An article accepted for publication in the journal “Monthly Notices of the Royal Astronomical Society” reports the discovery of a primordial galaxy that was cataloged as REBELS-25 and shows a rotating structure that is already ordered in a way that isn’t expected from such an ancient galaxy. A team of researchers used the ALMA radio telescope to study REBELS-25 and trace the movement of gas within it. The data collected will be invaluable in understanding how a galaxy that we see as it was when the universe was about 700 million years old can have a structure that is already becoming a spiral like the one of the Milky Way at that time.
Jacqueline Hodge, an astronomer at Leiden University, the Netherlands, and one of the authors of this study, explained that according to our models of galaxy formation, we expect most primordial galaxies to be small and messy. Mergers between these early galaxies are supposed to give rise to the massive and ordered galaxies we see in the nearby Universe, but a structure like the Milky Way’s is supposed to take a few billion years to become what it is today. REBELS-25 challenges this timescale.
Lucie Rowland, a doctoral student at Leiden University and first author of this study, stated that seeing a galaxy with such similarities to the Milky Way, which is strongly rotation-dominated, challenges our understanding of how quickly galaxies in the early Universe evolve into the ordered galaxies in today’s cosmos.
The REBELS-25 galaxy is one of those selected for the ALMA Reionization Era Bright Emission Line Survey (REBELS) conducted with the ALMA radio telescope on various primordial objects. Several members of the team that conducted the REBELS survey later joined the team that conducted the follow-up study of REBELS-25. This galaxy, which we see as it was when the universe was about 700 million years old, showed a more ordered structure than astronomers expected, and was interesting also because the observations suggested that it’s rotating.
Initial observations didn’t have sufficient resolution to confirm the rotation of the REBELS-25 galaxy, so this follow-up study led to more observations at a higher resolution. This confirmed the rotation of REBELS-25 making it the oldest known rotating galaxy. The observations also showed the presence of an elongated central bar and possible spiral arms like the Milky Way.
The researchers plan to continue their studies to verify that the structures observed are indeed spiral arms in the making. They also plan to conduct follow-up studies of other rotating galaxies observed during the REBELS survey to assess how far along they are in their evolution.
For years, observations of primordial galaxies led astronomers to develop certain models of their formation. However, these models seem insufficient to explain galaxies such as REBELS-25 and other primordial galaxies such as the three described in an article published in the journal “The Astrophysical Journal Letters” a few months ago.
The James Webb Space Telescope is making a significant contribution to the study of the early Universe and the ALMA radio telescope continues to prove to be an excellent instrument for this type of cosmological research. The new discoveries are leading to a reconstruction of the birth of the first galaxies different from the one known by astronomers and in the coming years we can expect to obtain a more complete and precise picture.
