Sky & Telescope - April 2021 - 11

GALACTIC

The Galactic " Fossil " Inside the Milky Way
THOUSANDS OF STARS that once

belonged to an ancient dwarf galaxy are
inside our own.
Graduate student Danny Horta and
his advisor Ricardo Schiavon (both at
Liverpool John Moores University, UK),
with an international team of colleagues, reached this conclusion using
two surveys that catalog millions of
stars: The Gaia mission, which pinpoints stars' positions, distances, and
velocities, and the APOGEE sky survey,
which provides chemical compositions.

DA NN Y HORTA (LIV ERPOOL JOHN M OORES UNIV ERSIT Y ) / ESA / G AIA / SDSS

IN BRIEF
Hayabusa 2 Returns Asteroid Sample to Earth
After a six-year journey of 5.24 billion kilometers (3.26 billion miles), the Japanese
Hayabusa 2 mission passed by Earth on
December 5th, jettisoning its sample-return capsule for a landing in the Woomera
Prohibited Area in southern Australia.
The capsule carried pieces of asteroid
162173 Ryugu. The team located and collected the capsule after a brief helicopter
search, then transported it to a facility in
Woomera for a " quick-look " inspection.
That inspection revealed that the capsule
contained gases from Ryugu. The capsule
and its contents then headed to Japan,
where the first chamber opening revealed
charcoal-colored rocks and dust. Study
of this pristine asteroid material will give
scientists a look back at the early solar

S Red rings superimposed over an image of
the Milky Way show the approximate extent
of the stars that came from the fossil galaxy
known as Heracles. (To the lower right are the
Large and Small Magellanic Clouds.)

Horta and colleagues zeroed in on
1,032 pristine and therefore ancient
stars within 13,000 light-years of
the galactic center. Gaia data reveal
unique, highly eccentric orbits for a
third of them, and APOGEE observations showed that those same stars have
unusual compositions.

system. After releasing the sample, Hayabusa 2 performed trajectory correction
maneuvers to fly past Earth as it continues
on an extended mission to two additional
asteroids, including 1998 KY26, a fast rotator that spins every 11 minutes.
DAVID DICKINSON

Surprisingly Mature Infant
Galaxies
Eight studies published in the November
Astronomy & Astrophysics show that
long before star formation peaked in the
universe, which occurred around 3 billion
years after the Big Bang, galaxies were
already churning stars out at prodigious
rates, and changing their environments as
they did so. An Atacama Large Millimeter/submillimeter Array (ALMA) program
dubbed ALPINE has gauged the dust, gas,
and star formation in 118 distant galax-

In the January Monthly Notices of the
Royal Astronomical Society, the researchers concluded that the stars are the
fossilized remains of an ancient galaxy
that collided with the Milky Way 10 billion years ago. The research team nicknamed the galactic fossil " Heracles. "
While some of the 300 or so stars
actually traveled in with Heracles, the
researchers speculate that the others
were born within the Milky Way, their
birth stimulated by the galactic merger.
They make up a sizable fraction of the
stellar halo that now surrounds our
galaxy's disk.
Heracles sounds similar to another
recently discovered ancient galaxy
dubbed " The Kraken " (S&T: Mar. 2021,
p. 11). Both would have encountered
our galaxy about 10 billion years ago
on a similar orbital path, and their
stars would have had similar chemical
compositions.
But Horta and Schiavon caution that
Heracles is about three times as massive
as the Kraken is thought to have been,
and they argue that the association
between the two isn't yet clear. A convincing identiﬁcation awaits additional
data and simulations.
MONICA YOUNG

ies. The observations show that even
when the universe was only 1-1.5 billion
years old, galaxies' starbirth was already
half-obscured in dust clouds. Dust takes
time to make - any atoms heavier than
hydrogen and helium have to be forged in
stars first - so astronomers didn't expect
to see so much dust and heavy elements
in these distant galaxies, says co-principal
investigator Andreas Faisst (Caltech).
ALPINE observations also show that
these youngsters were rambunctious: The
team classified 40% of the galaxies as
mergers. Yet, another 11% of the galaxies
were stable rotating disks. These orderly
young'uns confirm what other observations (S&T: Apr. 2018, p. 12; Sept. 2020,
p. 12) had already shown - that a number
of galaxies stabilize into disks early on,
even as others appear more chaotic and
swirly from previous or ongoing mergers.
MONICA YOUNG

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Sky & Telescope - April 2021

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