Four billion years ago and 40 million light years away, an act of galactic cannibalism was committed. No human eyes saw the smaller galaxy shredded and ripped apart by the gravitational might of the larger. No tears were shed when its remains were finally devoured, but clues of the catastrophe remain to this day.
High in the northern sky in the constellation Draco the Dragon ghostly ribbons of stars and dust swirling about the edge-on spiral galaxy NGC 5907 are all that remains of its one-time companion. Don’t expect to see those ethereal streams in a typical telescope; only long time-exposures reveal the remaining clots of dust and streams of stars torn from the companion galaxy during the close encounter and likely merger. The gigantic loops extend for more than 150,000 light years from NGC 5907, nicknamed the Knife-Edge or Splinter Galaxy.
Astronomers call these rivers of stars tidal streams. They’re created by disruptive gravitational tides induced when two galaxies pass near or through one another. As the companion orbited through the Splinter’s disk, repeated encounters stripped it of most of its goodies – stars, dust clouds and even dark matter – and flung the material into multiple tidal streams. When nothing but the compact stellar core was left it presumably merged (was eaten) by NGC 5907 and lost its individuality forever.
Or … it could have happened a different way when two equally mighty galaxies on a collision course yanked swirls of stellar debris from each other before their eventual merger into one serene spiral we see today. Here neither was the winner – two came together much as the Milky Way and Andromeda Galaxy will several billion years from now to meld into something altogether new.
As far as galactic cannibalism goes, we needn’t look any farther than our own Milky Way, which has been munching on the Sagittarius Dwarf Galaxy for at least the past billion years. Four separate whirls of stars peeling off above and below the plane of the Milky Way attest to our galaxy’s systematic preying on what was once a bright companion. Now half its contents are gone, dumped along the winding highway by an unrepentant galactic litter bug. Over time the dwarf and its contents will be fully absorbed by our galaxy with little left of its presence save a few stray globular clusters that once called it home.