This image shows the galaxy REBELS-25 as seen by the Atacama Large Millimeter/ submillimeter Array (ALMA), overlaid on an infrared image of other stars and galaxies. The infrared image was taken by ESO’s Visible and Infrared Survey Telescope for Astronomy (VISTA). In a recent study, researchers found evidence that REBELS-25 is a strongly rotating disc galaxy existing only 700 million years after the Big Bang. This makes it the most distant and earliest known Milky Way-like galaxy found to date. Credit: ALMA (ESO/ NAOJ/ NRAO)/ L Rowland et al/ ESO/ J Dunlop et al. Ack.: CASU, CALET. CC BY.

Astronomers using the Atacama Large Millimeter/ submillimeter Array have discovered a space oddity: the most distant rotating disc galaxy ever found.

Researchers have discovered the most distant Milky-Way-like galaxy yet observed. Dubbed REBELS-25, this disc galaxy seems as orderly as present-day 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, such early galaxies are expected to appear more chaotic.

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

Small and messy

The galaxies we see today have come a long way from their chaotic, clumpy counterparts that astronomers typically observe in the early Universe.

Co-author of the study, Jacqueline Hodge, an astronomer at Leiden University, said: “According to our understanding of galaxy formation, we expect most early galaxies to be small and messy looking.”

These messy, early galaxies merge with each other and then evolve into smoother shapes at an incredibly slow pace.

Current theories suggest that, for a galaxy to be as orderly as our own Milky Way – a rotating disc with tidy structures like spiral arms – billions of years of evolution must have elapsed.

The detection of REBELS-25, however, challenges that timescale.

In the study, astronomers found REBELS-25 to be the most distant strongly rotating disc galaxy ever discovered.

The light reaching us from this galaxy was emitted when the Universe was only 700 million years old – a mere 5% of its current age (13.8 billion) – making REBELS-25’s orderly rotation unexpected.

First author Lucie Rowland, a doctoral student at Leiden University, said: “Seeing a galaxy with such similarities to our own Milky Way, that is strongly rotation-dominated, challenges our understanding of how quickly galaxies in the early Universe evolve into the orderly galaxies of today’s cosmos.”

Antennas of the Atacama Large Millimeter/submillimeter Array (ALMA), on the Chajnantor Plateau in the Chilean Andes. The Large and Small Magellanic Clouds, two companion galaxies to our own Milky Way galaxy, can be seen as bright smudges in the night sky, in the centre of the photograph. Credit: ESO/ C Malin (christophmalin.com).

Exciting discovery

REBELS-25 was initially detected in previous observations by the same team, also conducted with ALMA, which is located in Chile’s Atacama Desert.

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 a higher resolution, which confirmed its record-breaking nature. 

Co-author Renske Smit, a researcher at Liverpool John Moores University, added: “ALMA is the only telescope in existence with the sensitivity and resolution to achieve this.”

Surprisingly, the data also hinted at more developed features similar to those of the Milky Way, like a central elongated bar, and even spiral arms, although more observations will be needed to confirm this.

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

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

The study has been accepted for publication in Monthly Notices of the Royal Astronomical Society.