In 1926, the famous astronomer Edwin Hubble developed a morphological classification of galaxies. This method divided galaxies into three basic groups - elliptical, spiral, and lenticular. Since then, astronomers have devoted a significant amount of time and effort to figuring out how galaxies evolve over several billion years and why they take on the exact shapes they ultimately take.
One of the most popular and widespread hypotheses in this regard is the one that explains the change in the shape of galaxies as a result of their merger, when more compact clusters of stars held by mutual gravity merge together and thereby form the shape and final appearance of galaxies over time. However, according to a new study by an international team of scientists, the shape and size of galaxies may actually be influenced by the appearance of new stars in their central regions.
The study was led by postdoctoral fellow Ken-Ichi Tadaki in collaboration with the Max Planck Institute for Extraterrestrial Physics and the National Astronomical Observatory of Japan (NAOJ). To get a more complete picture of galactic metamorphosis, scientists have conducted a series of observations of very distant galaxies.
Evolution diagram of galaxies
The study involved the use of multiple telescopes, with which astronomers observed 25 galaxies located about 11 billion light-years from Earth. At this distance, scientists actually observed galaxies when they looked exactly like this 11 billion years ago, that is, about 3 billion years after the Big Bang. This time is considered by astronomers as the period of peak activity of galactic formation in the Universe, when most galaxies were formed.
“It was believed that massive elliptical galaxies were formed by collisions of galactic disks. However, we are not sure if all elliptical galaxies were ever affected by such an intergalactic event. We believe that there is an alternative option,”Tadaki said in a press release posted on the Japanese Astronomical Observatory's website.
Capturing the subtle light of these distant galaxies proved to be a daunting task, requiring scientists to use two ground-based and one space telescopes. First, they used the 8.2-meter Subaru Telescope in Hawaii to search for 25 galaxies. The objects were then observed using the Hubble Space Telescope and the ground-based Atacama Large Millimeter-Wave Antenna Array (ALMA) located in Chile.
Hubble captured the light of galaxies to determine their shape (which they had 11 billion years ago), using ALMA, scientists studied the submillimeter waves emitted by cold clouds of dust and gas, the place where new stars were born. By comparing the results of both observations, astronomers were able to provide a detailed picture of how these galaxies looked 11 billion years ago, when their shape was still changing.
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Observing a galaxy 11 billion light years away
What the scientists found turned out to be very revealing. Hubble images indicated that early galaxies were dominated by the disk component rather than the central bar we are used to associating with spiral and lenticular galaxies. At the same time, ALMA images showed that massive reservoirs of gas and dust could be located near the center of these galaxies, within which very active star formation occurs.
To rule out the possibility that such intense star formation could be caused by galaxy mergers, the researchers also used data from the ESO's Very Large Telescope, located at the Paranal Observatory in Chile, to verify.
“Here we have strong evidence that dense galactic nuclei can form without galactic collisions. They can form due to very active star formation in the very heart of the galaxy,”Tadaki said.
The results of this study could force astronomers to rethink current models and theories of galactic evolution, as well as aspects such as how galaxies develop barriers and spiral arms. The exploration could also lead to a revision of cosmological models of evolution, not to mention the history of our own galaxy.
Who knows, maybe this will also force astronomers to rethink their predictions of what might happen when our Milky Way galaxy and Andromeda collide in a few billion years. The more and deeper scientists look into space, the more surprises it presents. And every time the observed observations do not correspond to our expectations, this forces scientists to revise the accepted hypotheses regarding the evolution of the Universe.
Nikolay Khizhnyak