Galaxies Get Hungry, Too

It was a damp, chilly night in uptown Manhattan, and the Milky Way was busy eating another galaxy.

The Sagittarius Dwarf galaxy, a satellite of the Milky Way, is being slowly torn apart and absorbed by the large aggressor that we call home in a process known as ‘galactic cannibalism.’ An animated simulation of this violent progression – compiled by Columbia University astronomer Kathryn Johnston – shows the galactic dwarf transforming in its orbit from a tight ball to a wispy thread, like a coil of rope being pulled from both ends.

Johnston delivered a lecture on the subject of galactic cannibalism to an audience of about 125 at Columbia’s Pupin Hall on October 15. A stargazing session was slated to follow the presentation, but both Harlem and the university’s partner observatory in the Virgin Islands were blanketed in rain clouds.

Johnston began with a joke:  “When gravity has a bad day, it can always go home, look in the mirror, and think, ‘I’m attractive.’ ” It is the very attractiveness of gravity that drives small galaxies into the patient jaws of large ones and also causes dazzling collisions between two equally-sized galaxies.

Since the timescale is cosmic, the galactic collisions and absorptions can only be visualized through computer animations. But, as Johnston says, these animations aren’t just Disney movies; they are simulations of stars interacting in complete accordance to the laws of physics.

In fact, the animations weren’t designed to portray galactic cannibalism at all. They were designed to show the theoretical evolution of galaxies, and the collisions and absorptions of galactic cannibalism were what resulted.

Johnston was quick to point out that these simulations do actually jive with observation. In modeling specific scenarios, such as the collision of two spiral galaxies, distinctive, lollipop-like shapes emerge. Astrophysicists reasoned that if galactic cannibalism were actually happening, we would be able to see these stellar suckers in the cosmos.

After explaining this logic, Johnston clicked her remote, causing an image from the Hubble Telescope to slowly fade in over top of a paused animation. A wave of quiet ‘whoas’ rippled through the audience as people realized that the Hubble image was not only of a real life galactic lollipop, but that it matched up perfectly with the animation.

Our Milky Way, the Sagittarius muncher, might be part of its own lollipop-like conglomeration one day. Neighboring galaxy Andromeda is moving ever closer in its “fatal attraction” to us. Scientists estimate it will be here in around five billion years.

To Johnston, a self-described hopeless romantic, the inevitable embrace between her home galaxy and its neighbor is somehow much sweeter than other instances of galactic cannibalism:  “It’s a story of galaxies, gravity, and love.”