Space oddity: Most distant rotating disc galaxy 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.

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 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, 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, 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. “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 also a 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, although 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 as a whole.

More 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 as part of 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 Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of ESO, the U.S. 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. 

The European Southern Observatory (ESO) enables scientists worldwide to discover the secrets of the Universe for the benefit of all. We design, build and operate world-class observatories on the ground — which astronomers use to tackle exciting questions and spread the fascination of astronomy — and promote international collaboration for astronomy. Established as an intergovernmental organisation in 1962, today ESO is supported by 16 Member States (Austria, Belgium, Czechia, Denmark, France, Finland, Germany, Ireland, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom), along with the host state of Chile and with Australia as a Strategic Partner. ESO’s headquarters and its visitor centre and planetarium, the ESO Supernova, are located close to Munich in Germany, while the Chilean Atacama Desert, a marvellous place with unique conditions to observe the sky, hosts our telescopes. ESO operates three observing sites: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope and its Very Large Telescope Interferometer, as well as survey telescopes such as VISTA. Also at Paranal ESO will host and operate the Cherenkov Telescope Array South, the world’s largest and most sensitive gamma-ray observatory. Together with international partners, ESO operates ALMA on Chajnantor, a facility that observes the skies in the millimetre and submillimetre range. At Cerro Armazones, near Paranal, we are building “the world’s biggest eye on the sky” — ESO’s Extremely Large Telescope. From our offices in Santiago, Chile we support our operations in the country and engage with Chilean partners and society. 

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