A bright star seen in the Southern Hemisphere reveals an ancient collision in the Milky Way

A bright star in the cluster, visible from the Southern Hemisphere, has revealed new insights into an ancient collision between our Milky Way and another, smaller dwarf galaxy, Gaia-Enceladus, early in our galaxy's history. An international team of scientists, led by the University of Birmingham, used a novel approach to apply the signature of an ancient bright star, called ν Indi, to the Milky Way's history. Stars carry a 'fossil record' of their history, and therefore of the environment in which they formed.

The team used satellite and ground-based telescope data to unlock this information about ν Indi, and their results are published in the journal Nature Astronomy. The star's natural oscillations (asteroseismology) were detected and aged in data collected by NASA's Transiting Exoplanet Survey Satellite (TESS) TESS. Launched in 2018, TESS is surveying stars across a large part of the sky to search for planets orbiting around them and to study the stars themselves. When combined with data from the European Space Agency's (ESA) Gaia mission, this detective story revealed the birth of this ancient star early in the life of the Milky Way.

But the collision changed its motion through our own galaxy, Bill Chaplin, Professor of Astrophysics at the University of Birmingham and lead author of the study, said: "Since ν Indi's motion was affected by the Gaia-Enceladus collision, the collision must have occurred after star formation. That's why we were able to use the age determined by asteroseismology to place new constraints on the timing of the Gaia-Enceladus event." Co-author Dr Ted MacLeith, also from Birmingham, said: "Because we see so many stars from Gaia-Enceladus, we think it must have had a big impact on the evolution of our own galaxy."

Understanding this is a very hot topic in astronomy right now, and this study is an important step towards understanding the timing of this collision. This study demonstrates the potential of TESS for asteroseismology, and what is possible when one has such a wide range of cutting-edge data about a bright star. This study clearly shows the strong potential of the TESS mission to gather rich new insights about the stars that are our closest neighbours in the Milky Way. This research was funded by the Science and Technology Facilities Council and the European Research Council through the Astronomical Timing programme.

Over the course of its history, the Milky Way has engulfed multiple smaller satellite galaxies, and while these accreted populations of stars can be identified as kinematically distinct structures within the Milky Way, it is generally difficult to accurately determine the age of any one merger. New results show that a large population of stars was accumulated through the collision of the Gaia-Enceladus 1 dwarf galaxy, resulting in a significant contamination of the chemical and dynamical properties of the Milky Way. This study identifies the very bright star ν Indi as an indicator of the age of the early "in situ population" of the Milky Way.

The study combined asteroseismology, spectroscopy, astrometry and kinematic observations to show that this metal-poor, alpha-rich star is a local member of the halo, and its age is measured to be \(11.0\pm 0.7\)(Stat)\(\pm 0.8\)(Sys) billion years. This star has features consistent with collisional motion heating, and its age means that the merger may have started as early as 11.6 billion years ago and 13.2 billion years ago, with 68% and 95% confidence, respectively. Calculations based on the hierarchical universe model slightly reduce the above restrictions.

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Bo Ke Yuan | Research/From: University of Birmingham

Reference journal Nature Astronomy

DOI: 10.1038/s41550-019-0975-9

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