The Gaia satellite is now pulling this blurry
history into focus by measuring the distance
and motion of billions of stars up to several
thousand light years away from the sun with
exquisite precision. The information it is
sending back to Earth amounts to a “huge
treasure chest” for astronomers trying to
piece together the details of the Milky Way’s
evolution, says Ralph Schoenrich, a theoretical
astrophysicist at University College London.
In April 2018, when Gaia released its second
tranche of data, Helmi and her colleagues
threw themselves into investigating a
particular subset: the fine motions of some
7 million stars as they wobble up and down
in the plane of the Milky Way’s disc. This isplunged through the Milky Way three times
in the past. But no one expected an imprint
of such an ancient collision to still be encoded
in the motions of stars today. “That was
genuinely a surprise,” says James Binney
at the University of Oxford.
Astronomers are still wrangling over the
precise origin of the snail shell. Theoretical
models and computer simulations have
since backed up Sagittarius as the prime
suspect, but it was more likely to have been
caused by one of the dwarf galaxy’s earlier
crossings a billion years ago, says Joss Bland-
Hawthorn at the University of Sydney.
“Sagittarius has been shedding a lot of mass
along its orbit. Today, it’s nowhere near
massive enough to do any of this,” he says.
Indeed, these days Sagittarius is a strung-out
shadow of its former self, hurtling back
towards the Milky Way as it traces the third
leaf of its clover-shaped orbit. Eventually,
like many galaxies before it, Sagittarius
will become part of us, having profoundly
shaped the Milky Way on its journey.
Regardless, the snail shell is one among
many previously hidden patterns that show
Gaia is ushering in a new age of “galactic
seismology”, says Bland-Hawthorn. No longer
restricted to fossil records, we can now observe
the faint tremors left behind by the impacts of
intruders past – in the same way that we learn
about Earth’s hidden interior by watching how
earthquakes propagate. Deciphering these
tremors, astronomers are sketching a new
panorama of the Milky Way and the riotous
saga of how it came to be.
A decade ago, it was proposed that an impact
with Sagittarius was the origin of the Milky
Way’s spiral arms – an idea that astronomers
are beginning to test with Gaia data. There
might even be clues to the true identity of
dark matter lurking in this surprisingly
dynamic galactic dance. The type of particles
that make up dark matter – whether they are
heavy or light, for example – would influence
the orbits of dwarf galaxies like Sagittarius
around the Milky Way and the patterns left in
their wake. “Two years ago, if you had asked
anyone in the field, they would not have
thought that anything like that was possible,”
says Schoenrich.
As if all that wasn’t enough, Ruiz-LaraDEEP
OL
BYPLAINPICTU
RE/NEST
ORRODANroughly akin to watching a human hair an
arm’s length away grow in 5 million times slow
motion. Walking to her car one night, Helmi
glanced at a data plot sent to her by a colleague,
who was searching for patterns in “phase
space” – an abstract, six-dimensional world
that combines the 3D positions and velocities
of stars. “I was like: ‘Oh my god! What is this?’
That plot was absolutely stunning,” says Helmi.
The team had uncovered a weird snail-
shaped pattern caused by something hitting
the Milky Way’s disc a few hundred million
years ago. Based on existing computer
simulations and observations of star streams,
the most likely culprit was a dwarf galaxy
called Sagittarius, which was known to haveThe part of the Milky
Way seen from Earth is
one of four spiral arms54 | New Scientist | 18/25 December 2021