Sky & Telescope - March 2020 - 38

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p MIND THE GAP Gaia data of the stellar stream GD-1 reveal clumping
and gaps (top), which are more obvious in the cleaned, zoomed-in version (bottom). Ana Bonaca and others suggest that the gap at the −40°
mark and the spur extending from it might be the result of a dark matter
clump punching through. (The gap at −20° is thought to mark where
GD-1's parent cluster disrupted.)

"A dark matter hurricane - we called it - is blowing
through the solar system," Evans says. "It's blowing through
the Earth."
And if dark matter is blowing through our planet, then it's
blowing through our laboratories. That could mean we might
actually spot a dark matter particle. Because our ability to
detect such particles increases with their speed, the fact that
they're zipping through our laboratories suggests that they
might be easier to see than previously thought. That said, it's
a hard problem to solve, and experiments are still ongoing.

q STELLAR STREAMS Shown are the streams Khyati Malhan and his colleagues have found in Gaia data so far using their algorithm. Some had
been previously detected, such as Jhelum and Indus; others were brand new and now have names incorporating "Gaia" or taken from ancient rivers
in Norse mythology.

Sylgr

GD-1

Orphan

Gaia-1

Svöl

Fjörm

Slidr

Distance
from
Sun
(light-years)

Gaia-4
Fimbulthul

50,000

Gaia-5
(red dots)

Ylgr

40,000

Gjöll
30,000

Indus
Phlegethon

Leiptr
20,000

Jhelum

Gaia-2
10,000

MARCH 2 02 0 * SK Y & TELESCOPE

G D -1 G A P: INITIA L DATA: A . M. PRICE-WHEL A N & A . BON ACA / ASTR OPHYSICAL JOUR NAL LE T TERS 863:L 20 (2018);
CLE A N DATA: A . BON ACA E T A L. / ASTR OPHYSICAL JOUR NAL 88 0:38 (2019); STELL A R STRE A MS: G A L A X Y: ESA / G AIA /
DPAC, STRE A MS DATA: K . M A LH A N E T A L. / MONTHLY NOTICES OF THE R OYAL ASTR ONOMICAL SOCIE T Y 2018 A ND R.
IBATA E T A L. / ASTROPHYSICAL JOUR NAL 2019

"Gaia has been a box full of chocolates for us," says Khyati
Malhan (Stockholm University, Sweden). Since Gaia's second
dataset was released, hundreds of papers have been published
using the satellite's observations.
"Most of us are very lucky to be in this place and in this
time," says Antoja. "This topic in general, galactic dynamics,
this will be the golden age."
For Malhan, it didn't take long to uncover evidence of
past collisions. Within ﬁve days, his team had harvested ﬁve
streams. Now, they've spotted more than a dozen. "It has
been a huge surprise," he says. "We weren't shocked to see
this many streams - but that Gaia's proper motions are so
bloody accurate and so bloody precise that it was so easy to
pick out even these very low-contrast features."
Although new streams are now commonplace (astronomers know of roughly 50 within the Milky Way's halo), each
ﬁnd is still a thrill. Every time a stream is announced at a
conference, it's met with a round of applause. That's in part
because these streams will enable astronomers to unveil our
galaxy's full formation history - down to the smallest mergers - but they also just might answer what is arguably the
largest mystery within astrophysics: dark matter.
Every time a dwarf galaxy falls into the Milky Way, it
deposits both stars and dark matter. Take the S1 stream as
an example. It's likely the remnant of a dwarf galaxy that
fell into the Milky Way roughly 9 billion years ago. Then, 1
billion years ago, it began passing the Sun like head-on trafﬁc
and depositing roughly 10% more dark matter than we would
otherwise experience in this section of the galaxy.

Degrees

Galactic Vandal

Sky & Telescope - March 2020

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