Space oddity: Most distant rotating disc galaxy found

7 October, 2024 / Read time: 9 minutes

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).

The galaxies we see today have come a long way from their chaotic, clumpy counterparts that astronomers typically observe in the early Universe. "According to our understanding of galaxy formation, we expect most early galaxies to be small and messy looking," says Jacqueline Hodge, an astronomer at Leiden University, the Netherlands, and co-author of the study.

These messy, early galaxies merge and then evolve into smoother shapes incredibly slowly. Current theories suggest that billions of years of evolution must have elapsed for a galaxy to be as orderly as our own Milky Way — a rotating disc with tidy structures like spiral arms. The detection of REBELS-25, however, challenges that timescale. 

In the study, accepted for publication in Monthly Notices of the Royal Astronomical Society, 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 five percent of its current age (13.8 billion) — making REBELS-25's orderly rotation unexpected. "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," says Lucie Rowland, a doctoral student at 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 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. "ALMA is the only telescope in existence with the sensitivity and resolution to achieve this," says Renske Smit, a researcher at Liverpool John Moores University in the UK and co-author of the study.

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. However, more observations will be needed to confirm this. "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," says Rowland. 

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.

Additional Information

This research is presented in a paper entitled "REBELS-25: Discovery of a dynamically cold disc galaxy at z=7.31" to appear in Monthly Notices of the Royal Astronomical Society.

The observations were conducted in the ALMA Large Program REBELS: Reionization Era Bright Emission Lines Survey.

The team is composed of L. E. Rowland (Leiden Observatory, Leiden University, the Netherlands [Leiden]), J. Hodge (Leiden), R. Bouwens (Leiden), P. M. Piña (Leiden), A. Hygate (Leiden), H. Algera (Astrophysical Science Center, Hiroshima University, Japan [HASC]; National Astronomical Observatory of Japan, Japan), M. Aravena (Núcleo de Astronomía, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Chile), R. Bowler (Jodrell Bank Centre for Astrophysics, University of Manchester, UK), E. da Cunha (International Centre for Radio Astronomy Research, University of Western Australia, Australia; ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions), P. Dayal (Kapteyn Astronomical Institute, University of Groningen, the Netherlands), A. Ferrara (Scuola Normale Superiore, Italy [SNS]), T. Herard-Demanche (Leiden), H. Inami (HASC), I. van Leeuwen (Leiden), I. de Looze (Sterrenkundig Observatorium, Ghent University, Belgium), P. Oesch (Department of Astronomy, University of Geneva, Switzerland; Cosmic Dawn Center, Denmark), A. Pallottini (SNS), S. Phillips (Astrophysics Research Institute, Liverpool John Moores University, UK [LJMU]), M. Rybak (Faculty of Electrical Engineering, Delft University of Technology, the Netherlands; Leiden; Netherlands Institute for Space Research, the Netherlands), S. Schouws (Leiden), R. Smit (LJMU), L. Sommovigo (Center for Computational Astrophysics, Flatiron Institute, USA), M. Stefanon (Departament d'Astronomia i Astrofísica, Universitat de València, Spain; Grupo de Astrofísica Extragaláctica y Cosmología, Universitat de València, Spain), P. van der Werf (Leiden).

The original press release was published by the European Southern Observatory (ESO), an ALMA partner on behalf of Europe.

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Organisation for Astronomical Research in the Southern Hemisphere (ESO), the US National Science Foundation (NSF) and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the National Science and Technology Council (NSTC) in Taiwan and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI).

ALMA construction and operations are led by ESO on behalf of its Member States; by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.

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