2019-06-01+Sky+and+Telescope

skyandtelescope.com • JUNE 2019 13

pLOFAR captured radio emission (orange) in

galaxy cluster Abell 1314, superimposed on

an inverted visible-light image (grayscale). The

extended radio emission in this image comes

from past collisions with other clusters.

SCIENTISTS STUDYING New Hori-

zons images of craters on Pluto and its

moon Charon have concluded that the

outer solar system contains fewer small

objects than expected.

In the March 1st Science, Kelsi Singer

(Southwest Research Institute) and

colleagues tallied up the craters on old,

smooth surfaces on Pluto and its largest

moon. If the Kuiper Belt is — or was —

crowded enough for frequent collisions,

astronomers ought to have seen many

more craters formed by small objects

than by big ones.

But what Singer’s team found was

that craters less than 13 km (8 miles)

across were surprisingly scarce on Pluto

and Charon. Geological processes such

as cryovolcanism and glacial activity

would have erased the bigger craters

along with the small ones, the scien-

tists say. Moreover, there are no known

processes that would preferentially erase

smaller craters. The dearth of small

craters translates to a lack of Kuiper

THE LOW FREQUENCY ARRAY

(LOFAR), which is exploring the sky

at low radio frequencies, has found

325,694 new radio sources, most of

them faraway galaxies.

These results represent the fi rst

phase of LOFAR’s ambitious Two-metre

Sky Survey and appeared February in a

special issue of Astronomy & Astrophys-

ics. The accompanying high-resolution

mosaic images, which include radio

emission at frequencies between 120

and 168 MHz, show details as fi ne as 6

arcseconds across. Under ideal circum-

stances, LOFAR may return images with

a resolution of 0.5 arcsecond.

About 70% of the new radio sources

have visible-light counterparts in the

Sloan and Pan-STARRS surveys, provid-

SOLAR SYSTEM

Pluto & Charon Missing Small Craters

uSinger and colleagues

counted craters on the

smooth, geologically stable

Vulcan Planitia on Charon,

shown here.

Belt objects spanning less than 1 to

2 kilometers.

The fi ndings appear to contradict

results from a recent star-monitoring

survey conducted by Ko Arimatsu

(National Astronomical Observatory

of Japan) and colleagues, who spotted

a single stellar occultation and inter-

preted it as a kilometer-size Kuiper

Belt object passing in front of a back-

ground star (S&T: May 2019, p. 9).

Since even a single fi nd

was unexpected, the

scientists argued that

the discovery points to

an excess of kilometer-

size objects in the outer

solar system. However,

Arimatsu acknowledges

that extrapolation from

a single event is diffi cult; one kilometer-

size object could still be consistent with

the New Horizons results.

If small objects — and thus colli-

sions — are truly scarce in the Kuiper

Belt, then its population contains

isolated and pristine remains of the

early solar system. Further study of the

population’s size distribution could help

scientists distinguish between different

theories of planet formation.

■ MONICA YOUNG

• Read more about both studies at
  https://is.gd/smallKBO.

ing astronomers with rough distance

estimates. Most of the sources are dis-

tant galaxies whose central black holes

power jets.

LOFAR sees jet activity on much

longer time scales than existing, higher-

frequency radio surveys. The radio

waves from the galaxies that LOFAR

has found are primarily generated when

electrons spiral along magnetic fi eld

lines that thread the jets. After these

electrons have been spiraling for a

while, they slow down and emit lower-

frequency radio waves.

One intriguing result is that all the

most massive galaxies appear to exhibit

long-lasting jet activity close to their

cores, which suggests that their central

black holes are feeding more or less

continuously. The new observations will

shed light on the evolution of super-

massive black holes over cosmic time.

Eventually, astronomers’ goal is to fi nd

the very fi rst supermassive black holes

in the universe.

There’s much more to come. The fi rst

phase of the survey covers 424 square

degrees, centered on the handle of the

Big Dipper. But that’s only about 2%

of the eventual survey area, which will

cover the whole northern sky.

■ GOVERT SCHILLING

GALAXIES

Radio Survey Finds

Hundreds of Thousands

of Galaxies

100 arcsec

Vulcan Planitia

150 km 20 km

SUR 93 miles 12 miles

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