ALMA image of an extremely distant galaxy that looks surprisingly like our Milky Way, SPT0418-47. It is gravitationally lensed by a nearby galaxy, appearing in the sky as a near-perfect ring of light.
Credit: ALMA (ESO/NAOJ/NRAO), Rizzo et al.

 
“This result represents a breakthrough in the field of galaxy formation, showing that the structures that we observe in nearby spiral galaxies and in our Milky Way were already in place 12 billion years ago,” says Francesca Rizzo, PhD student from the Max Planck Institute for Astrophysics in Germany, who led the research published today in Nature. While the galaxy the astronomers studied, called SPT0418-47, doesn’t appear to have spiral arms, it has at least two features typical of our Milky Way: a rotating disk and a bulge, the large group of stars packed tightly around the galactic center. This is the first time a bulge has been seen this early in the history of the Universe, making SPT0418-47 the most distant Milky Way look-alike.

“The big surprise was to find that this galaxy is actually quite similar to nearby galaxies, contrary to all expectations from the models and previous, less detailed, observations,” says co-author Filippo Fraternali, from the Kapteyn Astronomical Institute, University of Groningen in the Netherlands. In the early Universe, young galaxies were still in the process of forming, so researchers expected them to be chaotic and lacking the distinct structures typical of more mature galaxies like the Milky Way.

Reconstructed image of the gravitationally-lensed galaxy SPT0418-47.
Credit: ALMA (ESO/NAOJ/NRAO), Rizzo et al.

Gas motion of the galaxy SPT0418-47 (left: original lensed image, right: reconstructed image). The galaxy is a disk galaxy with a central bulge and the material in it rotates around the center. Gas moving away from us is shown in red, while gas moving in the direction of the observer is shown in blue.
Credit: ALMA (ESO/NAOJ/NRAO), Rizzo et al.

 
Studying distant galaxies like SPT0418-47 is fundamental to our understanding of how galaxies formed and evolved. This galaxy is so far away we see it when the Universe was just 10% of its current age because its light took 12 billion years to reach Earth. By studying it, we are going back to a time when these baby galaxies were just beginning to develop.

Because these galaxies are so far away, detailed observations with even the most powerful telescopes are almost impossible as the galaxies appear small and faint. The team overcame this obstacle by using a nearby galaxy as a powerful magnifying glass — an effect known as gravitational lensing — allowing ALMA to see into the distant past in unprecedented detail. In this effect, the gravitational pull from the nearby galaxy distorts and bends the light from the distant galaxy, causing it to appear misshapen and magnified.

The gravitationally lensed, distant galaxy appears as a near-perfect ring of light around the nearby galaxy, thanks to their almost exact alignment. The research team reconstructed the distant galaxy’s true shape and the motion of its gas from the ALMA data using a new computer modelling technique. “When I first saw the reconstructed image of SPT0418-47 I could not believe it: a treasure chest was opening,” says Rizzo.

“What we found was quite puzzling; despite forming stars at a high rate, and therefore being the site of highly energetic processes, SPT0418-47 is the most well-ordered galaxy disk ever observed in the early Universe,” stated co-author Simona Vegetti, also from the Max Planck Institute for Astrophysics. “This result is quite unexpected and has important implications for how we think galaxies evolve.” The astronomers note, however, that even though SPT0418-47 has a disk and other features similar to those of spiral galaxies we see today, they expect it to evolve into a galaxy very different from the Milky Way, and join the class of elliptical galaxies, another type of galaxies that, alongside the spirals, inhabit the Universe today.

This unexpected discovery suggests the early Universe may not be as chaotic as once believed and raises many questions on how a well-ordered galaxy could have formed so soon after the Big Bang. This ALMA finding follows the earlier discovery announced in May of a massive rotating disk seen at a similar distance. SPT0418-47 is seen in finer detail, thanks to the lensing effect, and has a bulge in addition to a disk, making it even more similar to our present-day Milky Way than the one studied previously.

Future studies will seek to uncover how typical these ‘baby’ disk galaxies really are and whether they are commonly less chaotic than predicted, opening up new avenues for astronomers to discover how galaxies evolved.

This article is based on the press release issued by the European Southern Observatory.

 
Paper and Research Team
These observation results were presented in F. Rizzo et al. “A dynamically cold disk galaxy in the early Universe” published in the journal Nature on August 12, 2020.

The team is composed of F. Rizzo (Max Planck Institute for Astrophysics, Garching, Germany [MPA]), S. Vegetti (MPA), D. Powell (MPA), F. Fraternali (Kapteyn Astronomical Institute, University of Groningen, the Netherlands), J. P. McKean (Kapteyn Astronomical Institute and ASTRON, Netherlands Institute for Radio Astronomy), H. R. Stacey (MPA, Kapteyn Astronomical Institute and ASTRON, Netherlands Institute for Radio Astronomy) and S. D. M. White (MPA).

 
The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Southern Observatory (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 Ministry of Science and Technology (MOST) 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.