My first Magazine Sky & Telescope - 02.2019

MILKY WAY Ancient Merger Wreckage in the Milky Way WE LIVE IN A BIG disk galaxy, a whirligig pancake that’s enshrouded in a halo of old stars. And increasingly, astronomers suspect that very early in our cosmic pancake’s history, a colli- sion messed up the serene stellar disk and donated the detritus that makes up much of the halo. Reporting in the November 1st Nature, Amina Helmi (University of Groningen, The Netherlands) and col- leagues conﬁrm that a previously noted horde of nearby stars in the halo is quite unusual. These stars rotate around the galactic center in the opposite, or retrograde, direction as the disk does. They also have different chemical compositions than those in the disk. The strange characteristics suggest that the stars aren’t indigenous to the Milky Way — rather, they’re probably crumbs from when our galaxy ate a galactic snack very early in its history. Helmi and colleagues took a closer look at the retrograde stars’ motions and compositions and noted three char- acteristics. First, the stars move together as a big unit. Second, their heavy- element levels suggest these stars didn’t all form in a single burst but over an extended period. Third, the stars have a range of ages. Taken together, these quirks set the stars apart from those born in the Milky Way, implying that they came from a cannibalized galaxy roughly 600 million times as massive as the Sun — about the same mass as the Small Magellanic Cloud. Simulations also conﬁ rm that a merger with such a gal- axy roughly 10 billion years ago could explain the stars’ properties. So many are the retrograde stars (about 30,000), that they form a huge swarm around the disk for at least thousands of light-years around the Sun. Helmi’s team estimates that roughly 80% of our galaxy’s halo could MILKY WAY Some Stars May Be Vis- iting From Elsewhere SOME HIGH-SPEED STARS in the Milky Way might actually be escapees from another galaxy. Speedy stars are useful indicators of extreme interactions with super- novae and black holes, as well as for probing the gravitational ﬁ eld of the galaxy. Combing through data from the European Gaia satellite, which is mapping the positions and speeds of over a billion stars in and around the Milky Way, Tomasso Marchetti (Leiden Observatory, The Netherlands) and col- leagues recently identiﬁ ed 20 stars that are moving so fast, they are more than 80% likely to break the gravitational bonds of our galaxy. To determine where these 20 stars originated, the researchers used the current orbits to follow their trajecto- ries backwards for up to 5 billion years. p Position and trajectories of 20 high-speed stars are shown in this illustration. Seven stars (red) are leaving our galaxy while the rest (orange) are coming toward the Milky Way, pos- sibly from another galaxy. Seven of the 20 stars can be traced back to the disk of our galaxy. However, the other 13 seem not to have originated in the Milky Way at all. The team reports its results in an upcoming Monthly Notices of the Royal Astronomical Society. It’s impossible to say where these stars came from with any certainty. p A computer simulation illustrates the posi- tions and motions (yellow arrows) of stars after a putative merger between the Milky Way and another smaller galaxy. be from this single collision. The merger would have also puffed up the galactic disk that existed at that time, creating the relatively thick disk we have today. ■ CAMILLE M. CARLISLE • For a video simulation of the merger, see https://is.gd/mwmerger. One possible source is one of our galac- tic neighbors, the Large Magellanic Cloud. If the stars came from there, their existence could tell us more about the presence of black holes or the his- tory of supernovae in that tiny galaxy. Alternatively, the stars could belong to the outer reaches of our own galactic halo, thrown inward by gravitational interactions with smaller galaxies eaten by the Milky Way long ago. Additional spectral data for these stars could help narrow down their likely origins by determining their ages and chemical compositions. Marchetti and colleagues plan to follow up with ground-based observa- tions in the near future, but they are also continuing to explore Gaia’s data for additional ways of identifying high- velocity stars. They hope to apply their methods on the complete set of 150 million stars with full 3D-velocity data expected in 2020. ■ SUMMER ASH sk yandtele scope.com • FE B RUA RY 2 019 13

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