stars, and star formation through study of open clusters and the OB associations. Detailed design of the LEGUE survey will be completed after a review of the results of the pilot survey in summer 2012.

• The paper presents the comprehensive LEGUE survey strategy using LAMOST to obtain millions of stellar spectra across the Galactic spheroid, anticenter, and disk.
• It details the methodological framework with precise target selection, observational planning using 4000 fibers, and multi-visit strategies to optimize spatial resolution.
• The study’s findings refine our understanding of Galactic formation, dynamics, and chemical evolution, setting the stage for future collaborations with missions like Gaia.

LAMOST Experiment for Galactic Understanding and Exploration (LEGUE): The Survey Science Plan

The LAMOST Experiment for Galactic Understanding and Exploration (LEGUE) represents an ambitious astronomical initiative aimed at performing an extensive spectroscopic survey of millions of stars in the Milky Way using the Guo Shou Jing Telescope (formerly known as LAMOST). The primary objective of LEGUE is to achieve a comprehensive understanding of the structure, kinematics, and chemical composition of the Milky Way's different stellar components – namely, the spheroid, disk, and anticenter regions. This spectroscopic endeavor aims to provide critical insights into the formation and evolution history of our Galaxy.

Survey Design and Execution

The survey is divided into three primary components, each targeting distinct regions of the Galaxy:

1. Spheroid Survey: This component will focus on obtaining spectra for at least 2.5 million stars in high Galactic latitude regions, leveraging data from SDSS imaging to perform weighted random sampling across various stellar types.
2. Anticenter Survey: Targeting the Galactic anticenter, this survey aims to analyze the significant volume of the thin and thick disk systems. Approximately 3.7 million stars are expected to be covered within the anticenter region, providing a deeper understanding of these disk components.
3. Disk Survey: Encompassing regions with low Galactic latitude, this component intends to explore structures within the dense stellar formations of the Galactic plane, including open clusters. This survey takes advantage of the bright screening criteria and uses overlap with the anticenter survey in certain regions.

Survey Implementation

Observational constraints, such as weather conditions and specific telescope capabilities, dictate the operational strategy. Observations are planned to cover optimal declinations and right ascensions that align with favorable meteorological and site conditions. Various observational priorities are set based on the galactic coordinate, focusing primarily on the anticenter direction, which enjoys better weather conditions, especially during winter months.

LEGUE employs LAMOST's capability to place as many as 4000 fibers on specified targets within a 20 $\mathrm{deg}^2$ sky field, achieving a fiber density comparable to that of SEGUE and capitalizing on multi-visit strategies to enhance spatial resolution in crowded areas such as the Galactic disk.

Science Goals

Key scientific objectives driving the LEGUE initiative encompass:

• Substructure and Merger History: Analyzing Galactic substructures to understand the Milky Way's assembly through mergers and accretion events.
• Stellar Populations and Metallicity Distribution: Mapping the chemical composition of stars over various galactic scales to unravel the Milky Way's chemical evolution.
• Galactic Disk Dynamics: Investigating the dynamic processes affecting the Galactic disks, including star formation sites, young stellar objects, and open clusters.
• Spheroidal and Halo Investigations: Characterizing spheroidal components and searching for halo streams using radial velocity data.
• Metal-poor Stars: Detection of highly metal-poor stars, including extremely and ultra-metal-poor varieties, to serve as cosmic probes of early Galaxy conditions and stellar formation.

Implications and Future Directions

The comprehensive nature of the LEGUE survey has profound implications both in terms of raw data yield and broader theoretical insights into Galactic astrophysics. The data collected will provide an unprecedented level of detail to help refine models of Galactic dynamics, star formation, and chemical evolution. Going forward, the LEGUE project could set the stage for collaborating with future missions like Gaia, leveraging combined datasets to enhance our understanding of the Milky Way’s vast and complex structure.

The LEGUE project's outcomes will undeniably enrich the astronomical community with actionable datasets, fostering advancements in understanding the dynamic and chemical state of our Galaxy and setting benchmark standards for interpreting other galactic systems.