Dr. Kelly Holley-Bockelmann, Vanderbilt University – The Milky Way’s Violent Merger

Jul 25, 2013

In today’s Academic Minute, Dr. Kelly Holley-Bockelmann of Vanderbilt University reveals the evidence for a violent merger of the Milky Way with another galaxy.

Kelly Holley-Bockelmann is an assistant professor of physics and astronomy at Vanderbilt University in Nashville, Tennessee. Her research interests include galaxy dynamics, N-body simulations, supermassive black holes, and gravitational waves.

About Dr. Holley-Bockelmann

Dr. Kelly Holley-Bockelmann – The Milky Way’s Violent Merger

Right now, the galactic center is a pretty calm place -- sure, there's a black hole lurking there, weighing in a 4 million times heavier than the Sun, but this supermassive monster isn't actively devouring stars or much gas, so it's dim and quiet. But, there's a wealth of forensic evidence that suggests that a violent event may have taken place at the heart of our Milky Way just 10 million years ago -- one that irrevocably changed the structure of our galactic core.

Clue number 1 is that our galactic center is teeming with new stars, all born roughly 10 million years ago. Clue number 2 is the newly-discovered Fermi Bubbles, two enormous galaxy-sized globes of gamma-ray light spouting from the galactic center straight above and below the galaxy disk. Clue number 3 is that the center is strangely missing tens of thousands of old stars.

My team and I suggest that all these clues can be explained by one single small galaxy that collided with our own about 10 million years ago. As this small galaxy plummeted toward our core, it was shredded until only a mere skeleton of stars and its own massive central black hole remained. This shredded satellite disturbed the gas near the Milky Way core, setting off a new burst of central star formation (Clue 1-- new stars, check!).  The remaining disturbed gas funneled into our supermassive black hole -- the Fermi Bubbles are just our black hole's energetic burp after a gas-rich meal. (Clue 2 --Fermi Bubble, check!)

Now what about those missing older stars? We suggest that when our supermassive black hole meets the one in the other galaxy, the two black holes pair up and spin aroun one another in a relativistic dance that flings out any stars that veer too close -- think of those really enthusiasic couples that manage to clear a crowded dance floor (Clue 3 -- missing stars, check!). Unlike a criminal trial, a verdict on the Milky Way mystery doesn't come from a jury of peers, but from more hard evidence -- evidence that either exonerates a satellite collision, or that confirms a prediction only matching our explanation of the events, a smoking gun if you will.

Our 'smoking gun' would be finding the missing old stars -- these stars would be flung from the galactic center at breakneck speeds by this binary black hole dance, and we're searching for these ejected hypervelocity stars now.


Production support for the Academic Minute comes from Newman’s Own, giving all profits to charity and pursuing the common good for over 30 years, and from Mount Holyoke College.