The GALAH Survey: Scientific Motivation (1502.04767v1)

Abstract: The GALAH survey is a large high-resolution spectroscopic survey using the newly commissioned HERMES spectrograph on the Anglo-Australian Telescope. The HERMES spectrograph provides high-resolution (R ~28,000) spectra in four passbands for 392 stars simultaneously over a 2 degree field of view. The goal of the survey is to unravel the formation and evolutionary history of the Milky Way, using fossil remnants of ancient star formation events which have been disrupted and are now dispersed throughout the Galaxy. Chemical tagging seeks to identify such dispersed remnants solely from their common and unique chemical signatures; these groups are unidentifiable from their spatial, photometric or kinematic properties. To carry out chemical tagging, the GALAH survey will acquire spectra for a million stars down to V~14. The HERMES spectra of FGK stars contain absorption lines from 29 elements including light proton-capture elements, alpha-elements, odd-Z elements, iron-peak elements and n-capture elements from the light and heavy s-process and the r-process. This paper describes the motivation and planned execution of the GALAH survey, and presents some results on the first-light performance of HERMES.

• The paper presents high-resolution spectroscopy to chemically tag stars, unveiling the Milky Way’s formation and evolutionary processes.
• It employs the HERMES spectrograph on the Anglo-Australian Telescope to measure 29 elemental abundances from around one million FGK stars.
• Findings aim to reconstruct ancient stellar clusters and clarify the chemical and dynamical structure of the Galaxy.

An Examination of the GALAH Survey: Scientific Motivation and Objectives

The GALAH survey represents a pivotal initiative in the domain of Galactic archaeology, aiming to resolve the formation and evolutionary history of the Milky Way through extensive spectroscopic data collection. Utilizing the HERMES spectrograph on the Anglo-Australian Telescope, this survey proposes to identify chemically distinct groups within the Galaxy through a process called chemical tagging. This technique seeks to track the dispersed remnants of ancient star-forming clusters previously indistinguishable based on spatial or kinematic data alone.

Survey Design and Objectives

The primary aim of the GALAH survey is to conduct a high-resolution spectroscopic analysis of approximately one million stars, extending to brightness magnitudes of V~14. By focusing on FGK stars, known for their rich absorption features from elements ranging from light proton-capture elements to n-capture elements, the survey anticipates gathering comprehensive chemical signatures across 29 elements. This approach establishes the framework for chemical tagging, enabling the differentiation of stars from ancient aggregated structures based solely on their unique elemental fingerprints.

The GALAH survey's strategic scientific goals include:

• Exploration of Galactic Components: The survey dissects the Galaxy's primary components, such as the stellar halo, thin and thick disks, and the bulge. It explores their chemical and dynamical interconnectedness, aiming to unravel the fundamental processes that have shaped the Milky Way's structure.
• Chemical Tagging and Galactic Archaeology: The survey endeavors to assign stars to their historical star-forming entities, effectively reconstructing the proto-galaxy's assembly narrative. In addressing the relative homogeneity of clusters and deciphering relics of ancient accretion events, GALAH promises to provide critical insights into the evolutionary dynamics of the Milky Way.

Numerically Measurable Outcomes and Characteristics

The HERMES spectrograph operates with a resolving power of R~28,000, enabling the simultaneous capture of spectra from 392 stars across a 2-degree field. It is tailored to achieve a limiting magnitude sufficient to ensure a signal-to-noise ratio of 100 per resolution element, effective in fields configured with stars maintaining 12≤V≤14 magnitude. The output spectra encompass four non-overlapping passbands tailored to detect the rich elemental diversity required for chemical tagging.

Implications and Future Prospects

The results derived from GALAH could significantly enhance our understanding of the Galaxy's structural evolution by resolving the chemical substructures of its stellar population. These findings may offer pivotal insights into the dynamical processes, such as radial mixing, and their role in the present-day arrangement of the Galaxy's components.

Future directions entail leveraging the survey's capabilities to refine the initial cluster mass functions and dimensionality of C-space. As a symbiotic effort with missions like Gaia and K2, the integration of kinematic data, parallax, and proper motion measurements from these platforms will further enrich the GALAH dataset, furnishing a detailed portrait of the Galaxy's formative history over cosmic timescales.

In conclusion, the GALAH survey epitomizes a methodological advancement in Galactic studies, offering a robust foundation for subsequent generations of astronomical research aimed at dissecting the underlying formation principles of the Milky Way and analogous spiral galaxies. Its dataset is poised to be a legacy resource, sculpting future explorations and theoretical models in the domain of astrophysics.