Sky & Telescope - February 2019 - 13

MILKY WAY

Ancient Merger Wreckage in the Milky Way
WE LIVE IN A BIG disk galaxy, a

MERG ER: CO MPOSITION: ESA; SIM UL ATION: KOPPELM A N, VILL A LOBOS A ND HELMI; G A L A X Y IM AG E: N ASA / ESA / HUBBLE
CC BY-SA 3.0 IGO; UNBOUND STA RS: CO MPOSITION: ESA , STA RS POSITIONS A ND TR A JECTORIES: M A RCHE T TI E T A L. 2018,
BACKG ROUND G A L A XIES N ASA / ESA / HUBBLE / CC BY-SA 3.0 IGO

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 collision 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 colleagues 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 characteristics. First, the stars move together
as a big unit. Second, their heavyelement 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
conﬁrm that a merger with such a galaxy 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 Visiting 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 supernovae and black holes, as well as for
probing the gravitational ﬁeld 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 colleagues recently identiﬁed 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 trajectories 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, possibly 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 positions 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 galactic 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 history 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 observations in the near future, but they are
also continuing to explore Gaia's data
for additional ways of identifying highvelocity 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

https://www.is.gd/mwmerger http://www.skyandtelescope.com

Sky & Telescope - February 2019

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