Sky & Telescope - July 2021 - 11

BLACK HOLES

A Supermassive Black
Hole on the Move
THE BLACK HOLE AT THE HEART of a
nearby galaxy is drifting. The 3 million-solar mass behemoth is moving
100 kilometers per second (230,000
mph) relative to its host galaxy. It's also
moving 50 km/s relative to the gas and
stars about 100 light-years out. Even
stranger, the gas and stars themselves
seem to be out of whack with galactic
material that's farther out.
The host galaxy J0437+2456 is a
barred spiral 220 million light-years
away, and it looks ordinary at ﬁrst
glance. But observations of its megamaser, a bright source of radio emission
orbiting the central black hole, showed
Dominic Pesce (Center for Astrophysics,
Harvard & Smithsonian) and colleagues
that the dark behemoth wasn't at rest
with respect to its galactic host.
Pesce's team followed up using
near-infrared spectroscopy to track the
motions of stars and gas as close as 110
light-years to the black hole. The team
also used the Arecibo Observatory (in

IN BRIEF

NOIRL A B / NSF / AUR A / J. DA SILVA

Asteroid Apophis Will
Miss Earth in 2068
Radar measurements taken in early
March rule out any chance of a collision
with 99942 Apophis over the next 100
years. When scores of asteroid specialists
convened virtually for a special three-day
conference dedicated to Apophis last
November, there was still a small chance of
a 2068 impact. That was in part because
its exact orbit was still uncertain, especially
since Apophis's close approach to Earth
on April 13, 2029, will alter the asteroid's
path significantly. Fortunately, another opportunity for orbital refinement occurred on
March 5th, when Apophis cruised by Earth
at a distance of 17 million kilometers (10½
million miles). Using ground-based radar to
determine the precise distance to Apophis
and its line-of-sight velocity as it sped by,
astronomers ruled out the already-remote
chance of an impact in 2068. So now, no

early 2019, before its catastrophic collapse) to image neutral hydrogen gas
farther out in the galaxy.
These observations, which appear in
the March 10th Astrophysical Journal,
show the black hole, the stars and gas
around it, and the galaxy at large all
moving at different speeds. The host
probably merged recently with another,
smaller galaxy, both of them bringing
their own central black holes to the
union. But what stage the merger's in is
still up for debate. The black hole could
just be falling into the galaxy, though
Pesce says that seems less likely since

S An artist's impression of a supermassive
black hole within a galaxy

longer worried about 2068, astronomers
are generating plans for how they'll observe
Apophis when it brushes by in 2029. When
closest, within 32,000 km (20,000 miles) of
Earth's surface, the asteroid will appear as
bright as a 4th-magnitude star.

the diffuse " atmosphere " of gas and dust
around the comet. Its high degree of
polarization indicates that the dust grains in
the coma are tiny, on the sub-micron scale,
says team member Ludmilla Kolokolova
(University of Maryland). Borisov's polarization pattern is unique among solar system
comets except for Comet Hale-Bopp
(C/1995 O1). The dust particles suffusing
Hale-Bopp's coma were likewise small
compared to those in other observed comets. Moreover, it was visible to the naked
eye for a year and a half, a record that jived
with scientists' assessment that Hale-Bopp
had only approached the Sun once before.
The similarities between the two suggest
Borisov's encounter with our Sun was the
first time it had come close to any star, including its own. If it originated far out from
its host before being ejected into interplanetary space, our Sun would have been the
first star to trigger its cometary activity.
MONICA YOUNG

J. KELLY BEATTY

Read the full story at https://is.gd/
Apophis2068.

Pristine Interstellar Comet
Initial observations indicated that the
interstellar comet 2I/Borisov, which zipped
nearest the Sun in late 2019 (S&T: Jan.
2020, p. 10), was a dark-reddish object
just like any of the long-period comets
that loop in from the Oort Cloud. But new
analysis appearing March 30th in Nature
Communications reveals that Borisov was
actually unique - the most pristine sample
of a planet-forming system ever observed.
Stefano Bagnulo (Armagh Observatory,
UK) and colleagues measured visible and
near-infrared light scattered by its coma,

it's already near the center. Or it could
be orbiting another, not-yet-detected,
supermassive black hole. Or perhaps the
two black holes have already coalesced,
and their merger resulted in recoil
motion away from the center.
Current data can't distinguish
between the options, but Pesce says
high-resolution radio observations
could reveal emissions from another
supermassive black hole in the system
- if it's there.
MONICA YOUNG

Read more Borisov ﬁndings at https://
is.gd/Borisovdust.

s k y a n d t e l e s c o p e . o r g * J U LY 2 0 2 1

https://www.is.gd/apophis2068 https://www.is.gd/apophis2068 https://www.is.gd/Borisovdust https://www.is.gd/Borisovdust http://www.skyandtelescope.org

Sky & Telescope - July 2021

Table of Contents for the Digital Edition of Sky & Telescope - July 2021