The GALAH Survey

ABOUT THE GALAH SURVEY

The formation and evolution of galaxies is one of the great outstanding problems of modern astrophysics. The goal of galactic archaeology is to uncover the history of the Milky Way and how it formed and evolved. The GALactic Archaeology with HERMES (GALAH) survey is a Large Observing Program using the HERMES instrument with the Anglo-Australian Telescope of the Australian Astronomical Observatory. HERMES provides simultaneous spectra for 400 stars at a time. This will obtain the highest spectral resolution multi-dimensional datasets for over a million stars of all ages and locations in the Milky Way to trace the full history of the Galaxy.

By chemically tagging stars, the GALAH team, led by CI Joss Bland-Hawthorn, will track which stars formed together in the past, but are now spread throughout the galaxy. This chemical tagging provides an archeological 3D record of how the Milky Way dynamically formed its elements and mass throughout its 13.7 billion year history.

The Hermes Instrument

AAO HERMES spectrograph on the Anglo-Australian Telescope (credit: AAO)

The GALAH survey uses the High Efficiency and Resolution Multi-Element Spectrograph (HERMES). This instrument allows us to take detailed spectra of 392 objects at a time over two degrees of the sky. HERMES works in conjunction with the 2-degree field (2dF) positioner; this instrument places a fibre at a star’s location for the light to pass to the HERMES instrument. Both are located at the Anglo-Australian Telescope (AAT) located at the Siding Spring Observatory in Coonabarabran, NSW.

Changes with Time

When fossil relic stars are discovered, we need to measure their ages to develop a picture of the assembly of the Milky Way. By working with Gaia, K2, and CoRoT, we will be able to estimate accurate ages for the GALAH sample. We will then place these reconstructed stellar clusters into an evolutionary sequence, i.e. a family tree, based on their chemical signatures. This allows us to estimate a timeline of the Galaxy’s chemical and dynamical evolution, including its accretion history. Additionally, we can investigate the variation in the star formation rate and the cluster mass function with time. Previously, this was only possible for a small volume of stars in the solar neighbourhood, and this was plagued by small sample size.

An all-sky view of stars in the first Gaia catalogue. With a dot for each star, the map outlines the Milky Way (horizontally) and the Magellanic Clouds (lower right). The curved features are artifacts due to Gaia’s scanning procedure.

Image credit: ESA/Gaia/DPAC.

3D artist’s impression of the Milky Way, A13 and Triangulum-Andromeda

IMAGE CREDIT: T. Mueller/NASA/JPL-Caltech

Accretion history

GALAH focuses its observations on the Milky Way disk and bulge, which contain almost all of the stellar mass of the Galaxy. These components, in addition to evolving dynamically, may also be influenced by the infall of small evolved satellite systems. For example, the Galactic disk near the Sun shows some kinematical clumping or substructure. Some of these clumps are likely the debris of old disrupted star clusters in the disk, which are now dispersed into extended regions of the Galaxy. Others may be remnants of satellites cannibalized by the Milky Way.

Dynamics

The GALAH sample will be useful for examining the dominant dynamical processes that occur during Galaxy development. Without radial mixing, the dispersed debris from open clusters detected by GALAH would be on near-circular orbits and confined to a fairly narrow annulus around the Galaxy, reflecting their early dynamical properties. Using GALAH, we can examine the radial extent of the chemically-tagged debris of disrupted clusters; we will then have a direct measurement of how important radial mixing, and other dynamical processes, has been to Milky Way evolution.

Nucleosynthetic Processes

The large, homogeneously observed and analyzed GALAH stellar sample will help to separate the different nucleosynthetic processes that lie behind the chemical evolution of the Galaxy. By examining how the different elements relate to one another using principal component analysis (PCA), we can examine the structure of the different dominant nucleosynthetic processes. Furthermore, we can learn about the contributions of the various nucleosynthetic processes in each Galactic component, as a function of position, velocity, and orbital integrals.