Astronomers have clarified how spherical structures formed in ancient galaxies.

The flat rotating disk of the Milky Way (and other modern galaxies) is made up of relatively young stars that orbit in nearly circular paths. The halo, on the other hand, consists of older stars moving on elongated orbits and has a spheroidal (ball-like) shape.

It is worth noting that a significant portion of stars in the universe is found within such elliptical structures. However, scientists were uncertain about how these dust-rich and star-forming "cores" formed — whether through galactic mergers, the collapse of gas clouds, or the gradual inflow of material from the disk.

Now, an international research team led by Qing-Hua Tan from the Zijinshan Observatory (China) and Emanuele Daddi from Paris-Saclay University (France) has uncovered an unexpected stage in the formation of large, spherical structures characteristic of modern massive galaxies — spheroids.

For the first time, the structure of cosmic inhabitants formed in the relatively early universe (when its age was less than half of its current age) was examined in detail using the highly sensitive ALMA radio telescope. The findings of this research are published in the journal Nature.

By analyzing archival observational data for 146 bright galaxies in the submillimeter range (terahertz radiation) at high redshifts
(that is, in the relatively ancient universe), the researchers employed computer simulations to reconstruct the shape and size of the glowing dust regions — the "cores" of distant star-forming systems.

It turned out that ancient galaxies formed their nearly spherical cores from stars and dust during periods of intense star formation (most often observed after the collision of two galaxies or a close encounter between them). These phenomena generally indicate a potential connection to the formation of massive spheroids, but observing them in optical and infrared wavelengths has been challenging due to dense dust clouds.

Ultimately, the study's authors concluded that during the so-called cosmic noon — approximately two to three billion years after the Big Bang — the key mechanism for forming spherical structures (stellar "cores") was intense bursts of star formation. Direct galactic mergers and unusual gas flows may have played a crucial role in this scenario, responsible for the "bulging" of structures and their rounded shape.

The results of this new scientific work represent an important step towards solving the long-standing puzzle of the origin of massive galactic spheroids. Further observations using the ALMA radio telescope and the James Webb Space Telescope will refine theoretical models of galaxy formation and provide more insights into the evolution of these stellar "cities."