Don’t mess with the Milky Way. A dim galaxy in the constellation Hercules has learned this lesson the hard way after diving into our galaxy and being torn asunder by its gravitational pull.
At least 50 galaxies orbit our own. The Hercules dwarf is currently 460,000 light years from Earth, nearly three times farther than the Milky Way’s brightest satellite galaxy, the Large Magellanic Cloud.
But Hercules’s stars spread over a great expanse of space, which suggests the Milky Way’s gravitational pull has yanked them away from one another.
Now Andreas Küpper and Kathryn Johnston at Columbia University in New York and their colleagues have used the observed positions and velocities of the galaxy’s stars to deduce its path through space.
“It’s plunging in from a really large distance to a really close distance,” Johnston says.
The astronomers calculate that, at its farthest, the dim galaxy voyages 600,000 light years from the Milky Way’s centre. At its closest, though, it’s just 16,000 light years out.
“That’s extreme,” Küpper says. It’s closer than any other satellite galaxy is known to come. It’s even closer than we are – the sun is 27,000 light years from the galactic centre.
But according to computer simulations the scientists conducted, the galaxy’s last passage proved fatal. As Hercules neared the Milky Way, half a billion years ago, material from our galaxy invaded the dwarf galaxy. The gravitational force of this additional matter pulled the galaxy’s stars and dark matter toward its centre.
Then, as the dwarf galaxy skirted away from the Milky Way’s centre, the stars and dark matter rebounded, causing Hercules to “explode” into its current distended state.
Today, Hercules’s stars owe their allegiance to the Milky Way rather than to one another, because they are so dispersed they no longer feel much of their siblings’ gravitational pull.
Still, the stars continue moving on similar paths, like parachutists jumping from the same aeroplane. The next time the galaxy ventures toward us, “you might be able to find it as a stream of stars zipping through at very high speed”, Johnston says.