The most distant and oldest rotating disk galaxy ever observed has surprised astronomers and could challenge our theories about galaxy evolution. Not only do they appear to exist very early in the universe to be rotating vigorously, but the galaxy also shows signs of spiral arms similar to highly evolved “young” galaxies like our own Milky Way.
The galaxy, called REBELS-25, is seen as it was just 700 million years after the Big Bang, at a time when astronomers expect galaxies to be young and chaotic. True to its name, this rebellious galaxy has defied the trend by appearing organized rather than chaotic.
REBELS-25 was discovered by a team of astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA), a network of 66 radio telescopes located in the Atacama Desert region of northern Chile.
“Based on our understanding of galaxy formation, we expect most early galaxies to be small and chaotic in appearance,” says team member Jacqueline Hodge, an astronomer at Leiden University in the Netherlands. He said in a statement.
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An elegant, rebellious young man
Modern galaxies, such as the Milky Way, which exist in the 13.8 billion-year-old universe, have had billions of years to develop distinctive shapes and characteristics, such as stunning spiral arms.
In the early universe, before it had that much time to organize, astronomers would expect to see chaotic, clumpy galaxies. These early galaxies are expected to have evolved the shape of modern galaxies over billions of years as they engaged in a series of collisions and mergers, developing properties such as disc shapes and spiral arms incredibly slowly.
The discovery of REBELS-25, when the universe was only 5% of its current age, casts doubt on this timeline.
“Seeing a galaxy similar to our own Milky Way, which is strongly rotation-dominated, challenges our understanding of how quickly galaxies in the early universe evolved into the galaxies organizing the universe today,” said study team leader Lucy Rowland, also from Leiden. The university said in the same statement.
When astronomers first discovered REBELS-25 using ALMA, the galaxy was already considered remarkable because it showed signs of rotation. However, this initial investigation was not high enough resolution to confirm that this is indeed the most distant and strongly rotating galaxy ever seen.
To more accurately reveal the structure and motion of this early galaxy, the team continued to study it using ALMA, but this time at higher resolution. This gas was shown in REBELS-25 moving toward and away from Earth. This is possible due to a phenomenon called blueshift and redshift.
When a light source moves toward Earth, the wavelength of that light is compressed. This ‘shifts’ the light towards the short-wavelength ‘blue end’ of the electromagnetic spectrum. Therefore, a light source moving toward us is “blue-shifted.” On the flip side of this, if a light source is moving away from us, the wavelength of the light it emits is stretched, shifting it towards the ‘red end’ of the electromagnetic spectrum.
The new study confirmed that REBELS-25 is a record-breaking galaxy, and it is the oldest and most distant strongly rotating galaxy ever.
“ALMA is the only telescope in existence with the sensitivity and resolution needed to achieve this,” team member Rinski Smit, of Liverpool John Morris University in England, said in the same statement.
It seems that REBELS-25 has other surprises that have not yet been fully revealed. For example, the early galaxy appears to have properties similar to those of the approximately 13.6 billion-year-old Milky Way. This includes an elongated “central bar” of stars with hints that it may have spiral arms. This would lead to REBELS-25 breaking another record as the most distant and oldest spiral galaxy ever. The current record holder is Cheers-2112, seen by the James Webb Space Telescope (JWST) when the universe was about 2.1 billion years old.
“Finding additional evidence of more advanced structures would be an exciting discovery, because it would be the most distant galaxy in which such structures have been observed to date,” Rowland concluded.
The team’s research has been accepted for publication in the Monthly Notices of the Royal Astronomical Society.
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