Our nearest galaxy, Andromeda or M31, lies 2.537 million light years (or 2.4 x 1019 km) away. However, scientists have agreed that it is on a collision course for our own Galaxy, the Milky Way, with an estimated speed of 116 km/s. Most studies agree that this will happen between 4 to 5 billion years from now forming a new galaxy dubbed ‘Milkomeda’. The goal is to simulate what would happen when these two galaxies collide, and when.
A recent study, involving a group led by Riccardo Schiavi, used N-body simulations to determine the collision time of the Milky Way and Andromeda and the effects of factors, such as the density of the interstellar medium (made of gas and matter) in Andromeda’s path, on the time till close encounter. Close encounter is determined by the distance at which each galaxy would be affected by the others gravitational pull. At this time, the gravitational attraction of the two galaxies will pull both galaxies ever closer together before merging fully and forming one galaxy, Milkomeda. The research paper went further to also investigate how the two supermassive black holes at the centre of the galaxies would interact and eventually merge.
The group found that there would be a close encounter of the galaxies in around 4.3 billion years, with the merger taking place over a period of 10 billion years. Whilst the timing of closest approach is in agreement to previous predictions, the merger itself was found to happen at a later time. This could be due to the uncertainties in the relative motion of the two galaxies, which is linked to their initial velocities and the density of the space they move in. All of these are hard to figure out an exact value for, and it means the timing of the mergers can differ between studies.
One of the areas which makes this hard to calculate is the unknown extent of either galaxy’s dark matter halo. Since dark matter interacts with other matter via gravity, this would affect what could be considered ‘close encounter’, described earlier, and would affect the distance at which each galaxy would feel the other’s gravity. It will be interesting to see if further research into the nature of dark matter and our own dark matter halo will help to constrain the values needed for a more accurate simulation on the formation of Milkomeda.
Returning to the supermassive black holes, the other main focus of the study, the Milky Way and Andromeda each have one at the centre of their galaxies, which will be affected massively by their future merger. A supermassive black hole is a black hole with a mass between 0.1 and 1 million times the mass of the Sun. So the gravitational pull of both is quite intense. The simulation from the Schiavi and their group showed that these black holes would spiral around each other and merge 16.6 million years after the merger of Andromeda and the Milky Way.
The gravitational waves emitted from the inspiraling of the supermassive black holes would be detected by any civilisation with the same power as our next generation gravitational wave detectors within a redshift value, z ≤ 2. The redshift value represents the change in wavelength of these gravitational waves, which changes as it travels through space. With our current abilities to detect gravitational waves, the civilisation would have to be 3.25 million light years away from our solar system. This suggests the gravitational waves would have the power of 1019 Lⵙ (the luminosity of the sun).
Although our Sun and world will be long gone before this merger happens, these and other future studies offer a peak into the future of our galaxy. With further observations of Andromeda and research into our own galaxy, the parameters could be better constrained and a more accurate model made. For now, this simulation does confirm that the black holes of M31 and the Milky Way will collide and merge, as well as the rest of the galaxy, and gives a better time frame on when this will be.
Written by Jessie Hammond and edited by Shona Richardson