They discover the most distant Milky Way-like galaxy observed so far

A team of researchers has discovered the most distant Milky Way-like galaxy observed so far. Nicknamed REBELS-25, this disk galaxy appears as ordered as galaxies today, but looks just as it did when the universe was just 700 million years old, the European Southern Observatory (ESO) has reported.

The rotation and structure of REBELS-25 have been revealed using the Atacama Large Millimeter/submillimeter Array (ALMA), of which the European Southern Observatory is a partner.

The galaxies seen today have come a long way from those chaotic, lumpy origins that the astronomical community often observes when studying the early universe. “Based on our understanding of galaxy formation, we expect most early galaxies to be small and look like train wrecks,” says Jacqueline Hodge, an astronomer at Leiden University in 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 ordered as our own Milky Way (a rotating disk with structures as defined as spiral arms), billions of years of evolution must have passed. The detection of REBELS-25, however, defies that time scale.

In the study, accepted for publication in the journal Monthly Notices of the Royal Astronomical Society, the team discovered that REBELS-25 is the most distant galaxy ever discovered with an intensely rotating disk.

The light arriving from this galaxy was emitted when the universe was only 700 million years old, just five percent of its current age (13.8 billion), making REBELS-25’s orderly rotation somewhat unexpected.

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

REBELS-25 was initially detected in previous observations by the same team, also made with ALMA (located in the Atacama Desert, Chile). At the time, it was an exciting discovery that showed hints of rotation, but the resolution of the data wasn’t fine enough to be sure.

To properly discern the structure and motion of the galaxy, the team conducted follow-up observations with ALMA at a higher resolution, confirming its record-breaking nature.

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

The data also offered hints of more developed features similar to those of the Milky Way, such as an elongated central bar and even spiral arms, although more observations will be needed to confirm this. “Finding more evidence of more evolved structures would be an exciting discovery, as it would be the most distant galaxy with such structures observed to date,” Rowland clarifies.

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