The GALAH Survey has been working for almost a decade to reveal the rich history of our Milky Way Galaxy by studying the current positions, motions, ages and chemistry of stars formed at different times and in different locations. This page summarizes some of the major discoveries that the GALAH Survey has enabled.
- Key results
- Data Releases
Data from the GALAH Survey has been used in over 170 publications, with these publications receiving nearly 2500 citations. Check out the full list of publications from the GALAH team and other astronomers working with GALAH data.
Below we highlight a few of the interesting science results found using GALAH.
Dissection of the stellar discs
Some major science highlights include the detailed dissection of the stellar discs in terms of age, velocity, action, and location with GALAH chemistry by Quillen et al. (2018) and Buder et al. (2019). GALAH performed the first detailed elemental abundance follow-up of Gaia’s discovery of the “phase spiral” perturbation in the local disc. This is a spiral pattern seen in phase space due to the Sagittarius dwarf’s interaction with the Galaxy.
GALAH has found evidence for differing abundance trends between the thin and thick disc as well as with metallicity and age (Buder et al. 2018, Duong et al. 2018). In Hayden et al. (2020), we showed that the velocity dispersions of stars varied smoothly with a function of chemistry and that there was no dramatic kinematic discontinuity between the chemical thin and thick disks; additionally it was found that nearly 40% of stars in the solar vicinity have metallicities higher than the local ISM and that radial migration is an important process in the evolution of disk galaxies. Khanna et al. (2019) showed that the velocity fluctuations in the Galaxy are quite small, contradicting previous measurements, and this has important consequences for determining the local standard of rest. In Sharma et al. (2020a,b), we deduced the fundamental relations governing the velocity dispersion of stars in terms of age, angular momentum, and metallicity. Bennett & Bovy (2019) in part used GALAH radial velocities to measure waves in the vertical density and velocities of stars near the Sun. Belokurov et al. (2020) used GALAH data to map out the properties of the principle Galactic components such as the ‘thin’ and ‘thick’ discs and the halo.
The accreted halo
The Gaia mission has enabled the remarkable discovery that a large fraction of the stars near the Solar neighbourhood are debris from one or more accretion events. The radial velocities and abundances results from GALAH has been at the forefront of helping to untangle the complicated kinematic structure of the halo.
At the small scale of stellar streams, GALAH has been used to chemically tagged the Fimbulthul stellar stream to the massive globular cluster ω Centauri; and explore the orbital and abundance properties of the Helmi streams. At the large scale, the GALAH r-process element measurements show that accreted stars from Gaia-Enceladus clearly show higher ratios of [Eu/Mg] than in-situ stars.
GALAH made the first demonstration of blind cluster chemical tagging (Kos et al. 2018a, Simpson et al. 2019) as well as disproving the reality of several high-latitude open clusters whose origin was a mystery (Kos et al. 2018b). Further, GALAH championed the use of unsupervised classification with t-SNE to detect peculiar objects using stellar spectra (Traven et al. 2017).
The ability to measure the lithium abundance of stars is a key capability of the GALAH survey. DR3 contains reliable lithium abundances for over 100000 stars all across the parameter space. Many researchers have used GALAH to investigate too long-standing problems related to lithium: dwarf stars with too little lithium; and giant stars with too much lithium.
The Spite Plateau is the observation that metal-poor halo dwarfs all have about the same lithium abundance irrespective of their metallicity. Initially thought to represent the primordial lithium abundance, Big Bang Nucleosynthesis models now predict a primordial lithium abundance 3 to 4 times higher than the Spite Plateau value. This discrepancy is known as the cosmological lithium problem. The GALAH team found that warmer stars show a lithium abundance actually consistent with the primordial value — a conclusion only possible with the large dataset of GALAH. Researchers both inside and outside of the GALAH team investigated the lithium abudance of accreted dwwarf stars, finding them to exhibit the same Spite Plateau as likely in situ stars. This shows that the cosmological lithium problem is not the result of some galactic formation environment effect.
Another lithium mystery are lithium-rich giant stars. The canonical model of stellar evolution is that giant stars will be very efficient at destroying lithium at their stellar surfaces — but about one per cent of giant stars are found to have very high lithium abundances. The large stellar survey revolution has changed the the picture of lithium-rich giants. Censuses of lithium-rich giants, including by the GALAH team have found that red clump stars are 2.5 times as likely to be lithium-rich as red giant branch stars. In fact, there appears to be a more general relationship between the core-helium burning stage of stellar evolution and lithium enrichment: these core-helium burning should have little-to-no-observable lithium, but several papers has shown with GALAH that there is ubiquitous lithium production in low-mass stars.
Calibrating other surveys
As a large catalogue of radial velocies, stellar parameters, and elemental abundances, the GALAH Survey is crucial as a cross-check and calibration of other large surveys. We delivered the largest high quality catalogue of radial velocities of its time, reaching an impressive precision of 0.1 km/s (an order of magnitude better than Gaia DR2 for stars at the same magnitude). GALAH forms part of the TESS Input Catalog (Stassun et al. 2018), the StarHorse Catalog (Queiroz et al. 2018). It was used by the RAVE, Gaia, and SkyMapper teams to help produce their catalogues.
The GALAH Survey has had three data releases, each aimed to leverage data releases from the Gaia mission.
Data Release 1
The GALAH survey published our First Data Release on 9 September 2016. It contains data for 9860 stars that were likely to be included in the first Gaia data release as part of its TGAS data set. It contained stellar parameters (Teff, log g, [Fe/H], [α/Fe]), radial velocity, distances and reddening. Martel et al. (2017), which describes DR1 has been cited over 100 times.
Data Release 2
The GALAH survey published our Second Data Release on 18 April 2018. It contains data for 342,682 stars with up to 23 elemental abundances per star. Buder et al. (2018), which describes DR2 has been cited nearly 200 times, and is higlighted by Monthly Notices of the Royal Astronomical Society in their collection of highly-cited papers that showcase the breadth of top-quality research published by MNRAS.
The GALAH survey published our Third Data Release on 6 November 2020. GALAH DR3 contained stellar parameters and elemental abundances from 678,423 spectra for 588,571 stars that have been observed with the HERMES spectrograph at the Anglo-Australian Telescope between November 2013 and February 2019. DR3 was the first GALAH data release to include reduced one-dimensional spectra for the stars. There are two main catalogues that contain stellar parameters and elemental abundances for up to 30 elements per star: Li, C, O, Na, Mg, Al, Si, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Rb, Sr, Y, Zr, Mo, Ru, Ba, La, Ce, Nd, Sm, and Eu. Buder et al. (2021), which describes DR3 already has over 30 citations.