For the first time, astronomers have found evidence proving that the structure of a galaxy can change over the course of its lifetime, demonstrating that a large proportion of them have gone through a significant “metamorphosis” after initially being formed.
The study, which has been published in the Monthly Notices of the Royal Astronomical Society, used the Hubble and Herschel telescope to observe roughly 10,000 galaxies and classified each one into two primary types: flat, rotating, disc-shaped galaxies (like the Milky Way); and large, spherical galaxies with a swarm of disordered stars, according to the authors.
As lead author and Cardiff University Professor Steve Eales told redOrbit via email, “The rate at which stars are forming in a galaxy is proportional to the energy output of the galaxy. Essentially we measured the total energy output of all the galaxies in a small region of sky, out in space and therefore back in time, and found that most of the energy output was from disk galaxies.”
“We used this calorimetric measurement to calculate that at least 81 percent of the stars that had ever formed had formed in disk-galaxies like our own,” he added. “However, in the Universe today only 49 percent of the stars are in disk galaxies. Therefore, there must have been a major transformation of disk galaxies into spheroidal galaxies (ellipticals and galaxies with huge stellar bulges) after most of the stars had formed.”
Two main theories to explain this metamorphosis
According to Professor Eales, although experts have previously claimed that this transformation had occurred, he and his colleagues are the first to actually measure its size. They hope that by detecting the first direct evidence of this phenomenon, they will be able to shed new light on the processes responsible for causing these changes to happen in the first place.
Professor Eales told redOrbit that there are two main possible causes for this metamorphosis: “(a) galaxy mergers in which two disk galaxies are scrambled together into a elliptical; or (b) the gradual motion of newly formed stars in a disk into the center of a galaxy, gradually building up a big pile of stars.” He added that the cause may be “something we haven’t thought of.”
The first theory proposes that the transformation was caused by a series of cosmic catastrophies in which two disk-dominated galaxies wandered too close to each other, and were forced by the graviational pull to merge into a single entity, which would destroy the disks in the process. The second is a less violent theory in which the stars eventually moved to the galaxy’s center.
Professor Eales also emphasized that the research would not be possible without the Herschel Space Observatory, which is larger than Hubble, has a 3.5m mirror and operates in the infrared part of the spectrum instead of the optical. Herschel, he added, “has made it possible to measure the 50 percent of the energy from galaxies that is obscured by interstellar dust.”
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Feature Image: This artist’s concept illustrates the two types of spiral galaxies that populate our universe: those with plump middles, or central bulges (upper left), and those lacking the bulge (foreground). (Credit: NASA)
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