Astronomy

10.75

CL J1449+0856 (farthest galaxy cluster)
10.51 billion light-years

WWW.ASTRONOMY.COM 55

many of its members — including M84,

M86, M87, and the Sombrero (M104) and

Black Eye (M64) galaxies — through mod-

est backyard telescopes.

Because Virgo has so many galaxies

at approximately the same distance from

Earth, astronomers use the cluster as a lab-

oratory to study galaxy evolution. Just as

an alien visitor would learn more about

human development by looking at a sta-

dium full of different people than at a sin-

gle family, so too can astronomers learn

more about galaxies from a huge batch like

the one in Virgo.

One thing they learned recently deals

with star formation — more specifically,

stars that are not forming as fast as many

expected. A 2014 study found that turbu-

lence — the kind that shakes airplanes

— in the center of Virgo shakes up star-

making gas, keeping it hot for billions of

years. That turbulence, which comes from

active black holes in galaxy centers shoot-

ing out powerful jets, prevents the gas from

becoming stable enough to form stars.

“The energy in such slow motions is

more than enough to stop gas from cata-

strophic cooling and star formation,” says

lead researcher Irina Zhuravleva of Stan-

ford University in Palo Alto, California.

Understanding why stars stop forming at

the centers of galaxies and clusters helps

astronomers understand how their lives

progress and how the life of our own gal-

axy and group might unfold.

Fish and bears

Even farther out, a 59-million-light-year

jaunt, lies the Dorado Group. This large

collection comprises 70 galaxies in the

direction of the southern constellation

Dorado the Dolphinfish (a species that

appears on menus as “mahi-mahi”). One

of its most prominent members, NGC

1483, shows a luminous central bulge and

jumbled spiral arms that host bright star-

forming regions and young star clusters.

At approximately the same distance but

in the direction of Ursa Major, two groups

form a defined band. Most of its major

galaxies are spirals, and these com-

bine with the minor ones to pro-

duce 30 percent as much light

as the Virgo Cluster. Thirty

percent may not sound

impressive, but consider

that the Ursa Major groups contain just 5

percent of Virgo’s mass.

Thirty-two galaxies call the Ursa Major

North Group home, including the bright

spirals NGC 3631, NGC 4088, NGC 3953,

and M109. The last of these looks most

similar to the Milky Way. Of all 109 objects

in the Messier catalog, it lies farthest away,

meaning that 18th-century telescopes

couldn’t decipher things beyond this point.

Now, however, large scopes on Earth and

in space give us glimpses of galaxies bil-

lions of light-years more distant.

Moving targets

But classifying those galaxies into catego-

ries has always been a problem. They orient

themselves differently relative to us, and

a face-on spiral looks quite different from

one seen edge-on. We only have a 2-D

sense of galaxies, as if they’re projected on

a screen, rather than a true 3-D picture.

The problem is particularly acute for disk-

shaped galaxies, such as spirals, and sphe-

roidals. You can tell the difference, though,

by watching how their stars move. If they

rotate slowly and disorderly, they’re sphe-

roidal. If they rotate fast and in an orderly

manner, they are disk galaxies.

A team of astronomers led by Nicholas

Scott of the University of Sydney in Aus-

tralia set about to study the stellar motions

in the Fornax I Cluster, 62 million light-

years away. They found that 93 percent of

the cluster’s galaxies were fast rotators (spi-

rals) and just 7 percent were slow rotators

(spheroidals). Combining this study with

those of other clusters, including the larger

one in Virgo, Scott’s team found that the

spheroidals tended to hover near a cluster’s

center — either because they began there

or migrated there.

“One of the long-debated ideas is the

question of nature versus nurture in galaxy

evolution,” says Scott. Is how a galaxy looks

today totally determined by its properties

in the early universe, or do its surround-

ings play a role too? “This study shows that,

for at least some galaxies, the environment

does play a major role.” He adds that many

questions still exist, which further cluster

studies can answer.

More questions always exist about

the cosmos — its stars, galaxies, groups,

clusters, superclusters, and beyond. And

because the universe is organized into

superclusters, their constituents stay “close”

together. Astronomers can swing their

telescopes and find a stadium alive with

variety. They learn not just what the more

distant universe is like, but also what it

used to be like, what it will be like, and

what has happened and will happen even

closer to home.

The giant Virgo Cluster contains more than a

thousand galaxies. The central region seen here

includes the cluster’s most massive member,

M87, below center. TERRY HANCOCK

At 62 million light-years, the Fornax I Galaxy

Group (and NGC 1427A, seen here) is one of the

Local Supercluster’s most distant members.

NASA/ESA/THE HUBBLE HERITAGE TEAM (STS

CI/AURA)