10 AUSTRALIAN SKY & TELESCOPE February | March 2019
WELIVEINABIGdisk galaxy, a whirligig
pancakethat’senshroudedinahaloof
oldstars.Andincreasingly,astronomers
suspectthatveryearlyinourcosmic
pancake’shistory,acollisionmessedup
theserenestellardiskanddonatedthe
detritusthatmakesupmuchofthehalo.WAcomputersimulationillustratesthepositionsandmotions(yellowarrows)ofstarsaftera
putativemergerbetweentheMilkyWayandanothersmallergalaxy.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.
Seven of the 20 stars can be traced back
to the disk of our galaxy. However, the
other 13 seem not to have originatedSPosition 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.ReportingintheNovember
issue of the journalNature,Amina
Helmi (University of Groningen, The
Netherlands) and colleagues confirm
thatapreviouslynotedhordeofnearby
starsinthehaloisquiteunusual.
These stars rotate around the galactic
centre in the opposite, orretrograde,
directioncomparedtootherstarsinthe
disk.Theyalsohavedifferentchemical
compositions to those in the disk. The
strange characteristics suggest that
these stars aren’t indigenous to the
MilkyWay—rather,they’reprobably
crumbsfromwhenourgalaxyatea
galactic snack very early in its history.
Helmiandcolleaguestookacloser
lookattheretrogradestars’motions
and compositions and noted three
characteristics. First, the stars move
togetherasabigunit.Second,their
heavy-element levels suggest they didn’t
allforminasingleburstbutoveran
extendedperiod.Third,thestarshaveaSome stars may be intergalactic visitors
in the Milky Way at all. The team will
report its results in an upcoming issue
of the Monthly Notices of the Royal
Astronomical Society.
It’s impossible to say where these
stars came from with any certainty.
One possible source is one of our
galactic neighbours, 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, butthey 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 ASHrangeofages.
Takentogether,thesequirksset
thesestarsapartfromthoseborninthe
MilkyWay,implyingthattheycame
fromacannibalisedgalaxyroughly
600milliontimesasmassiveasthe
Sun—aboutthesamemassasthe
SmallMagellanicCloud.Simulations
alsoconfirmthatamergerwithsuch
agalaxyroughly10billionyearsago
couldexplainthestars’properties.
So many are the retrograde stars
(about 30,000), that they form a huge
swarmaroundthediskforatleast
thousandsoflight-yearsaroundthe
Sun.Helmi’steamestimatesthat
roughly80%ofourgalaxy’shalocould
be from this single collision. The merger
wouldhavealsopuffedupthegalactic
diskthatexistedatthattime,creating
the relatively thick disk we have today.
■CAMILLE M. CARLISLE- Foravideosimulationofthemerger,
seehttps://is.gd/mwmerger.
MERGER: COMPOSITION: ESA; SIMULATION: KOPPELMAN, VILLALOBOS AND HELMI; GALAXY IMAGE: NASA / ESA / HUBBLE CC BY-SA 3.0 IGO; UNBOUND STARS: COMPOSITION: ESA, STARS POSITIONS AND TRAJECTORIES: MARCHETTI ET AL. 2018, BACKGROUND GALAXIES NASA / ESA / HUBBLE / CC BY-SA 3.0 IGO
Ancient merger wreckage
in the Milky WayNEWS NOTES