Galactic mergers result when two or more galaxies collide, and these smash-ups are the most violent type of galaxy interaction in the Universe. Indeed, the gravitational dance between colliding and merging galaxies, and the friction between the dust and gas, have dramatic effects on the doomed galaxies involved. These mergers are important because the merger rate is a fundamental measurement of galaxy evolution, and it also provides astronomers with clues that can help them solve the mystery about how galaxies grew over time. In June 2019, a team of astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, observed the earliest combined signals of carbon, oxygen, and dust from a galaxy in the Universe, seeing it as it appeared 13 billion years ago. By comparing the different signals, the astronomers found that the galaxy is really a duo of merging galaxies–making it the earliest example of galactic mergers yet observed in the 13.8 billion-year-old Universe.

Dr. Takuya Hashimoto of Waseda University in Japan, and his colleagues, used ALMA to observe a remote object with the telephone-book-sounding name of B14-65666, which is situated about 13 billion light-years from Earth in the constellation Sextans. Because of the finite speed of light, the signals that astronomers now receive from B14-65666 had to travel through Space and Time for 13 billion years in order to reach telescopes on Earth. Hence, the observations reveal to astronomers what the duo of doomed dancing galaxies looked like less than 1 billion years after the Big Bang birth of the Universe.

In cosmology, long ago is the same as far away. Because of the expansion of the Universe, and the universal speed limit set by light, the more distant an observed celestial object is in space, the older it is in time–hence, Spacetime. Time is the fourth dimension. It is impossible to locate an object in space, without also locating it in time. No known signal can travel faster than light in a vacuum, although space itself can. The light emitted by a distant object billions of years ago is seen by observers on our planet as it looked at the time it was first emitted.

ALMA managed to achieve the earliest observation of radio emissions from carbon, oxygen, and dust in B14-65666. The discovery of multiple signals enabled astronomers to determine additional relevant information.

Galaxy Smash-ups

During a galaxy merger, both stars and dark matter within within each of the doomed galaxies, fall under the gravitational influence of the approaching galaxy. Dark matter is a mysterious substance that is thought to account for most of the matter content of the Cosmos. It is believed to be composed of exotic non-atomic particles that do not interact with light or any other form of electromagnetic radiation and, as a result, is transparent and invisible. However, most astronomers think that it is really there because of its gravitational influence on objects that can be observed.

Toward the later stages of a galaxy merger, the gravitational potential (the shape of the galaxy) starts to change so rapidly that the orbits of its constituent stars are affected, and they lose any “memory” of their previous orbits. This process is termed violent relaxation. Hence, if two disk galaxies blast into one another, the entire devastating process starts off with their populations of stars orbiting in an orderly rotation in the plane of their disks. As the merger progresses, however, the orderly motion experiences a sea-change into random energy. The galaxy becomes dominated by stars that orbit the galaxy in a random, complex web of disordered orbits. Elegant and orderly galaxies include spirals like our own Milky Way, and lenticular galaxies which are sometimes referred to as “spirals without arms.” Astronomers observe stars on random chaotic orbits in elliptical galaxies, which are usually very large football-shaped galaxies populated by elderly red stars.

A fury of fiery baby star birth comes in the wake of a galaxy merger. The star formation rate (SFR) that occurs during a major merger can produce thousands of solar masses of bright new stars every year, depending on the amount of gas contained in each of the merging galaxies, as well as each galaxy’s redshift. SFRs are less than 100 new solar-masses of stars each year during a typical merger. This is a large number when compared to that of our own Galaxy, which gives birth to only a few (about 2) fiery new baby stars every year. Even though the distance between stars is normally great enough to prevent individual stars from blasting into each other when their host galaxies merge, frigid molecular clouds quickly tumble down to the heart of the galaxy where they collide with other molecular clouds. The collisions that occur between these dark and gigantic star-birthing clouds, trigger condensations of these clouds into baby stars. Astronomers have seen this happen in merging galaxies that are relatively near to us in the Universe. However, this process was more intense during the mergers responsible for creating the elliptical galaxies that dwell in our Universe today. The elliptical galaxies that we now observe probably formed between 1 and 10 billion years ago, when there was much more gas available–and, hence, many more molecular clouds–contained within galaxies. In addition, in regions away from a galaxy’s center, clouds of gas will bump into one another. This produces shocks which also trigger the birth of new stars within gas clouds. As a result, galaxies start to run out of gas, and will produce fewer new stars. Therefore, if a galaxy experiences a major merger with another of its kind, and then a few billion years pass, the galaxy will have very few young stars left. This is why ellipticals are populated by elderly stars–they have scanty amounts of molecular gas and very few young stars. It has been proposed that elliptical galaxies are the end stage of major galaxy mergers which use up most of the gas during the merger. For this reason, there is little additional star-birth after the merger is quenched.

Our Milky Way Galaxy and the Andromeda galaxy are the two largest members of what is called the Local Group of Galaxies. Both galaxies are spirals–twirling starlit pin-wheels in Spacetime. The Local Group, in turn, is located near the outer edge of the Virgo Cluster of galaxies, whose big, bright elliptical galactic heart is currently approximately 50 million light years from us.

Billions of years from now, our Milky Way and Andromeda will blast into each other and merge to become one gigantic galaxy. After the collision, what was once a galactic duo, will undergo a metamorphosis. When this smash-up occurs, the merger of this pair of spirals will produce an entirely new galaxy. The new galaxy that emerges from the wreckage of the Milky Way and Andromeda will likely sport an elliptical shape, instead of the elegant–and much more orderly– pin-wheel shape that is displayed by the two wandering galaxies today. The new galaxy will rise from the funeral pyre of its spiral progenitors, and this future galaxy already has a name–the Milkomeda Galaxy.

Today, our Milky Way and Andromeda are rushing towards one another through the space between galaxies at the breathtaking clip of 250,000 miles per hour. For a long time, astronomers have speculated that the duo of large starlit spirals are doomed to suffer a violent collision in the distant future. This catastrophe will likely be messy. Alas, the Andromeda galaxy is flying straight in our Milky Way’s direction, and when it finally hits our galaxy, it will eat it. Technically, Andromeda will devour our Milky Way because it is the more massive of the pair. Like fish swimming around in a celestial sea, the bigger fish dine on their smaller kin.

What Goes Around Will Come Around

An analysis of the new ALMA data, indicated to the team of astronomers from Waseda University that the emissions are actually separated into two distinct blobs. Earlier observations with the Hubble Space Telescope (HST) showed two-star clusters in B14-65666. Afterwards, using the three emission signals spotted by ALMA, the team was able to demonstrate that the two blobs formed a single system, but with different speeds. This indicated that the blobs were two merging galaxies–as well as the earliest known example of merging galaxies. The team of astronomers calculated that the total stellar mass of B14-65666 is less than 10% that of our Milky Way. This means that it’s in its earliest phases of evolution. Despite its relative youth, B14-65666 is giving birth to bright new fiery baby stars 100 times more actively than our Galaxy. Such an active star formation rate is another sign of galactic mergers. This is because the gas compression in colliding galaxies naturally results in bursts of stellar formation.

“With rich data from ALMA and HST, combined with advanced data analysis, we could put the pieces together to show that B14-65666 is a pair of merging galaxies in the earliest era of the Universe. Detection of radio waves from three components in such a distant object demonstrates ALMA’s high capability to investigate the distant Universe,” Dr. Hashimoto explained in the June 17, 2019 ALMA Press Release.

The galaxies that dwell in the Universe today, such as our own Milky Way, have gone through countless, often violent, mergers. During some of the mergers, a more massive galaxy swallowed a smaller one–as will likely be the case when our own Milky Way meets up with Andromeda in about 5 billion years. However, galaxies with smaller sizes also merged to form new, much larger galaxies. Indeed, mergers are essential for galaxy evolution, and for this reason many astronomers are eager to trace them back in time.

Dr. Akio Inoue, who is also of Waseda University, commented in the June 17, 2019 ALMA Press Release that “Our next step is to search for nitrogen, another major chemical element, and even the carbon monoxide molecule. Ultimately, we hope to observationally understand the circulation and accumulation of elements and material in the context of galaxy formation and evolution.”

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