Hubble captures details of ‘imminent’ galaxy fusion

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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 a unit called Arp 91. This fascinating galactic interaction has been captured by Hubble: in the images of the telescope we can clearly see that one of the galaxies attracts the other. However, the merger will not be complete until hundreds of millions of 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. In fact, although the universe is huge, galaxies are often interconnected by filaments of intergalactic gas, veritable “highways” on which cosmic matter circulates, inevitably leading to the approximation of these space monsters.

Most astronomers believe that fusions between spiral galaxies lead to the formation of another type of galaxy, called elliptical galaxies. But unfortunately, we will not see the result of this encounter: 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 favors the formation of stars

Arp 91 is only at the beginning of the fusion: 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 – actually still appears relatively intact. NGC 5954 is clearly drawn towards NGC 5953 and appears to extend an arm in its direction, as shown in the following animation:

Experts note that their proximity has already resulted in an explosion of star formation in each of them; the new gas entering the galactic clouds causes interactions and shocks in the interstellar gas, which generates the formation of new stars. The phenomenon is also favored by the fact that each of the galaxies has a supermassive black hole in the center, which actively absorbs matter and generates strong black hole winds in the process, which further disrupts 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 Antenna galaxies – a pair of interacting galaxies about 75 million light-years away, in the constellation Raven. Their study, published in 2017, shows that the rate of star formation in this system is about ten solar masses per year, mainly in the non-nuclear region (known as 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 seen many such galactic fusions, each ultimately – on our human scale – just a snapshot of a very long phenomenon. However, by jointly studying the different interactions, the researchers were able to reconstruct the entire course of events, as illustrated by this simulation proposed by ESA in 2016:

In a few hundred million years, NGC 5953 and NGC 5954 would eventually merge into a single elliptical galaxy, characterized by a population of “old” low-mass stars, a sparse interstellar medium, and dazzling activity. Stars minimal. The probable merger of our Milky Way and Andromeda should meanwhile happen in about 4.5 billion years, experts say.

Note that when the fusion involves two galaxies of very different sizes, where one is much smaller than the other, the larger galaxy generally remains unchanged while completely absorbing the other, which gradually breaks down. This is what happens to the Milky Way, which is currently engulfing two of its satellite galaxies, the dwarf galaxies Sagittarius and Canis Major.

It also happens that two galaxies collide, but each has 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 can above all significantly affect the shape and orbit of the galaxies involved.

Source: ESA / Hubble

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