OUT THERE
707070 DISCOVERMAGAZINE.COMDISCOVERMAGAZINE.COMDISCOVERMAGAZINE.COMNEARLY A CENTURY after Edwin Hubble
established the nature of galaxies
as distant “island universes,” our
understanding of their birth and
evolution remains incomplete. While
much current astronomical research
focuses on galaxy formation, a growing
body of evidence suggests that galaxy
destruction is also common. Countless
galaxies have met their demise over the
13.8 billion-year history of the cosmos.
The carnage is everywhere.EAT AND BE EATEN!
Galaxies are a gregarious bunch —
where there’s one, there are usually
others. This makes isolated giant
galaxies like NGC 1132 and ESO
306–017 cosmic oddities. Even more
strangely, these hermits reside in vast
seas of dark matter and hot gas that
normally would be home to tens or
hundreds of other galaxies.
NGC 1132 and ESO 306–017 have
a sinister secret: They’re cannibals.
Evidence suggests that these behemoths
grew to their bloated sizes by devouringtheir neighbors in a galactic feeding
frenzy that would make Hannibal
Lecter proud.
And they’re not the only ones.
Telescopes capture images of large
galaxies devouring smaller ones in
brazen acts of cosmic cannibalism, a
single moment of a billion-year struggle
frozen in time. Other galaxies are ripped
apart by violent gusts of gravity from
passing neighbors, scattering luminous
shards along their path like a jet fighter
breaking up in flight. The universe
is littered with disembodied streams
of stars, gas, and dust — the ghostly
remains of once normal galaxies.
Galaxy clusters, the urban centers of
the cosmos, are especially dangerous
places. Here, hundreds of galaxies
are crammed into a region roughly
spanning the distance between the
Milky Way and Andromeda galaxies.
Trapped in such close confines, they
turn on each other. And as a cannibal
moves up through the weight classes, its
increased gravity makes grabbing new
victims easier. Survival of the biggest is
the rule.
It’s a harsh reality
for small galaxies. Tiny
“ultracompact dwarfs,” an
intriguing class of galaxies
discovered less than two
decades ago, appear to
have been slowly flayed by
repeated encounters with
larger brutes until all that
remains are their exposed
innards — a compact
nucleus. Astronomers have
now spotted hundredsof ultracompact dwarfs, most in the
nearby Fornax, Virgo, and Coma
galaxy clusters.
The surviving nuclei of some stripped
galaxies might even masquerade as
globular clusters. Some researchers have
speculated that several of the Milky
Way’s largest globular clusters, such as
47 Tucanae and Omega Centauri, were
once small galaxies.
G1, the brightest globular cluster
in the Andromeda Galaxy — and
indeed the entire Local Group — has
a number of puzzling features that set
it apart from other globulars. These
include an unusually elongated shape
and the presence of multiple genera-
tions of stars within it. (Most globular
clusters are composed of members with
uniform ages.) There’s even evidence
that G1 might harbor a black hole with
a mass equal to 20,000 suns, something
not seen in normal globular clusters.When Galaxies
Become Cannibals
Some galaxies harbor a sinister secret: They’ve eaten their siblings.
BY MICHAEL WESTLEFT: NASA/ESA/THE HUBBLE HERITAGE TEAM (STScI/AURA)-ESA/HUBBLE COLLABORATION (2). ABOVE: ESO/JOSE FRANCISCOGravity shepherds galaxies together
to form systems ranging from small
groups to immense cosmic cities with
thousands of members, like the Fornax
galaxy cluster. But monsters lurk
among them: cannibal galaxies.Left: Isolated giant galaxies like NGC 1132 are believed to
be cannibals that have devoured their neighbors. Most of
the smaller galaxies seen in this image are foreground or
background objects not associated with NGC 1132. Right:
The Hubble Space Telescope captured this image of cosmic
cannibal ESO 306–17, a gargantuan galaxy about 500 million
light-years away.