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 in situ spheroid formation in distant starburst galaxies – announced in a paper published today in the journal 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 8 and 12 billion years ago 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 the study, researchers 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 deviate 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 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 vast 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. In addition, the capabilities of Euclid, the James Webb Space Telescope (JWST), and the China Space Station Telescope (CSST) to map the stellar components of galaxies will complement this approach, offering a more complete view of their evolution. Together, these tools promise to revolutionise our understanding of galaxy formation in the early Universe.

The research was presented in a paper entitled “In situ spheroid formation in distant submillimetre-bright Galaxies” published in Nature.

The team is composed of Q.Tan (Purple Mountain Observatory, Chinese Academy of Sciences, China [PMO] and Commisariat pour l’Energie Atomique, Saclay, France [CEA Paris-Saclay]), E.Daddi (CEA Paris-Saclay), B.Magnelli (CEA Paris-Saclay), C.A.Correa (CEA Paris-Saclay), F.Bournaud (CEA Paris-Saclay), S. Adscheid (Argelander-Institut fur Astronomie, Universitat Bonn, Germany), S.Zhang (PMO), D. Elbaz (CEA Paris-Saclay), C. Gomez-Guijarro (CEA Paris-Saclay), B.S. Kalita (Kavli Institute for the Physics and Mathematics of the Universe, The University of Tokyo, Japan [Kavli-IPMU], and Kavli Institute for Astronomy and Astrophysics, Peking University, China), D.Liu (PMO), Z.Liu (Kavli-IPMU, and Department of Astronomy, School of Science, The University of Tokyo, Japan [UTokyo]), J.Pety (Institut de Radioastronomie Millimetrique, France, and LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universites, France), A.Puglisi (School of Physics and Astronomy, University of Southampton, UK, and Center for Extragalactic Astronomy, Department of Physics, Durham University, UK), E.Schinnerer (Max-Planck-Institut fur Astronomie, Germany), J.D.Silverman (Kavli-IPMU, UTokyo, and Center for Astrophysical Sciences, Department of Physics & Astronomy, Johns Hopkins University, USA), and F.Valentino (European Southern Observatory, Germany, and Cosmic Dawn Center, Denmark).

Schematic diagram illustrating the process of spheroid formation in distant submillimetre-bright galaxies and the possible link with the evolution of giant elliptical galaxies in the present-day Universe. On the far left, the infrared images captured by the JWST (using F444W for red, F227W for green, and F150W for blue) are followed by a zoom on their central submillimetre regions, obtained using ALMA. The diagram also provide a classification of the intrinsic shapes of galaxies. The mean shape parameters are shown for: the entire sample studied (green ellipse), a subsample of compact galaxies at submillimetre wavelengths (orange ellipse), and a subsample of extended galaxies at submillimetre wavelengths (blue ellipse). These parameters are compared with those of local early-type galaxies (red ellipse) and late-type galaxies (represented by purple and cyan spiral shapes). (Credit: Tan et al. 2024)