Galaxy fusions are beautiful sights, but ultimately fatal. During the collision, the combined galaxy will glow brighter than ever before. But that fame comes with a price: all of these new stars use up all available fuel, and star formation comes to a standstill.
Our own galaxy, the Milky Way, will merge with our closest neighbor, Andromeda, in about 4 billion years. And our galaxy is not alone: galaxies are merging with one another throughout the universe, as evidenced by the abundance of galaxies caught in the act (so to speak) through our observations.
It is difficult to find out the details of the fusion process and its effects on the galaxies. We cannot see a single galaxy fusion event happening in real time because it takes hundreds of millions of years to complete. However, from the multitude of snapshots that we can see, we can observe different stages of the process.
A recent study of a merged galaxy known as ID2299 adds to the intricate portrait. As far as we can tell, it is a glorious sight when galaxies merge for the first time: star formation increases from all collisions of gas clouds and extreme gravitational interactions. In short, the merged galaxies can shine up to ten times brighter than individually.
But to make stars, you need fuel in the form of cold gas reserves. And unfortunately, the fusion event is heating the galaxy in different ways. On the one hand, the central supermassive black hole feeds on fresh rounds of material that are brought into the core. This feeding event triggers the release of intense amounts of radiation that flood the surrounding galaxy. Second, all these new stars contain tons of large, hot, bright stars that also flood the galaxy with high-energy radiation, especially when they go into supernova.
Taken together, the intense heat and lack of raw materials suppress the formation of new stars, and the merged galaxy eventually retreats into galactic retirement and disappears from the cosmic state.
At least it was fun while it lasted.
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