Our Tiny Galactic Neighbor Hosts Ancient Stars – Sky & Telescope

Astronomers have found {that a} tiny galaxy, a satellite tv for pc of our Method, homes a few of the most historic identified stars and is held collectively by a large, prolonged darkish matter . The outcomes appeared this week’s Nature Astronomy.

Tucana II
A number of the stars on this picture belong to Tucana II, an ultra-faint dwarf galaxy 163,000 light-years away.
Anirudh Chiti/MIT

Astronomers first found Tucana II, a really faint dwarf galaxy about 163,000 light-years from Earth, in 2015. Since then, scientists have discovered that the galaxy homes a few of the oldest stars identified, primarily based on their composition. “These stars fashioned early on within the universe, when iron and different heavy components had been distinctly not as current,” explains graduate pupil Anirudh Chiti (MIT), who led the report.

As a result of universe started with largely hydrogen and helium, heavier components — what astronomers name metals — first needed to be cast in stars. Subsequently, the the star, the less metals it has, and the celebs first noticed in Tucana II had iron abundances on common about 1/500 of that discovered within the Solar.

Chiti’s advisor, Anna Frebel (MIT), has made a profession of looking for probably the most historic stars within the universe — and Tucana II was an apparent place to look. Chiti, Frebel, and colleagues used Australia’s SkyMapper Telescope to acquire that contained 10 stars beforehand recognized as belonging to Tucana II. However working these photographs via a brand new , the crew discovered 9 new stars with low steel content material, indicating that these additionally belong to Tucana II.  

Utilizing the Magellan telescope in Chile, Chiti obtained spectra of the newly recognized stars each to verify their affiliation with the galaxy and to measure their composition. The information confirmed that the celebs’ steel content material was even decrease — lower than one-thousandth the Solar’s iron abundance — which signifies that these stars are much more historic.

Intriguingly, whereas 4 of the newly found stars had been present in Tucana II’s core, 5 had been discovered on the perimeter, so far as 3,600 light-years from the core. And the celebs within the core even have a barely steel content material than these on the sting — in different phrases, the oldest stars hang around on the dwarf’s outer rim.

Chiti and colleagues counsel that this might need occurred if Tucana II might need fashioned from a merger. When two even smaller galaxies got here collectively, the celebs of the smaller and older galaxy would have been flung to the outskirts.

John Sensible (Georgia Institute of Know-how), who was not concerned within the examine, agrees with the proposed situation. “The observations counsel galaxy mergers, however not more than a handful, occurred in its previous, implying that even smaller galaxies had been forming within the early universe,” he says.

One other rationalization is that supernovae within the core of younger Tucana II spewed out heavier components, permitting for barely extra metal-enhanced stars to type within the middle of the galaxy, whereas stars on the perimeter had been nonetheless born with fewer metals.  

Tucana II-like galaxy in simulation
This simulation body reveals a dwarf galaxy like Tucana II. The simulated darkish matter halo is proven as a shaded background and the galaxy’s stars seem as shiny dots. Prior to now, astronomers have solely been in a position to see the denser clump of stars within the galaxy’s middle. However because of new methods, Anirudh Chiti (MIT) and colleagues detected the spray of stars on the sting of the simulation — the farthest out that they might count on to straight measure darkish matter.
Coral Wheeler et al. / Monthly Notices of the Royal Astronomical Society 2019

Due to its small measurement, the internal of the galaxy itself doesn’t have sufficient gravity to carry the outer stars near the core. Scientists have lengthy identified that darkish matter have to be accountable for holding collectively even tiny galaxies, however this discovery is the primary direct proof of a large, spatially prolonged darkish matter halo in a tiny relic galaxy like Tucana II.

Timothy Beers (Notre Dame College), who was not concerned within the examine, can be obsessed with its outcomes. “That is an thrilling new examine that expands our information of the character of the ultra-faint dwarf galaxies and opens new avenues for additional detailed work,” he says.


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