Galaxies in the Universe exhibit a variety of shapes and can be broadly classified into two categories. The first category includes younger, disk-like spiral galaxies, such as the Milky Way, which are still undergoing star formation. The second category comprises older elliptical galaxies, characterized by a prominent central bulge. These galaxies no longer form new stars and mostly lack gas. While these spheroidal galaxies contain very old stars, the exact process of their formation has remained a mystery, but recent joint research by scientists in China, Europe and Japan has shed new light on this question.

Specifically, the researchers discovered evidence of in situ spheroid formation in distant starburst galaxies. Their findings were based on analysis of data from the Atacama Large Millimeter/submilllimeter Array (ALMA) on over 100 Submillimeter Bright Galaxies (SMGs).  The SMGs featured redshifts from the “Cosmic noon” era of the Universe—between 8–12 billion years ago—when many galaxies were actively forming stars.

The associated study, titled “In situ spheroid formation in distant submillimetre-bright galaxies,” was published in Nature on Dec. 4, with Qing-Hua Tan of China’s Purple Mountain Observatory of the Chinese Academy of Sciences as first author.

This research provides the first solid observational evidence that spheroids can form directly through intense star formation within the cores of highly luminous starburst galaxies in the early Universe, as revealed by data from the submillimeter band. This breakthrough is poised to significantly impact models of galaxy evolution.

In this study, researchers conducted a statistical analysis of the surface brightness distribution of dust emission in the submillimeter wavelength range, utilizing a novel analytical technique. They discovered that the submillimeter emission in most of the sample galaxies was quite compact, with surface brightness profiles that deviated significantly compared to exponential disks. This finding suggests that the submillimeter emission originates from structures resembling spheroids.

Further evidence supporting the spheroidal shape of galaxies comes from detailed analysis of their three-dimensional geometry. Modeling based on the skewed distribution of high axis ratios indicates that the ratio of the shortest to the longest of a galaxy’s three axes is, on average, 0.5 and increases with spatial compactness. 

This suggests that many of these intensely star-forming galaxies are inherently spherical rather than disk-shaped. Backed by numerical simulations, this discovery reveals that the primary mechanism behind the formation of these spheroids involves the simultaneous processes of cold gas accretion and galaxy interactions. This phenomenon is believed to have been common in the early Universe, a period during which most spheroids were created. As a result, it has the potential to redefine galaxy formation.

This research received support from the A³COSMOS and A³GOODSS archival projects, which enabled the researchers to collect data from a large number of galaxies with sufficiently high signal-to-noise ratios for detailed analysis. Future analysis of extensive ALMA data accumulated over the years, along with new submillimeter and millimeter observations with higher resolution and sensitivity, will enable systematic studies of the cold gas present in galaxies. This research will provide new insights into the distribution and movement of the raw materials that fuel star formation.

In addition, the capabilities of Euclid, the James Webb Space Telescope (JWST), and the China Space Station Telescope (CSST) for mapping the stellar components of galaxies will complement this approach, offering a more complete view of their evolution. These tools are expected to significantly enhance the understanding of galaxy formation in the early Universe.

Schematic diagram illustrating the process of spheroid formation in distant submillimeter-bright galaxies and the possible link to the evolution of giant elliptical galaxies in the present-day Universe. (Image by Tan et al. 2024)