Discovery of most distant rotating disk galaxy challenges current formation theories

Researchers have discovered the most distant Milky Way-like galaxy ever observed. Dubbed REBELS-25, this disk galaxy appears as orderly as today’s galaxies, but we see it as it was when the universe was only 700 million years old.

This is surprising since, according to our current understanding of galaxy formation, these early galaxies should appear more chaotic. The rotation and structure of REBELS-25 were revealed using the Atacama Large Millimeter/submillimeter Array (ALMA), of which the European Southern Observatory (ESO) is a partner.

The galaxies we see today have come a long way from their chaotic, clumped counterparts that astronomers typically observe in the early universe. “Based on our understanding of galaxy formation, we expect most early galaxies to be small and disordered in appearance,” says Jacqueline Hodge, an astronomer at Leiden University, the Netherlands, and co-author of the study.

These early disordered galaxies merge with each other and then evolve into smoother forms at an incredibly slow rate. Current theories suggest that for a galaxy to be as orderly as our own Milky Way – a rotating disk with ordered structures like spiral arms – billions of years of evolution must have passed. The detection of REBELS-25, however, calls this deadline into question.

In the study “REBELS-25: Discovery of a dynamically cold disk galaxy at z = 7.31”, accepted for publication in Monthly Notices of the Royal Astronomical Societyastronomers found that REBELS-25 was the most distant strongly rotating disk galaxy ever discovered. The light reaching us from this galaxy was emitted when the universe was only 700 million years old, or only five percent of its current age (13.8 billion), making the orderly rotation of the universe unexpected. REBELS-25. The results are available on the arXiv preprint server.

“Seeing a galaxy with such similarities to our own Milky Way, which is strongly dominated by rotation, challenges our understanding of how quickly the galaxies of the early universe evolved into the ordered galaxies of today’s cosmos.” , explains Lucie Rowland, doctoral student in Leiden. University and first author of the study.

REBELS-25 was initially detected in previous observations by the same team, also conducted with ALMA, located in the Atacama Desert in Chile. At the time, it was an exciting discovery, showing hints of rotation, but the resolution of the data was not fine enough to be sure. To properly discern the structure and motion of the galaxy, the team performed follow-up observations with ALMA at higher resolution, which confirmed its record-breaking nature.

“ALMA is the only existing telescope with the sensitivity and resolution to achieve this goal,” says Renske Smit, a researcher at Liverpool John Moores University in the United Kingdom and also a co-author of the study.

Surprisingly, the data also hints at more developed features similar to those of the Milky Way, such as an elongated central bar and even spiral arms, although additional observations are needed to confirm this.

“Finding further evidence of more evolved structures would be an exciting discovery, as this would be the most distant galaxy with such structures observed to date,” says Rowland.

These future observations of REBELS-25, along with other discoveries of early rotating galaxies, will potentially transform our understanding of the formation of early galaxies and the evolution of the universe as a whole.