Sky & Telescope - January 2020 - 13

GALAXIES

Exotic Messenger Probes
Galactic Halo
ASTRONOMERS UTILIZED a fast radio

burst, a powerful 40-microsecond-long
ﬂash of radio waves, to evaluate the
state of the nearly invisible gas around
an intervening galaxy.
The mass of the hot, gaseous halo
that surrounds most massive galaxies is often on par with the mass of
all the stars in the galaxy itself. Yet
it largely evades detection. It's both
very hot - "cooler" clouds of some
10,000K (17,500°F) ﬂoat within a hotter, million-degree atmosphere - and
sparse, with only a couple hundred
atoms within the space of a child's balloon. So astronomers observe the gas
indirectly, by the way it absorbs the light
of background sources.
Previous observations have suggested that halos tend to be turbulent,
with high-density clouds embedded in
the rareﬁed gas. Yet when astronomers
reported in the October 11th Science
that a powerful fast radio burst had
passed through the halo of an intervening galaxy, they found that the radio
waves seemed almost entirely undisturbed, indicating an unexpectedly
calm and sparse halo.

IN BRIEF

ESO / M. KOR NMESSER

Physicists Detect Black
Hole Ringdown
A re-analysis of LIGO's first detection of
gravitational waves reveals that scientists can
pick up the shudder in spacetime that follows
a black hole merger. This shudder, called
the ringdown, is like the dying vibration of a
struck bell. Physicists had typically looked for
the ringdown signal long after a merger, but
graduate student Matthew Giesler (Caltech),
Maximiliano Isi (MIT), and their colleagues discovered that the ringdown could be detected
right away. The key, they found, is to look for
overtones, like additional tones in the ringing
bell. Based on the ringdown's fundamental
vibration and its overtones, the researchers
calculated the black hole's mass and spin.
They confirmed that these two parameters
encapsulate everything you need to know

FRB 181112
host galaxy

Pulse of
radio waves

Intervening
galaxy
Earth

p Astronomers utilized a 40-microsecond fast radio burst, designated FRB 181112, to probe a
galaxy's outer reaches.

J. Xavier Prochaska (University of
California, Santa Cruz) and colleagues
probed the halo using FRB 181112, a
fast radio burst detected by the Australian Square Kilometer Array Pathﬁnder.
ASKAP immediately pinpointed the
source to a speciﬁc location on the sky.
As the powerful packet of radio waves
traversed billions of light-years toward
Earth, it had passed within 95,000
light-years of a foreground galaxy.
That's close enough to pass through the
galaxy's halo, but whatever medium the

about an astrophysical black hole, an idea
known as the no-hair theorem. The team
reports the results in the September 12th
Physical Review Letters and in an upcoming
Physical Review X.
■ CAMILLE M. CARLISLE

NASA Launches ICON
to Explore Ionosphere
On October 10th Northrop Grumman's
Pegasus XL rocket, ejected from the fuselage
of a L-1011 Stargazer aircraft, lofted NASA's
Ionospheric Connection Explorer (ICON) into
low-Earth orbit. ICON will explore the boundary between Earth and space by studying the
ionosphere, the region in Earth's atmosphere
that's ionized by incoming sunlight. In the
ionosphere, rarefied ions and electrons flow,
their motions governed by winds, daytime
heating and nighttime cooling, and solar
activity. The movements of ions affect Earth's

radio waves passed through barely made
a dent in the signal.
The intervening galaxy has a central, supermassive black hole that's still
somewhat active. It's possible that this
black hole once pushed out jets of material that evacuated the inner halo. Or,
it's also possible that the gas that some
galaxies swim in is simply more serene
than expected. The team plans to follow
up on other fast radio bursts to test
these scenarios in other galaxies.
■ MONICA YOUNG

magnetic field, radio communications, the
operation of low-Earth satellites, and many
other aspects of Earth-space interactions.
Slated for a two-year primary mission, ICON
will characterize the ionosphere indirectly, by
observing airglow, as well as directly via in
situ measurements. ICON's launch came after
a series of delays since its first scheduled
launch from the Kwajalein Atoll in late 2017.
The delays were due to problems with the
Pegasus XL rocket, but ultimately the launch
went off without a hitch. Science measurements will begin at the current minimum of
solar activity, enabling scientists to focus on
terrestrial drivers of space weather. If all goes
well, ICON may observe solar maximum, too,
when the Sun's activity - and its effect on
Earth - is more variable.
■ DAVID DICKINSON

Read more about ICON's mission at:
https://is.gd/ICONlaunch.
sk yandtelescope.com * JANUARY 2 02 0

https://www.is.gd/ICONlaunch http://www.skyandtelescope.com

Sky & Telescope - January 2020

Table of Contents for the Digital Edition of Sky & Telescope - January 2020