If another large galaxy merged with the Milky Way how would it affect Earth and what can we do to prepare/escape in time?

Spiral galaxies like the Milky Way collide much more frequently than once assumed. Because of the expansion of the universe most galaxies are being propelled away from ours, but galaxies collide when their shared gravity force is stronger than the force that is causing the expansion of the universe.  

When galaxies collide/merge together a sudden alteration of the formation and content of the galaxies is unavoidable. When two large galaxies collide, their original shapes will be distorted, resulting in the formation of a supermassive elliptical galaxy. During these collisions large sections of each galaxy are torn off while they are orbiting and colliding resulting in extensive shock waves penetrating both galaxies, this is also one of the causes of the formation of the new elliptical galaxy. The combination of gases from both merging galaxies can cause the formation of new stars, another reason for the galaxies change in shape. While this would be a beautiful sight, it could entirely alter life on Earth.  

The change in galaxy shape could have upsides in supporting life as spiral galaxies are more prone to dangerous cosmic events compared to other shapes, an example being elliptical galaxies. While the result of a galaxy merger between the Milky Way and another could potentially be more beneficial by reducing the risk of cosmic events there are still many more risks and downsides that could take place during the merge. One theory is that the sun could be pushed further away from the spiral arm that our solar system resides on that is 27,000 light-years away from the center of the galaxy. The only star that has the capacity to provide Earth with the light, heat, and energy needed to maintain life is the sun, so if it moved further away the temperature of the earth could drop drastically, potentially freezing oceans and other large and small bodies of water. While it is unlikely that a star would pass close enough to our solar system to alter the planetary orbits, if the orbit of Earth was swapped the glaciers would melt, resulting in rising sea levels, essentially flooding the Earth. An example of a potentially harmful aspect of a galaxy merge is if an older large star passed by close to the Earth. If the star were to die creating a supernova the Earth’s atmosphere would likely be destroyed, resulting in the end of life on Earth.  

Large amounts of radiation will be released by the black holes in each of the merging galaxies as they collide as they absorb gas from their surroundings. The radiation released by black holes during galaxy merging is called gravitational radiation. Gravitational radiation does not have much of an effect on humans as one would have to be very close to the source to feel any disturbance. Unlike humans, anything energic residing on Earth would be greatly affected by the amounts of gravitational radiation emanating from the black holes. The radiation would be absorbed and transformed into real physical energy present in our world.  

The best way to prepare for a collision and/or merger between the Milky Way and another galaxy would be interstellar voyage and/or colonising other planets. Though the Earth may survive the collision it is unknown whether it would still be able to support life. A solution to whether Earth would still be habitable could be building a biosphere on an inhabitable planet or the Earth. A variation of a biosphere would also be useful if in an attempt to escape the galaxy collision humans were to go into space and live on a planet that could be colonized. If Earth became inhabitable a biosphere could also be a solution and the large amounts of radiation released while the two galaxies collide could be used to power the biospheres. Some possible habitable exosolar planets are Proxima Centauri b, Alpha Centauri A, and Tau Ceti. With current-day technology it would take thousands or millions of years to reach any of these planets. To partake in any type of space travel to flee the collision of the two galaxies all passengers must be willing to make the choice to spend the rest of their lives and future generations’ lives inside a spaceship. In reality if there was a catastrophic Earth destroying event humans would most likely perish as interstellar travel still resides in the realm of science fiction.