The spiral galaxies NGC 5953 and NGC 5954, located about 100 million light years away, are merging. They are so close that they now form an entity called Arp 91. This fascinating galactic interaction has been captured by Hubble: on the telescope pictures, we can clearly see that one of the galaxies is attracting the other. . However, the merger will not be complete for several hundred million years.
Moreover, the same fate awaits the Milky Way. According to experts, our galaxy will also eventually merge with the nearest galaxy, the Andromeda galaxy. Such gravitational interactions are common and form an important part of galactic evolution. Indeed, although the Universe is immense, galaxies are often connected to each other by filaments of intergalactic gas, real “highways” on which cosmic matter circulates, leading irreparably to the rapprochement of these space monsters.
Most astronomers believe that mergers between spiral galaxies lead to the formation of another type of galaxies, called elliptical galaxies. But unfortunately we will not see the outcome of this meeting: these extremely energetic and massive interactions take place over hundreds of millions of years! Arp 91 will therefore not really change in appearance during our short lives.
An interaction that promotes star formation
Arp 91 is only at the beginnings of the merger: the spiral structure of each of the two galaxies – NGC 5953 located at the bottom of the image, then NGC 5954 located at the top right – appears in fact still relatively intact. NGC 5954 is clearly drawn to NGC 5953 and appears to extend an arm towards it, as shown in the following animation:
Experts note that their coming together has already resulted in an explosion of star formation in all of them; the new gas entering galactic clouds causes interactions and shocks within the interstellar gas, resulting in the formation of new stars. The phenomenon is also favored by the fact that each of the galaxies has a supermassive black hole at its center, which actively absorbs matter, generating in the process powerful black hole winds that further disturb the surrounding gas.
Several studies have highlighted this explosion of stars inherent in galactic interactions. Using the ALMA submillimeter array, a team of astronomers from the Harvard & Smithsonian Center for Astrophysics, studied gas in the central regions of the Antennae galaxies – a pair of interacting galaxies about 75 million away. light years away, in the constellation Raven. Their study, published in 2017, shows that the star formation rate in this system is around ten solar masses per year, largely in the non-nuclear region (called the “overlap region”) of the two galaxies; the two nuclear regions themselves appear to have lower star formation rates.
Very common phenomena in the Universe
Astronomers have spotted many galactic fusions of this type, each ultimately – on our human scale – just a snapshot of a very long phenomenon. But by collectively studying the different interactions, the scientists were able to reconstruct the entire sequence of events, as illustrated by this simulation proposed by ESA in 2016:
In a few hundred million years, NGC 5953 and NGC 5954 are expected to eventually merge into a single elliptical galaxy, characterized by a population of low-mass “old” stars, a sparse interstellar medium, and heavy production activity. ‘minimal stars. The probable fusion of our Milky Way and Andromeda, meanwhile, is expected to occur in around 4.5 billion years, according to experts.
Note that when the merger involves two galaxies of very different size, one being much smaller than the other, the larger galaxy generally remains unchanged while it completely absorbs the second, which gradually breaks up. This is what is happening with the Milky Way which is currently engulfing two of its satellite galaxies, the dwarf galaxies of Sagittarius and Canary Islands.
It also happens that two galaxies collide, but each have sufficient speed to each continue their path after the collision. This type of collision also generates an explosion of stars (due to the interaction of clouds of gas and dust), but above all, can significantly impact the shape and trajectory of the galaxies involved.