Astronomy

billion light-years

9.25 9.5

Twin Quasar (Q 0957+561)

9.18 billion light-years

52 ASTRONOMY • DECEMBER 2015

dominated by dark matter and gas that
never formed stars. “Due to its proximity,
the Sculptor Group was the most obvious
choice for this study,” he says, “enabling
observations at sufficient resolution and
sensitivity to detect potential HVCs.”
They found the population they
expected around NGC 55 but none around
NGC 300. This discovery automatically
knocked out the idea of dark satellite gal-
axies because both objects, being around
the same size, should have formed a simi-
lar set of satellites. The lack of clouds, then,
suggested that HVCs come from gas flow-
ing into or out of the main galaxy, which
happens when bigger galaxies eat smaller
ones or when stars form.
The big surprise from the study, how-
ever, was about wonkiness in the big galax-
ies’ gaseous disks. They appear
to wobble as they travel
through space under
the inf luence of a force
called ram pressure.
This could happen

only if the galaxy slammed into a dense

intergalactic medium. While astronomers

know that large clusters bathe in dense

soups of material, they thought smaller

groups sat in thin broths.

The pressure blows gas away, meaning

stars can’t form as well — or at all in the

case of small satellites. “As gas constitutes

the fuel for future star formation, its

removal could potentially shut down star-

formation activity in dwarf galaxies living

in group environments,” says Westmeier.

This discovery helps solve a mystery

called the “missing satellites problem,”

which arose about 15 years ago when com-

puter simulations showed that galaxies like

the Milky Way should have more dark

dwarf-sized companions than they do.

“This discrepancy suggests that either there

is something wrong with the simulations

or some mechanism has prevented the

majority of dark matter halos from turning

into proper galaxies,” says Westmeier. If

pressure strips gas away, as it seems to

in the Sculptor Group, small galaxies

wouldn’t be able to form stars, meaning the

satellites aren’t missing — they’re invisible.

The unusual suspects

The Centaurus A Group lies at a similar

distance as Sculptor, some 12 million light-

years from Earth. The group’s dominant

member, Centaurus A, is the closest radio

galaxy. This massive object swallowed a

spiral galaxy about 500 million years ago.

The digestion is ongoing, though, and

it causes radio waves to blast into space.

Centaurus A serves as an example of what

could ensue when the Milky Way and

Andromeda galaxies collide 4 billion

years from now.

A more visually recognizable crowd

— the M81 Group — resides at the same

distance. Located inside the boundaries of

Ursa Major and Camelopardalis, it con-

tains 1 trillion solar masses and 34 known

galaxies, the most famous of which are

M81 and M82. The two tug on each other,

which has turned M82 into a star factory.

The galaxy’s center glows 100 times

brighter than the Milky Way’s as hydrogen

gas falls from intergalactic space toward

the gravitational hub.

The galaxy causing some of this ruckus,

M81, holds the title of “Biggest Galaxy in

its Group.” At the center of its perfect spiral

arms lurks a supermassive black hole

weighing 70 million solar masses.

Chaotic concentrations

In contrast to the order in the M81 Group,

the Canes Venatici I Group seems chaotic.

Located 15 million light-years away in the

constellations Canes Venatici and Coma

Berenices, this group’s members are bound

only loosely to one another. Not all of its

approximately 20 galaxies have settled

into stable orbits, and they seem to be just

weakly connected gravitationally.

Several million light-years farther out

and back in the direction of Ursa Major,

the M101 Group has the same loose, dis-

connected air. This group’s namesake

keeps the family together. Most of the

other galaxies in the group are merely

companions of this Milky Way twin.

M101 spans 170,000 light-years, has tightly

wound spiral arms, and holds 100 billion

solar masses’ worth of material.

Less than 20° to the southwest, we

find the Canes Venatici II Group. Thirty

Galaxies Maffei I and Maffei II are the centers

of the two closest galaxy groups. This infrared

image penetrates material in our galaxy that

hides the two in visible light. NASA/JPL-CALTECH/WISE TEAM

Centaurus A dominates its group from a distance

of 12 million light-years. The massive galaxy

shines brightly in both visible light and radio

waves, the latter a result if its having consumed

a companion galaxy 500 million years ago. ESO

NGC 300 resides on the near side of the Sculptor
Galaxy Group, whose bulk lies 12 million light-
years from Earth. The galaxy appears nearly as
big on the sky as the Full Moon. ESO

NASA/ESA/HUBBLE (TWIN QUASAR, EINSTEIN’S CROSS)

Maffei I

Maffei II