Sky & Telescope - March 2020 - 36

Galactic Vandal

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p GAIA'S REACH The previous mapping satellite, Hipparcos, charted
stars' positions out to 300 light-years from the Sun - less than a dot at
this diagram's scale. Gaia surpasses that by a factor of 100.

36

MARCH 2 02 0 * SK Y & TELESCOPE

The dwarf galaxy hurtled toward the
Milky Way and punched through the
disk of our galaxy, only to pop back
out the other side.
and back again - a signature of a single major crash.
"I think it is really hard to think of another explanation,
to be honest," team member Evans says. Because the plotted velocities formed a sausage shape, the team dubbed the
ancient dwarf galaxy the Gaia Sausage.

Shakes and Tremors
More than a year later, when Gaia's second data release
arrived on April 25, 2018, astronomers across the world were
poised at their laptops, ready to download the data immediately and begin the search - even those on the West Coast of
the U.S., where it was 3 a.m. local time. In the Netherlands,
where it was a sunny spring day, Helmi and her colleagues
analyzed 20,000 Gaia stars within the Milky Way's halo.
They quickly discovered that half of them appear to be moving on eccentric orbits that travel in the opposite direction
as the Sun does around the galaxy - suggesting that they did
not form in the Milky Way at all.
To pin down whether that was the case, the team analyzed
the chemical compositions of some 600 of those stars. In
every galaxy, the abundance of heavy elements increases over
time as more and more supernovae erupt and unleash new
material into the cosmos. The exact increase works much like
a fingerprint, creating a pattern that is specific to each galaxy
and allowing astronomers to determine whether a group of
stars belongs to the Milky Way - and if it doesn't, the type of
galaxy it originated within.
Helmi's group of stars, for example, contain a different
pattern of heavy elements compared with what's made in
the Milky Way, supporting the notion that they didn't grow
up here. Instead, the stars likely formed within a single large
dwarf galaxy that was pulled into a dangerous dance with
the Milky Way roughly 10 billion years ago. That's when the
dwarf galaxy hurtled toward the Milky Way and punched into
the disk of our galaxy, only to pop back out the other side. For
a brief moment it looked as though it might continue racing
away from the Milky Way, but it could not escape such a
massive galaxy's gravitational pull. It slowed down, reversed
course and plunged back into the Milky Way's disk. It danced
back and forth like a pendulum for a little more than a billion years, but was eventually swallowed.
The team named this long-gone dwarf galaxy Gaia-Enceladus in honor of Gaia, both the Greek personification of Earth
and the space observatory, and Enceladus, one of the children
of Gaia and Uranus, the sky. It is particularly apt given that
the merger likely rocked the Milky Way in a similar manner
to the ancients' Enceladus, who was once believed responsible
for earthquakes below Mount Etna in Italy.

R. HURT / N ASA / JPL- CA LTECH

The first issue is that the streams begin to blend into the
Milky Way over time, blurred out like muddy rills mixed into
a much larger river. But even then, a few telltale scars remain.
Stars that were captured in a collision, for example, often
move against the status quo - traveling together along erratic
orbits compared with the majority of stars in our galaxy. Now
that astronomers can untangle the orbits for a billion stars
(thanks to those positions and velocities), uncovering which
stars are newcomers has become much easier.
"That is the miracle that Gaia has given to us," says Wyn
Evans (University of Cambridge, UK).
The second issue is that prior work had charted the
galaxy out to roughly 300 light-years, a small corner of the
Milky Way. Gaia, on the other hand, is able to map all the
stars within 30,000 light-years - well past the distance to
the galactic center. "Before we could explore only the solar
neighborhood - and it was really a small neighborhood," says
Teresa Antoja (University of Barcelona, Spain). "Now we can
study beyond our city."
And that has made all the difference. When Gaia's preliminary data release became available in September 2016,
Vasily Belokurov (University of Cambridge, UK) used it along
with positions in the Sloan Digital Sky Survey to find some
10,000 stars that travel together in elongated orbits. The stars
eventually move from the center of the galaxy to the outskirts



Sky & Telescope - March 2020

Table of Contents for the Digital Edition of Sky & Telescope - March 2020

Contents
Sky & Telescope - March 2020 - Cover1
Sky & Telescope - March 2020 - Cover2
Sky & Telescope - March 2020 - 1
Sky & Telescope - March 2020 - Contents
Sky & Telescope - March 2020 - 3
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