Sky and Telescope - June 2015 - 14

News Notes

IN BRIEF

GALAXIES I Dusty Galaxy in Early Universe

Dwarfs Nicked Oort Cloud. A red dwarf
and its brown dwarf companion buzzed
through the outer Oort Cloud some 70,000
years ago, Eric Mamajek (University of
Rochester) and colleagues suggest in the
February 10th Astrophysical Journal Letters.
The team used a combination of position
and motion data gathered by Adam Burgasser (University of California, San Diego)
and others to simulate 10,000 orbits for the
M dwarf WISE J072003.20-084651.2, called
Scholz's Star. Of all those simulations,
98% had the star passing through the
outer Oort Cloud. Its closest approach was
probably between 0.6 and 1.2 light-years
away (38,000 to 75,000 astronomical units),
where it scraped the Oort Cloud. A second
star, Gliese 710, has a more precisely calculated trajectory that shows it ﬂying roughly
1.1 light-years from the Sun 1.4 million years
from now. Theorists expect that stellar
passes closer than 0.8 light-year happen
roughly every 100,000 years.

Astronomers have directly detected dust
in a galaxy shining at us from only 700
million years after the Big Bang (redshift
of 7.5). And given its mass, it's as dusty as
a mature star-forming galaxy today.
Darach Watson (University of Copenhagen, Denmark) and colleagues explored
the galaxy A1689-zD1 using ALMA and
the Very Large Telescope (VLT). The
galaxy is only bright enough to study
because the galaxy cluster Abell 1689
gravitationally lenses A1689-zD1's light,
magnifying it by more than nine times.
The optical light VLT detects from
A1689-zD1 began as ultraviolet radiation,
emitted by young, massive stars and was
then stretched to visible wavelengths by
the universe's expansion. Thus, the team
can use the light to estimate the galaxy's
mass in stars: 1.7 billion solar masses, a
hundredth the Milky Way's stellar mass.
The optical spectrum from VLT and
the dust emission detected by ALMA
both suggest A1689-zD1's star-formation
rate is at least a few times higher than the
one-Sun-per-year rate of the Milky Way
- not unusual for this cosmic era.

■ SHANNON HALL

Chang'e 3 Landed on Lunar Layers.
When China's Chang'e 3 spacecraft came
to rest atop northern Mare Imbrium on
December 14, 2013, it achieved the ﬁrst soft
landing on the Moon since 1976 (see page
54). The main spacecraft and its wheeled
rover, called Yutu, each carried four instruments. Although Yutu traveled only 114
meters before suﬀering a mobility malfunction (S&T: May 2014, p. 12), its ground-penetrating radar system operated well. In the
March 13th Science, mission scientist Long
Xiao (China University of Geosciences and
Macau University of Science and Technology) and colleagues report ﬁnding at least
nine discrete rock layers extending to about
400 meters (0.25 mile) beneath the surface.
Most are thought to be solidiﬁed lava or ash
ﬂows from regional eruptions that occurred
2.5 to 3.3 billion years ago. But the topmost
pair, which together are about 4 meters
thick, appear to be rubble thrown out when
an impact excavated a nearby 450-meterwide crater 27 to 80 million years ago.
■ J. KELLY BEATTY

June 2015 sky & telescope

What is unusual is that the galaxy contains between half and a few times the
ratio of dust to gas in the Milky Way. This
value suggests the galaxy has already
burned through half of its gas, only 150
million years after the universe's galaxies
started churning out stars in earnest.
But because A1689-zD1 is only a hundredth as massive as the Milky Way, the
little galaxy has only about a hundredth
as much dust. It could have built up that
dust by forming stars at a moderate rate
for the last 150 million years, or perhaps
it had an extreme "starburst" phase earlier and is now calming down. Per unit
area, its star-formation rate is on par with
many starburst galaxies, the authors note
March 2nd in Nature. It's also consistent
with upper limits calculated for other
galaxies in the same cosmic era.
As in today's galaxies, most of the dust
grains likely formed in the winds from
old, swollen stars, or when massive stars
went supernova. Because giant stars don't
live long, there's no need to have a very
old galaxy to see dust.
■ CAMILLE M. CARLISLE

BLACK HOLES I Midsize Candidate Found
Astronomers have discovered what
appears to be a black hole of intermediate
mass in the arm of NGC 2276, a spiral
galaxy 100 million light-years from Earth.
The source, called NGC-2276-3c, is shooting out a powerful radio jet 6 light-years
long, with further radio emission extending out to 2,000 light-years. The black
hole is potentially about 50,000 solar
masses, Mar Mezcua (Harvard-Smithsonian Center for Astrophysics and Universidad de La Laguna, Spain) and colleagues
estimate in the April 1st Monthly Notices
of the Royal Astronomical Society.
Scientists know of only two credible black hole candidates in the 100 to
100,000 solar-mass range. One is in a
galaxy 300 million light-years away, and
the other is in the starburst galaxy M82.
Their masses range from a few hundred
to ten thousand solar masses. If proved
legitimate, NGC-2276-3c "would be a rare

and important ﬁnd," says Roberto Soria
(Curtin University, Australia).
There are several other cosmic objects
that on ﬁrst glance also appear to be
intermediate-mass black holes (IMBHs),
but that's because they are accreting
material at unnaturally high rates, boosting their luminosity. Such objects are
actually stellar-size, but they can also
have powerful jets. If NGC-2276-3c were
accreting at a rate far higher than normal, its mass could be smaller.
What's unclear is whether a stellarmass black hole could support a radio
jet steady enough to carve out the region
seen around NGC-2276-3c. Black holes
tend to cycle through diﬀerent accretion
states, but if stellar-mass black holes can
truly sustain jet-powering super accretion, that would make the IMBH explanation for NGC-2276-3c less of a shoo-in.
■ EMILY POORE

Sky and Telescope - June 2015

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