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EVERYTHING YOU NEED TO KNOW ABOUT THE UNIVERSE THIS MONTH...

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On March 24, 2017,

Proxima Centauri

experienced a huge

stellar flare, calling

the habitability of its

planet into question.

RED DAWN

The Opportunity

rover has now

witnessed more

than 5,000 sunrises

on Mars.

GOLDEN AGE

NASA’s Global-scale

Observations of the Limb and

Disk (GOLD) satellite powered

up January 28, despite a launch

anomaly that will delay arrival

in its final orbit.

SNAPSHOT

Centaurus A

contradicts

dark matter

models

New observations confirm

what astronomers have seen

elsewhere ... and challenge

current dark matter theories.

Large galaxies, including our

own, maintain systems of smaller

satellite galaxies through gravity.

According to the current stan-

dard cosmological model, these

satellites orbit within a halo of

dark matter stretching far past

the visible portion of the parent

galaxy. Satellites also should be

randomly distributed in orbit and

positioned around their parent

galaxy — but new observations

have just shown, for the third

time, that this is not the case.

The results, published February

2 in Science, show that the satellite

galaxies surrounding Centaurus A

(NGC 5128), an elliptical galaxy

13 million light-years away, are

not orbiting randomly. Instead,

they are orbiting in a nice, orderly

fashion in a well-defined plane.

Such observations confirm what

astronomers have already seen

around the Milky Way and the

Andromeda Galaxy, but f ly in the

face of the standard model, which

says that such ordered systems of

satellites should be rare — as in,

0.5 percent of galaxies should have

them. Instead, astronomers have

now seen them in 100 percent of

observed systems.

Of course, outliers that don’t

follow the current model’s pre-

dictions should exist. And three

galaxies is an incredibly small

sample. Even so, these new

observations now confirm that

satellites are much more likely to

orbit in an orderly fashion than

believed. “Coherent movement

seems to be a universal phenom-

enon that demands new explana-

tions,” said Oliver Müller of the

University of Basel in

Switzerland, and lead author of

the study, in a press release.

Müller’s team discovered that

Centaurus A’s satellites appear

arranged neatly on a thin plane

seen edge-on when viewed from

Earth. Such an orientation means

any Doppler shifting of the light

received from the satellite galax-

ies is due to their motion around

the galaxy’s center. Of 16 satellite

galaxies observed in the study, 14

are rotating together around the

center of the galaxy. This is con-

sistent with previous observa-

tions of the distribution and

motion of satellites around the

Milky Way and Andromeda.

The ultimate conclusion is that

it’s much more common for satel-

lite galaxies to move together than

current dark matter models pre-

dict. While these results put

added force behind a blow to

astronomers’ understanding of

dark matter, they don’t necessarily

mean that dark matter is no more.

What they do mean, however, is

that current models of the way

dark matter interacts with normal

matter are not completely correct

— which makes sense, given that

astronomers have yet to detect it

directly. Challenges to current

models are the best way to hone

and improve those models, push-

ing them to better match the uni-

verse we observe. — Alison Klesman

ORDERED MOTION. Centaurus A is an elliptical galaxy whose satellites orbit in an ordered fashion on a well-defined plane —

an observation at odds with predictions from dark matter models.

CHRISTIAN WOLF & SKYMAPPER TEAM/AUSTRALIAN NATIONAL UNIVERSITY; TOP FROM LEFT: ROBERTO MOLAR CANDANOSA/CARNEGIE INSTITUTION FOR SCIENCE, NASA/SDO, NASA/JPL; NASA/JPL-CALTECH/CORNELL/ARIZONA STATE UNIV./TEXAS A&M; NASA GODDARD’S CONCEPTUAL IMAGE LAB/CHRIS MEANEY