The Milky Way Galaxy’s spiral arms play a prominent role in ensuring the habitability of Earth. Recent simulations demonstrate that this spiral structure relies on regular collisions between the Milky Way Galaxy and other smaller galaxies. Too few or too many collisions would disrupt the spiral arms, as would collisions with galaxies that are too large. Thus, the collision rate and the size of the colliding galaxies exhibit fine-tuning.
Like most parents, my wife and I struggle to teach our kids to eat the right amount of the right kinds of foods. Too little food results in them not having the strength they need. Too much food or the wrong kind of food leads to myriad other problems. But a balanced diet in the proper portions equips them to tackle their daily tasks. Likewise, the Milky Way Galaxy’s beautiful spiral arms show how our galaxy has maintained a healthy diet for the last 10 billion years.
In galactic terms (at least as far as life-friendly galaxies go), the proper “food” comes in the form of smaller galaxies. If the Milky Way (MWG) had “eaten” (collided with) too few smaller galaxies over its history, star formation would have ceased long ago. But in order to host life, a galaxy must form multiple generations of stars. This way a life-hosting solar system can form after previous stars produce elements heavier than helium (like carbon, oxygen, iron, and uranium) and scatter these elements throughout the galaxy. On the other hand, a collision with a galaxy that was too large would have disrupted the MWG’s spiral structure and led to conditions hostile to life.
New research now indicates that, in addition to driving star formation, this collision process also produces our galaxy’s spiral arm structure. A paper published in the Astrophysical Journal describes simulations of the Sagittarius dwarf galaxy’s (Sgr) interactions with the MWG.1 The simulations determine Sgr’s orbital history and reveals how the Sgr impacts the MWG’s structure. The results of the study demonstrate that this interaction, and others like it, produces significant effects on the MWG’s structure while the smaller galaxy’s material is incorporated into the MWG. Specifically, the collision results in spiral arm formation, affects the central bar, and adds energy to the outer part of the MWG’s disk.
A steady, healthy diet of smaller galaxies—like Sgr—ensured that the Milky Way could handle all the problems that had to be solved in order for life to be possible. Although galaxy collisions were (and are) frequent throughout the history of the universe, these results suggest that the life-friendly characteristics of our galaxy may not be common.
1. Chris W. Purcell et al., “The Sagittarius Impact as an Architect of Spirality and Outer Rings in the Milky Way,” Nature 477 (September 15, 2011): 301–3