Sky and Telescope - October 2017 - 13

IN BRIEF

BLACK HOLES

Supermassive Black Holes in Close Dance

MOST LARGE GALAXIES host central
black holes, and those galaxies frequently collide, so supermassive duos
should abound in the universe. Yet,
despite observing thousands of galaxies, astronomers have only found one
close pair of supermassive black holes,
separated by 24 light-years in elliptical galaxy 0402+379. Now, a team has
measured the plane-of-sky motions of
this pair for the first time, making it the
first visual black hole binary.
Karishma Bansal and Gregory Taylor
(both at University of New Mexico)
and colleagues used radio observations spanning 12 years to create sharp
images of the galaxy's core at multiple
frequencies, publishing the results in
the July 1st Astrophysical Journal.
The set of images shows the motion
across the sky of two bright spots of
radio emission coming from each black
hole's jets. Bansal and colleagues mea-

GALAXIES

Ring Around a Black Hole
150

Star velocity (km per second)

BL ACK HOLES: JOSH VA LENZUEL A / UNIV. OF NE W ME XICO; G A L A XIES: STEPH A NIE JUNE AU E T A L.

p This artist's concept shows two black holes
at the center of a nearby galaxy.

100
50
0
-50
-100
-150

sured the black holes' relative motion
to be 1,500 km/s, or 0.5% the speed of
light. The duo will take some 30,000
years to complete a single orbit and, the
authors suggest, millions of years to
merge. (The team plans additional radio
observations in 2019 or later to pin
down precise orbital parameters.)
But black hole mergers aren't guaranteed. Just as Earth has circled the
Sun for about 4.5 billion years, two
black holes will continue to orbit unless
something siphons away their angular
momentum. Initially, gravitational
interactions between the black holes
and surrounding material do the trick;
eventually, when the black holes are
within spitting distance, they'll emit
gravitational waves that drive their
inward spiral. But in between there's a
"final parsec" problem: When the black
holes are a few light-years apart, simulations show that they'll already have
slingshotted away all the surrounding
stars and other material, yet the gravitational waves they emit won't be strong
enough to change their orbits.
Even so, coauthor Taylor speculates,
the pair in 0402+379 will probably merge
if enough material continues to fall into
the center. Although we'll never see it
happen, this pair nevertheless presents a
tantalizing target for understanding how
black hole mergers work.

Titan's Geologic History
A topographic analysis suggests something has erased part of Titan's geologic
past. Benjamin Black (City University of
New York) and colleagues used global
drainage patterns to determine the likelihood of recent tectonic activity. Drainage
patterns will align with ancient topographic
gradients, unless tectonism disrupts them.
By blurring out small-scale features, the
team determined how often fluvial features
"conformed" to the underlying topography.
In general, surface material on Titan seems
to migrate poleward - hydrocarbons in
the atmosphere travel from mid-latitudes to
the pole, and five out of six rivers drain to
the poles. Reporting in the May 19th Science, the team finds that Titan's drainage
networks follow the prevailing slopes in
mid-latitude and equatorial regions, but not
near the north pole.
■ JANINE MYSZKA

Mass Makes the Star
Trent Dupuy (University of Texas, Austin)
and Michael Liu (University of Hawai'i) defined stardom in a study appearing in the
Astrophysical Journal Supplement Series.
The team used the Hubble Space Telescope, Keck Observatory, and CanadaFrance-Hawaii Telescope to monitor 31
binary systems, consisting of brown dwarfs
and low-mass stars, for almost a decade.
The period and size of each pair's orbit
determines each object's mass. Based on
their sample, Dupuy and Liu confirmed that
an object must have at least 70 times Jupiter's mass to ignite fusion; anything less is
fated to brown dwarf status.
■ SUMMER ASH

■ MONICA YOUNG

t A TEAM OF ASTRONOMERS has taken
a close look at nearby spiral galaxy NGC 7582
and discovered an unusual structure circling
its supermassive black hole: a ring of dust,
gas, and stars spanning 2,000 light-years.
Stephanie Juneau (NOAO) presented new
data from the MUSE instrument on the Very
Large Telescope in Chile at June's meeting of
the American Astronomical Society.
Each of the image's 90,000 pixels includes a spectrum, its Dopper shift revealing
how that part of the galaxy is moving. Red
indicates stars moving away along our line of
sight, while blue indicates stars moving toward us. Juneau's team separated the overall
rotation of stars around the galactic center

(red-to-blue gradient) from the even faster
rotation of an inner ring, seen edge-on (blue
and yellow dots in the galaxy's core).
The ring spans a thousand times the width
of the gaseous accretion disk that's feeding
the black hole. In addition to blocking some
of the disk's intense radiation, the ring shapes
the wind emanating from the disk and protects the galaxy from its destructive power.
The authors speculate that the ring might
have formed during a recent merger with a
dwarf galaxy once in orbit around NGC 7582.
■ MONICA YOUNG

* Learn more about the study at https://
is.gd/ringaroundbh.

s k y a n d t e l e s c o p e . c o m * O C T O B E R 2 0 17

13


http://www.is.gd/ringaroundbh http://www.is.gd/ringaroundbh http://www.skyandtelescope.com

Sky and Telescope - October 2017

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