October 2017 Discover

October 2017^ DISCOVER^71

FAR RIGHT FROM TOP: R. CARRASCO ET AL./GEMINI OBSER

VATORY/A

URA; NASA/ESA/HUBBLE HERITAGE TEAM (STScI/AURA)

THE LOCAL BULLY

Astronomers began to suspect that our

own galaxy might be a cannibal back in

1978 when Leonard Searle and Robert

Zinn noticed that globular clusters in

the Milky Way’s outer regions have a

surprisingly wide range of ages. This

could be explained, they reasoned, if

our galaxy had snacked on smaller

companions, inheriting their globular

clusters in the process. The evidence

was, admittedly, circumstantial.

Then, in 1994, Rodrigo Ibata, Mike

Irwin, and Gerry Gilmore found the

smoking gun, proof of the Milky Way’s

cannibalistic ways. Hidden against the

dense backdrop of stars toward the

Milky Way’s center was the battered

body of a small galaxy. The Sagittarius

Dwarf, as the trio of astronomers

named it, is a barely discernible pile

of stars slowly relinquishing its four

remaining globular clusters to the

Milky Way like lunch money to a

schoolyard bully. Some evidence

suggests that the Sagittarius Dwarf

may have survived several trips around

the Milky Way, allowing our galaxy

to savor its snack over hundreds of

millions of years.

There’s now overwhelming evidence

that our galaxy is a serial cannibal.

Astronomers have discovered more

than a dozen long streams of stars in

the Milky Way, vestiges of past victims

that it pulled apart like taffy before

devouring.

Researchers have dubbed one

particular patch of sky (observed as

part of the Sloan Digital Sky Survey)

the “Field of Streams” because it has

several crisscrossing ribbons of stars,

including one or more that snake

back to the Sagittarius Dwarf. Others,

like the appropriately named Orphan

Stream, have no known ancestor galaxy.

The Pan-Andromeda Archaeological

Survey, an international effort by

researchers to map the Andromeda

Galaxy using the Canada-France-

Hawaii Telescope on Mauna Kea, has

found similar stellar streams there.

Which object will be the Milky

Way’s next victim? It’s hard to say

— our galaxy doesn’t appear to be a

picky eater. The Canis Major dwarf

galaxy, whose very existence is con-

troversial because of its low density

of stars, is the nearest potential target

and may already be feeling the Milky

Way’s gravitational fury. It would add

an estimated 1 billion new stars to our

home galaxy.

Beyond that, dozens of small

galaxies surround the Milky Way, and

new ones are still being discovered,

Caught in the act! Lurking in the heart of

the galaxy cluster Abell 3827 is one of the

most massive galaxies in the local universe,

a behemoth that could swallow dozens

of galaxies the size of the Milky Way.

The partially digested remains of several

recently cannibalized galaxies are still

visible in its center.

47 Tucanae is one of the largest globular

clusters in the Milky Way. Or is it? Some

evidence suggests that 47 Tucanae might be all

that remains of what was once a dwarf galaxy.