6 AUSTRALIAN SKY & TELESCOPE April 2019
NEWS NOTESPICTURE A SPIRAL GALAXY and
you probably see something like most
artist illustrations of the Milky Way:
a top-down view of a pinwheel. But
from the side, these disk galaxies appear
drastically different. For one, they’re
much thinner than they are wide —
the typical aspect ratio is like that of a
pancake. And, like pancakes wobbling
as they flip through the air, these
galaxies are often warped.
In 1957 radio observations first
revealed a warp in the Milky Way’s
disk of hydrogen gas, its reservoir of
star-making materials. Since then
astronomers have seen hints of this
warp in other sources, including
galactic pulsars, neighbourhood stars,
and even in the cosmic dust floating
throughout our galaxy.
Now, Xiaodian Chen (Chinese
Academy of Sciences) and his
colleagues, including Professor Richard
de Grijs at Macquarie University in
Sydney, have pinpointed luminous stars
across two-thirds of our galaxy’s stellar
disk, mapping out its warp in exquisitedetail. The results can help us figure out
how our galaxy got its shape.
Three years ago Chen set out looking
for variable stars known as Cepheids.
Cepheids are giant stars that can have 20
times the mass of the Sun and 100,
times its brightness. They become
unstable as they near the end of their
lives, pulsating in a way that ties directly
to their luminosity. That trick makes
them invaluable to astronomers, who
can use them as objective measures of
distance. For this reason, astronomers
are always seeking more of them.
Because Cepheids are the kind of
massive,fast-burningstarslikelytobe
found in sites of recent star formation,
Chenwasoriginallyhopingtomap
outthespiralstructureoftheMilky
Way.Butasheplottedthelocationsof
thousands of Cepheids, he realized it
wasn’tthespiralarmshewasseeingbut
the warp of our galaxy’s stellar disk.
Atleasthalfofallgalaxiesare
warped, but what’s twisting them
remains unclear. Twisted shapes may
arise during gravitational interactions
withsatellitegalaxies:theMagellanic
Clouds, for example, are close enough
that they could warp our galaxy’s disk.
Theinfallofotherdwarfgalaxieslong
ago—nowseenasstreamsofstars
flittingaroundandoutofthedisk—
couldalsohaveplayedarole.Otheroptions include intergalactic magnetic
fields, an interstellar wind blown by
starbirth and death, or a misalignment
between our galaxy’s disk and the much
larger halo that surrounds it.
In other words, the problem is not
the lack of possible explanations — it’s
the lack of data.
Chen picked out 1,339 Cepheid
variable stars from the Widefield Infrared
Survey Explorer (WISE) catalogue and a
number of visible-light surveys. While
lots of Cepheids can be found by their
visible light, infrared light enables
astronomers to peer through dust into
places like star-forming regions.
The stars trace a warped disk that
extends 65,000 light-years from the
galaxy’s centre. (The Sun, for reference,
is 26,000 light-years out.) The shape
is like thrown pottery gone wrong —
one side wings upward while the other
splays asymmetrically downward.
Because the stars’ distances are known
so well, Chen and colleagues can
precisely replicate the shape they trace
using a mathematical model.
Analysingthemodel,theresearchers
suggest that multiple pulls and pushes
maintain our galaxy’s shape. Within
50,000light-yearsofourgalaxy’s
centre (in other words, within the main
portionoftheMilkyWay),theinner
stellar disk pulls on and distorts the
outerdiskastheybothturnintheir
orbits. But at greater distances from the
centre, there’s something else at play:
“mostlikelyaccreteddwarfgalaxiesor
misaligned angular momentum of disk
andhalo,”Chensays.
The current map shows stars mostly
onoursideofthegalaxy,butinfrared
surveys continue to find Cepheids
on the far side of the Milky Way.
Eventually, astronomers will be able to
mapoutthewholeofthediskforbetter
insightintonotonlyonitsstructure
but also its history.
■ MONICA YOUNGThe Milky Way is warped in more ways than one
WThe edge-on spiral galaxy ESO 510-13,
pictured here, shows a distinct warp. Our
galaxy’s warp is more subtle — and even
harder to make out because we’re seeing it
from the inside.WMacquarie University’s Professor Richard
de Grijs was part of the team that determined
the Milky Way’s warped structure. GALAXY: NASA/SPACE TELESCOPE SCIENCE INSTITUTE; RICHARD DE GRIJS: MACQUARIE UNIVERSITY