Sky and Telescope - August 2018 - 10

NEWS NOTES

14 Galaxies Might Have
Become Largest Cluster in
the Universe
AT LEAST 14 GALAXIES congregating

just 1.4 billion years after the Big Bang
might have become one of the most
massive structures in the universe - if
we could observe it to the present day.
Each galaxy in this protocluster is
alight with stars forming 50 to 1,000
times more quickly than the Milky Way;
the complex as a whole has the mass
of 10 trillion Suns. The fact that these
galaxies came together in such a massive
structure so early on challenges our
ideas of how clusters form.
Clusters, the largest gravitationally
bound structures in the universe, contain hundreds or thousands of galaxies
tethered together by their massive halos

p ALMA imaged 14 galaxies swarming like
fireflies in a massive protocluster that existed
1.4 billion years after the Big Bang.

of dark matter. As galaxies are coming
together, such a group is called a protocluster, and its future isn't guaranteed - depending on its environment,
it might yet fall apart. But in the case
of protocluster SPT 2349-56, the team
reports in the April 26th Nature, the 14
galaxies are close enough together that
the still-growing structure would probably have survived to the present day.
The protocluster was ﬁrst seen as a
bright, millimeter-wavelength smudge
by the South Pole Telescope. Follow-up

GALAXIES

Do Supermassive Black Holes Wander the Milky Way?
NEW SIMULATIONS SUGGEST that

galaxies like the Milky Way could be
home to a dozen big black holes.
A gigantic black hole sits at the
center of the Milky Way and most other
large galaxies. But when galaxies merge,
their central black holes can ﬁnd themselves with an eviction notice.
Astronomers have increasingly
realized that evicted black holes only
migrate to the new galaxy's core in a
small fraction of mergers. The problem is especially acute when one of the
galaxies is much smaller than the other.
By implication, then, the outskirts of
big galaxies should play host to a large
number of supermassive black holes.
To understand the implications for
galaxies like the Milky Way, Michael
Tremmel (Yale University) and colleagues used the Romulus set of simulations, which follows the growth of
cosmic structure in a cube of computerized space some 80 million light-years
on a side. The team watched the development of Milky Way-mass galaxies
and their interaction with their fellow
10

AUGUST 2 018 * SK Y & TELESCOPE

stellar metropolises from soon after the
universe's birth (when galaxy mergers
were far more common) to today.
The team reports in the April 20th
Astrophysical Journal Letters that, of 26
Milky Way-mass galaxies, each plays
host to on average 4 to 20 supermassive
black holes; roughly ﬁve of the black
holes (again, on average) lie within
30,000 light-years of the galaxy's center.
When a merger in the simulation
deposited a black hole near the plane
of the galaxy's disk, the black hole
gravitationally interacted with stars
and gas and quickly sank to the core.
Black holes following orbits that kept
them far from the galaxies' disks, where
they encountered few stars or gas, never
migrated inward.
But if the Milky Way does host giant
black holes in its outskirts, they're likely
well outside the disk. Furthermore, the
team estimates that our solar system
would come across one of these wanderers every 100 billion years or so -
nearly 10 times the age of the universe.
■ CAMILLE M. CARLISLE

observations by the Atacama Large Millimeter/submillimeter Array (ALMA),
combined with Spitzer observations
at infrared wavelengths, resolved the
smudge into at least 14 galaxies forming
stars out of huge gas reservoirs.
While most protoclusters discovered
so far are about as bright as theoretical
models predict, SPT 2349-56 is 10 times
brighter. Computer simulations haven't
been able to make protoclusters producing as many stars as SPT 2349-56 does.
That said, the authors acknowledge theory's limited scope. Allison
Noble (MIT), who wasn't involved in
the study, agrees that current models
haven't yet probed large enough scales
to predict such an extreme case. Nevertheless, this protocluster, and any others like it, will help astronomers better
understand how the universe grew its
largest structures.
■ MONICA YOUNG

IN BRIEF
Helicopter to Fly with
Mars 2020 Rover
NASA has announced that a small,
autonomous helicopter, named Mars
Helicopter Scout, will make the journey
to the Red Planet with the Mars 2020
rover. It may become the first mission
to fly on another planet. Tucked away
under the rover during descent, the
helicopter will be deposited on the
Martian surface shortly after landing. Weighing in at 1.8 kg (4 lbs), the
baseball-size helicopter will whip its
rotor blades at 3,000 rpm (10 times the
rate of a helicopter on Earth) to create
lift in the tenuous Martian atmosphere.
As a technology demonstrator, the
helicopter won't carry science instruments, but it will have two cameras
that could prove valuable in mapping
terrain. Funded for $23 million early
this year, the helicopter is expected to
last 30 days, during which NASA plans
at least five short flights. The first flight
will make an initial vertical climb of 3
meters (10 feet). Subsequent sorties
will feature journeys up to 90 seconds
in duration that range over several
hundred meters.
■ DAVID DICKINSON

A LM A (ESO / N AOJ / NR AO), B. SA X TON (NR AO / AUI / NSF )

STARS

Sky and Telescope - August 2018

Table of Contents for the Digital Edition of Sky and Telescope - August 2018