Free Astronomy Magazine November-December 2024

45 NOVEMBER-DECEMBER 2024 ASTRO PUBLISHING T his image shows the motion of cold gas in the REBELS- 25 galaxy as seen with the Atacama Large Mil- limeter/sub- millimeter Array (ALMA). Blue colour- ing indicates movement towards Earth and red indicates movement away from Earth, with a darker shade repre- senting faster movement. In this case, the red-blue divide of the image shows clearly that the object is rotating, making REBELS-25 the most distant rotating disc galaxy ever discovered. [ALMA (ESO/NAOJ/NRAO)/L. Rowland et al.] The galaxies we see today have come a long way from their chaotic, clumpy counterparts that as- tronomers 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 Nether- lands, and co-author of the study. These messy, early galaxies merge with each other and then evolve into smoother shapes at an incredi- bly slow pace. Current theories sug- gest 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, how- ever, challenges that timescale. In the study, accepted for publica- tion in Monthly Notices of the Royal Astronomical Society, astronomers found REBELS-25 to be the most dis- tant 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 per- cent of its current age (13.8 billion) — making REBELS-25’s orderly rota- tion unexpected. “Seeing a galaxy with such similarities to our own Milky Way, that is strongly rotation- dominated, challenges our under- standing of how quickly galaxies in the early Universe evolve into the orderly galaxies of today’s cosmos,” says Lucie Rowland, a doctoral stu- dent 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 excit- ing 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 ob- servations with ALMA at a higher resolution, which confirmed its record- breaking nature. “ALMA is the only telescope in exis- tence with the sensitivity T his video summarizes the discovery discussed in these pages. [ESO] and resolution to achieve this,” says Renske Smit, a researcher at Liver- pool 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 cen- tral 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 ex- citing discovery, as it would be the most distant galaxy with such struc- tures observed to date,” says Row- land. These future observations of REBELS-25, alongside other discov- eries of early rotating galaxies, will potentially transform our under- standing of early galaxy formation, and the evolution of the Universe as a whole. !