Seismic measurements of an old, bright star improve estimates of when a dwarf galaxy merged with the Milky Way.
Our galaxy did not form in one piece, but grew by enveloping numerous smaller galaxies. Notably, recent evidence shows that the Milky Way devoured a relatively large galaxy, named Gaia-Enceladus, early in its history. This collision helped shape the Milky Way that we see today.
An international team of researchers led by William Chaplin at the University of Birmingham, UK, has narrowed estimates1 of when this merger occurred through a detailed study of a star called ν Indi, which is bright enough to be visible from Earth with the naked eye.
The team used spectroscopic data from several ground-based telescopes, and detailed measurements of ν Indi’s position and movement from the European Space Agency’s Gaia space observatory.
Most importantly, the researchers acquired asteroseismology data from NASA’s new Transiting Exoplanet Survey Satellite (TESS). Asteroseismology examines how stars oscillate – a behaviour shown by our Sun. These oscillations, which are formed by trapped waves, allow astronomers to determine the internal structure of a star.
“Asteroseismology is beginning to play an important role in studies of the evolution of the Galaxy, because it provides high-fidelity characterisations of individual stars,” says Chaplin.
The researchers combined all the data into stellar simulations, whose outputs suggested that ν Indi is 11 billion years old and most likely formed as a member of the existing Milky Way rather than Gaia-Enceladus. The simulations also showed that its velocity increased due to the galaxies merging. Because ν Indi was clearly affected by the merger, and the merger was assumed to take around two billion years, the researchers concluded that the merger itself likely began no earlier than 11.6 billion years ago.
“Our study offers a novel approach where the forensic characterisation of a bright, individual star may be used to probe the evolution of populations of stars in our galaxy,” says Chaplin. “It highlights the excellent potential of TESS, which is targeting the brightest stars in the sky.”
Chaplin praises the diverse expertise of his co-authors, including those from New York University Abu Dhabi who played an important role in extracting the oscillation frequencies of ν Indi from the TESS data. The team plans to extend the work to other old, bright stars to reveal further details of this crucial event in our galaxy’s history.
Carme Gallart at Instituto de Astrofísica de Canarias, Spain led a different study2 in 2019 identifying the stars that remain from the Gaia-Enceladus collision. “The accurate asteroseismic age for ν Indi places a tight new constraint on the earliest time at which the merger could have occurred,” she says. “This shows the great prospects offered by asteroseismology, not only for the study of stellar interiors, but also for Galactic exploration.”