Some Stars Around Galaxy May Be From Elsewhere

Data from the Gaia satellite reveal 20 new high-speed stars, 13 of which appear to have originated outside of the Milky Way.

Illustration of stars racing toward Milky Way

An illustration of 20 high-velocity stars sprinting through the Milky Way. Seven stars (red) are racing away from the Galaxy while 13 stars (orange) could be from other galaxies.
NASA / ESA/ Hubble / Marchetti et al. 2018

Last summer, researchers identified a handful of stars that had been kicked out of the center of our Galaxy, one of which seemed destined to escape the Milky Way. Now, the same team has found more runaway stars that might actually be escapees from a galaxy besides our own.

High velocity stars are of interest to astronomers for two primary reasons: the stars can indicate extreme interactions with supernovae and black holes, and they can help probe the gravitational field of the Galaxy by mapping the distribution of dark matter.

However, identifying fast-moving stars with respect to the billions of background stars is quite difficult. Stars in the Milky Way travel at a range of speeds depending on their location and any interactions with nearby massive objects. The majority of stars live in a thin disc or in a bulge around our Galaxy’s central supermassive black hole. Stars in the disc all rotate in the same direction while the orbits of stars in the bulge and halo can be randomly distributed.

Gaia

Artist's impression of Gaia mapping the stars of the Milky Way.
ESA / ATG medialab; background: ESO / S. Brunier

That’s where the Gaia satellite comes in. Launched in 2013, it is currently mapping the stars in our Galaxy with the goal of having precise measurements for over a billion of them by 2020. Gaia’s second data release was made public in April and consists of 1.7 billion stars, 7 million of which have proper motion, position, parallax, and radial velocity data. Using these 7 million as a starting point, Tomasso Marchetti (Leiden Observatory, Netherlands) and colleagues began their search for stars moving at speeds approaching the escape velocity of the Galaxy: roughly 400 – 600 km/s (about 900,000 – 1.3 million mph), depending on the star's location.

Of the 7 million stars in their sample, the team found 125 high-velocity stars that were moving fast enough to be more than 50% likely to escape the Milky Way. Of those 125, twenty of them were more than 80% likely to break the gravitational bonds of our Galaxy and become what are called unbound stars.

Running the clock backwards

To determine where these 20 unbound stars originated, the researchers used the current orbital parameters and then “ran the clock backwards” for up to 5 billion years (for reference, the Milky Way rotates once every 225 million – 250 million years). This is usually enough to ensure that if a star originated in the Milky Way, researchers will see its orbit intersect the galaxy.

Seven of the 20 stars can be traced back to the disc of our Galaxy. However, the surprise was that the other 13 seem not to have originated in the Milky Way at all. This subset of stars had greater than an 80% chance of being unbound and less than a 50% chance of intersecting the Galaxy in the past 5 billion years.

The paper reporting these results appeared online September 20th in Monthly Notices of the Royal Astronomical Society.

It’s impossible to say where these stars came from with any certainty without further study. One possible source is the Large Magellanic Cloud (LMC), a dwarf galaxy orbiting and being accreted by the Milky Way. If the stars came from the LMC, their existence could tell us more about the presence of black holes or the history of supernovae in our galactic neighbor. 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 would help narrow down their likely origins by determining their ages and chemical compositions.

Marchetti and colleagues plan to follow up with ground-based observations 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.

Reference:

T. Marchetti, E.M. Rossi, and A.G.A Brown. "Gaia DR2 in 6D: Searching for the Fastest Stars in the Galaxy.Monthly Notices of the Royal Astronomical Society. September 20, 2018.

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