Sky & Telescope - June 2020 - 10

NEWS NOTES
t This nearby massive galaxy, like XMM-2599,
has shut down its star formation.

GALAXIES

How Did This "Monster" Galaxy
Snuff Out Star Formation?
WITHIN THE FIRST several billion
years of the universe, galaxies grew like
newborns, turning gas into stars at
fantastic rates. But astronomers have
discovered one massive galaxy that furiously formed new stars only to abruptly
quench starbirth altogether. The result
could change how we think about galaxy evolution.
Ben Forrest (University of California, Riverside) and colleagues published
follow-up observations of this monster galaxy, which was discovered in
a larger survey, in the February 10th

Astrophysical Journal Letters. The team
took spectra using the Keck I telescope
on Mauna Kea, Hawai'i, to measure the
galaxy's stellar mass and its star formation history.
The galaxy, dubbed XMM-2599,
already had five times the Milky Way's
mass in stars just 1.8 billion years after
the Big Bang. Such a gargantuan galaxy
is by itself rare in the early universe, as
most galaxies hadn't had enough time
to produce so many stars. But some
galaxies worked overtime, at least for
a little while. Chemical fingerprints in

Rover

the galaxy's spectrum indicate that,
for a period of several hundred million
years around 1 billion years after the
Big Bang, XMM-2599 was forging more
than 1,000 stars per year. (The Milky
Way only produces about one or two
stars every year.)
However uncommon, massive starforming galaxies have been seen in both
observations and simulations of galaxy
evolution. But not only is XMM-2599
massive, all the data collected so far
point to a complete shutdown of its
star-formation factory. Astronomers
have spotted only a few massive galaxies
that have doused starbirth so early on
in the universe. Moreover, simulations
can't reproduce them.
Katherine Whitaker (University of
Massachusetts, Amherst), who was not
involved in the study, says this discovery
highlights tensions between observations and computer models. "I don't
know that we need to totally rethink
models," she adds. "But these tensions
are precisely what we need to identify
and revisit the assumptions and prescriptions driving the simulations."
■ MONICA YOUNG

MOON

What Lies Beneath
the Lunar Farside
Lander

~12m

~24m

~40m

10

JUNE 2 02 0 * SK Y & TELESCOPE

farside of the Moon is providing a look
at the lunar subsurface. Data from the
Yutu 2 rover's Lunar Penetrating Radar
has revealed three discrete layers of
regolith underneath the landing site in
Von Kármán Crater in the vast South
Pole-Aitken Basin.
Chunlai Li (Chinese Academy of Sciences) and colleagues published results
t The farside's subsurface can be divided into
three units: Unit 1 (down to 12 m, or 40 feet)
consists of fine lunar regolith, unit 2 (12 to 24 m)
consists of coarser materials with embedded
rocks, and unit 3 (24 to at least 40 m) contains
alternating layers of coarse and fine materials.

G A L A X Y: N ASA / ESA / R. FOLE Y; M OON SUBSURFACE:
CHUNL AI LI E T A L. / SCIENCE ADVANCES 2020

CHINA'S CHANG'E 4 MISSION to the



Sky & Telescope - June 2020

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