An international collaborative research team, including Assistant Professor Hanae Inami and researcher Hiddo Algera from Hiroshima University’s Astrophysical Science Center, has discovered the most distant Milky Way-like galaxy yet observed, using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile.

Current understanding of the formation and evolution of galaxies predicts that young galaxies in the early Universe gradually grow by repeated violent collisions and mergers, and therefore are smaller and have more complex shapes and motions than modern galaxies. For a galaxy to be as orderly as our own Milky Way, present theories suggest billions of years of evolution must have elapsed. However, the detection of REBELS-25 challenges that timescale.

The researchers observed that the light reaching us from REBELS-25 was emitted when the Universe was just 700 million years old — a mere five percent of its current age (13.8 billion). Despite its young age, REBELS-25 has a rotating disk structure comparable in size to the Milky Way, a fully grown galaxy.

“It is surprising that a galaxy with a rotating disk existed so soon after the birth of the Universe. It's an exciting discovery that could change our understanding of how galaxies form," Inami said of the significance of this discovery.

This research was carried out as part of the ALMA large-scale program "REBELS: Reionization Era Bright Emission Line Survey.” The original ALMA observations from the REBELS survey hinted at possible rotation in REBELS-25, 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. 

“REBELS-25 immediately struck us as a remarkable galaxy among the 40 targets of the REBELS survey, and these follow-up ALMA observations confirm this first impression in a spectacular fashion,” Algera said. 

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 of REBELS-25 will be needed to confirm this.

“Future observations with the powerful James Webb Space Telescope (JWST) will uncover the light from its young stars at a resolution comparable to that of ALMA,” Algera said.

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. Inami expected that “more rotating disk galaxies similar to REBELS-25 could be found as we continue exploring the early Universe.” In the near future, we may have even more evidence that early galaxies evolved much faster than we had previously thought.

This research will be published in the Monthly Notices of the Royal Astronomical Society under the title "REBELS-25: Discovery of a Dynamically Cold Disk Galaxy at z=7.31."

This article is a revised version. Please check the original press release by the European South Observatory here.

Journal: Monthly Notices of the Royal Astronomical Society
Title: REBELS-25: Discovery of a dynamically cold disc galaxy at 𝑧 = 7.31
Authors: Lucie E. Rowland, Jacqueline Hodge, Rychard Bouwens, Pavel Mancera Piña, Alexander Hygate, Hiddo Algera, Manuel Aravena, Rebecca Bowler, Elisabete da Cunha, Pratika Dayal, Andrea Ferrara, Thomas Herard-Demanche, Hanae Inami, Ivana van Leeuwen, Ilse de Looze, Pascal Oesch, Andrea Pallottini, Siân Phillips, Matus Rybak, Sander Schouws, Renske Smit, Laura Sommovigo, Mauro Stefanon, Paul van der Werf
DOI: 10.1093/mnras/stae2217