Playing detective on a galactic scale

Huge new dataset will solve Milky Way mysteries.

Australian Telescope
Night and day at the Anglo Australian Telescope. Credit: Ángel R. López-Sánchez / Australian Astronomical Optics / Macquarie University / ASTRO 3D

An Australian-led team of “galactic archaeologists” has just released the largest set of stellar chemical data ever compiled, containing information from 600,000 stars.

This new dataset will help astronomers solve many questions about the structure and evolution of the Milky Way, unravelling mysteries about star formation, chemical enrichment, migrations and galaxy mergers. 

The 500GB of data is the result of 342 nights of observing over the last seven years by HERMES, a spectrograph attached to the Anglo Australian Telescope (AAT) in rural New South Wales. HERMES can collect light from more than 300 stars at once and separate out their light into spectra, from which astronomers discern the unique “fingerprints” of the chemical elements within the star.

“It’s a bit like a galactic version of the game Cluedo,” says Sven Buder, an astrophysicist from the Australian National University and a member of the Galactic Archaeology with HERMES (GALAH) collaboration.

“The chemical information we’ve gathered is rather like stellar DNA – we can use it to tell where each star has come from. We can also determine their ages and movements and furnish a deeper understanding of how the Milky Way evolved.”

Sarah Martell, another member of the collaboration from UNSW Sydney, explains that the chemical abundance patterns of stars can also “tell us what has been happening in the galaxy before they formed – were there supernova explosions? What elements have previous generations of stars been able to eject into the environment? Has fresh gas fallen in from outside the Milky Way?”

The dataset is the third to be released by the GALAH project. The previous observations, released in 2018, fuelled research into areas ranging from the evolution of the Milky Way to the existence of exoplanets. The project anticipates observing another 250,000 stars before the end of 2022, shooting for one million in total.

Most of the stars observed were within 10,000 light-years of the Sun – a conscious choice by the team in order to delve into the nitty-gritty details of our own stellar neighbourhood.

“There are good reasons to think that our side of the galaxy is very similar to the far side in terms of stellar ages, orbits, and chemical compositions,” Martell says, “so a thorough dataset in the solar neighbourhood lets us make a lot of inferences about the rest of the galaxy.”

Interestingly, the team found that more than 20,000 of these nearby stars don’t share the same age or composition as our Sun and many of its neighbours – because they’re interlopers from another galaxy.

“We know that roughly eight billion years ago the shape of the Milky Way changed drastically when it collided with another, smaller galaxy, which contained millions of stars,” Buder explains.

“We’ve now used the stellar DNA to identify some of the prime suspects for the assault. These stowaways are so different they can only have come from somewhere else.”

This new collection of stellar information may also answer a puzzle that has plagued astronomers for years: the cosmological lithium problem.

Lithium was forged in the hot, dense conditions of the early Universe, but according to standard models of the Big Bang, we should observe a lot more of this element than we actually do.

“Basically, a lot of the oldest stars have burned much of the Big Bang lithium, so our measurements for this element come out lower than the amount that was initially synthesised in the early Universe,” says Sanjib Sharma, another collaborator from ASTRO 3D and the University of Sydney.

“We have found that one type of star, known as evolved giants, should have burned through pretty much all of their lithium by now, but a lot of them have much more of it than we expected. The GALAH data will help us discover why.”

The massive dataset is now freely available for astronomers around the world to use in their own research, and it will be particularly useful in conjunction with other complementary datasets.

“GALAH is just one of several surveys going on at the moment that are collecting chemical abundance data on the scale of hundreds of thousands to a million stars, and there are great synergies between those projects,” Martell explains.

GALAH thoroughly samples the Milky Way, while the APOGEE survey, for example, uses infrared to peer into the dusty plane of the galaxy, and the LAMOST survey can observe much more distant stars.

“We are working on clever ways to combine the datasets from the different surveys to build a comprehensive catalogue for the Milky Way,” says Martell.

Source: Cosmos Magazine