New observations show ancient stars lining the edges of the ultra-faint dwarf galaxy Tucana II, a tiny satellite of the Milky Way.
Astronomers have discovered that a tiny galaxy, a satellite of our Milky Way, houses some of the most ancient known stars and is held together by a massive, extended dark matter halo. The results appeared in this week's Nature Astronomy.
Astronomers first discovered Tucana II, a very faint dwarf galaxy about 163,000 light-years from Earth, in 2015. Since then, scientists have found that the galaxy houses some of the oldest stars known, based on their composition. “Those stars formed early on in the universe, when iron and other heavy elements were distinctly not as present,” explains graduate student Anirudh Chiti (MIT), who led the report.
Because universe began with mostly hydrogen and helium, heavier elements — what astronomers call metals — first had to be forged in stars. Therefore, the older the star, the fewer metals it has, and the stars first observed in Tucana II had iron abundances on average about 1/500 of that found in the Sun.
Chiti’s advisor, Anna Frebel (MIT), has made a career of hunting for the most ancient stars in the universe — and Tucana II was an obvious place to look. Chiti, Frebel, and colleagues used Australia’s SkyMapper Telescope to obtain images that contained 10 stars previously identified as belonging to Tucana II. But running these images through a new filter, the team found nine new stars with low metal content, indicating that these also belong to Tucana II.
Using the Magellan telescope in Chile, Chiti obtained spectra of the newly identified stars both to confirm their association with the galaxy and to measure their composition. The data showed that the stars’ metal content was even lower — less than one-thousandth the Sun's iron abundance — which indicates that these stars are even more ancient.
Intriguingly, while four of the newly discovered stars were found in Tucana II’s core, five were found on the fringe, as far as 3,600 light-years from the core. And the stars in the core actually have a slightly higher metal content than those on the edge — in other words, the oldest stars hang out on the dwarf’s outer rim.
Chiti and colleagues suggest that this might have happened if Tucana II might have formed from a merger. When two even smaller galaxies came together, the stars of the smaller and older galaxy would have been flung to the outskirts.
John Wise (Georgia Institute of Technology), who was not involved in the study, agrees with the proposed scenario. “The observations suggest galaxy mergers, but no more than a handful, occurred in its distant past, implying that even smaller galaxies were forming in the early universe,” he says.
Another explanation is that supernovae in the core of young Tucana II spewed out heavier elements, allowing for slightly more metal-enhanced stars to form in the center of the galaxy, while stars on the fringe were still born with fewer metals.
Because of its small size, the inner region of the galaxy itself does not have enough gravity to hold the outer stars close to the core. Scientists have long known that dark matter must be responsible for holding together even tiny galaxies, but this discovery is the first direct evidence of a massive, spatially extended dark matter halo in a tiny relic galaxy like Tucana II.
Timothy Beers (Notre Dame University), who was not involved in the study, is also enthusiastic about its results. “This is an exciting new study that expands our knowledge of the nature of the ultra-faint dwarf galaxies and opens new avenues for further detailed work,” he says.