Astronomers witness the in situ spheroid formation in distant submillimetre-bright galaxies

Galaxies in today’s Universe are diverse in morphologies and can be roughly divided into two categories: younger, disk-like spiral galaxies, like our own Milky Way, that are still forming new stars; and older, elliptical galaxies, which are dominated by a central bulge, no longer forming stars and mostly lacking gas. These spheroidal galaxies contain very old stars, yet how they formed has remained a mystery — until now.

The discovery of the birth sites of giant, elliptical galaxies — announced in a paper published today in Nature — come from analyzing data from the Atacama Large Millimeter/submillimeter Array (ALMA) on over 100 Submillimeter Bright Galaxies (SMGs) with redshifts dating to the “Cosmic noon” era, when the universe was between around 1.6 and 5.9 billion years old and many galaxies were actively forming stars. This study 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, based on a new perspective from the submillimeter band. This breakthrough will significantly impact models of galaxy evolution and deepen our understanding of how galaxies form and evolve across the Universe.

In this study, researchers led by Chinese Academy of Sciences Purple Mountain Observatory Associate Researcher Qinghua Tan, and including Kavli IPMU Professor John Silverman, Project Researcher Boris Kalita, and graduate student Zhaoxuan Liu, used statistical analysis of the surface brightness distribution of dust emission in the submillimeter band, combined with a novel analysis technique. They found that the submillimeter emission in most of sample galaxies are very compact, with surface brightness profiles deviating significantly from those of exponential disks. This suggests that the submillimeter emission typically comes from structures that are already spheroid-like. Further evidence for this spheroidal shape comes from a detailed analysis of galaxies’ 3D geometry. Modeling based on the skewed-high axis-ratio distribution shows that the ratio of the shortest to the longest of their three axes is, on average, half and increases with spatial compactness. This indicates that most of these highly star-forming galaxies are intrinsically spherical rather than disk-shaped. Supported by numerical simulations, this discovery has shown us that the main mechanism behind the formation of these tri-dimensional galaxies (spheroids) is the simultaneous action of cold gas accretion and galaxy interactions. This process is thought to have been quite common in the early Universe, during the period when most spheroids were forming. It could redefine how we understand galaxy formation.

This research was made possible thanks to the A3COSMOS and A3GOODSS archival projects, which enabled researchers to gather a large number of galaxies observed with a high enough signal-to-noise ratio for detailed analysis. Future exploration of the wealth of ALMA observations accumulated over the years, along with new submillimeter and millimeter observations with higher resolution and sensitivity, will allow us to systematically study the cold gas in galaxies. This will offer unprecedented insight into the distribution and kinematics of the raw materials fueling star formation. With the powerful capabilities of Euclid, the James Webb Space Telescope (JWST), and the China Space Station Telescope (CSST) to map the stellar components of galaxies, we will gain a more complete picture of early galaxy formation. Together, these insights will deepen our understanding of how the Universe as a whole has evolved over time.