Leveraging technical and observational advancements, an international team led by researchers from the Department of Astrophysics at IRFU (CEA Paris-Saclay) has unraveled the mystery of the formation of spheroids, which are found in the bulges of spiral galaxies and in giant elliptical galaxies.
These structures, long considered primarily the product of late galactic mergers in cosmic history, might also form directly in the distant universe. Their spherical shape would result from intense star formation induced by a dynamic process combining the accretion of cold gas and galactic interactions.
Figure 1 - Examples of images captured with the JWST, from the galaxy sample analyzed in this study.
The color images were reconstructed by combining three filters: F444W (red), F227W (green), and F150W (blue). The region outlined by cyan dashes corresponds to the best fit of the surface brightness profiles of the submillimeter emission. The white bar at the bottom of the thumbnails indicates the scale, while the source name and redshift (z) of the galaxies are mentioned at the top of each thumbnail.
Credit: Tan et al. 2024
These discoveries represent a major advance in our understanding of galaxy evolution, impacting current models that will also benefit from high-resolution observations thanks to next-generation telescopes (JWST, Euclid, etc.).
This research was presented in an article titled "
In situ spheroid formation in distant submillimetre-bright Galaxies", published in the journal
Nature.
Technical and observational limitations finally overcome
Galaxies in the universe are divided into two major morphological categories. On one side, spiral galaxies, disk-shaped, like our Milky Way. They are young, gas-rich, and continue to form stars. On the other side, spheroidal galaxies, which include elliptical galaxies and the bulges of spiral galaxies. They are gas-poor, composed of very old stars, and hardly form any new stars; they are like "dead." While the formation of spiral galaxies is perhaps better understood, that of spheroidal galaxies remained a mystery until now, despite the existence of several theories, which were limited by our previous observational and technical means.
To understand the formation of these spheroids, we must go back to the birth of the stars that compose them, up to the era of "Cosmic Noon," when the universe was 1.6 to 4.3 billion years old. At that time, many galaxies were actively forming stars and were rich in dust and gas, making them opaque in the visible spectrum but extremely bright in millimeter and submillimeter wavelengths. The arrival of the Atacama Large Millimeter/submillimeter Array (ALMA), capable of observing in this part of the spectrum, thus opened the possibility of studying galactic bulges. These observations are complemented by the infrared vision of the powerful James Webb Space Telescope (JWST), which provides a global view of galaxies (cf. Figure 1).
Figure 2 - Diagram illustrating the process of spheroid formation in distant submillimeter-bright galaxies and its link with the evolution of giant elliptical galaxies in the current universe.
On the far left, the infrared images captured by the JWST (see caption Fig. 1) are followed by a zoom on their central regions in submillimeter, obtained thanks to ALMA. The diagram also proposes a classification of the intrinsic shapes of galaxies. The average parameters of the morphologies are represented for: the entire sample studied (green ellipse), a subsample of submillimeter-compact galaxies (orange ellipse), and a subsample of submillimeter-extended galaxies (blue ellipse). These parameters are compared to those of local early-type galaxies (red ellipse) and late-type galaxies (represented by purple and cyan spiral shapes).
Credit: Tan et al. 2024. This research was also made possible by a significant technical advancement. In a previous publication (Tan et al. 2024, A&A), the researchers developed a new method for fitting surface brightness profiles to interferometric observations, such as those produced by ALMA. Before this innovation, extracting information from these data was complex, and existing methods introduced too much bias, making it difficult to conduct an in-depth analysis of spheroidal systems.
New perspectives on the formation of giant elliptical galaxies in the early universe
This study relies on ALMA observations collected over the years by various projects. Thanks to the archival projects
A3COSMOS and A3GOODS, the researchers were able to assemble a sample of over a hundred intensely star-forming galaxies, very bright in the submillimeter domain, with a high signal-to-noise ratio (S/N > 50). These galaxies come from the early universe, then aged only 1.6 to 4.3 billion years (redshift between z = 1.5 and 4). Such a wealth of data would have been impossible to obtain within the framework of a standard observation time request, highlighting the importance of exploiting archives for studies of this scale.
The first discovery concerns the morphology of the submillimeter components at the centers of these galaxies, which correspond to the sites of star formation. The study indicates that most of the centers of these galaxies are intrinsically spherical, and not disk-shaped as previously thought. Indeed, the researchers found that the submillimeter emission of these galaxies is very compact, with surface brightness profiles significantly deviating from those typical of disks. This conclusion is reinforced by detailed modeling of their 3D geometry, which shows that the ratio between the shortest and longest axes is on average half, increasing with spatial compactness (cf. Figure 2).
The second revelation of this study concerns the mechanism of formation of spheroidal galaxies. It was long thought that spheroids formed late in the history of the universe, mainly through coalescence, i.e., the merger of two galaxies after a collision. However, this study brings a new perspective: spheroids have been observed forming directly from star bursts, likely due to the simultaneous action of cold gas accretion and interactions between galaxies, without requiring a merger. These processes lead to intense star formation concentrated in the three-dimensional cores of these galaxies, and this, from the earliest epochs of cosmic history.
Possible access to the birthplaces of giant elliptical galaxies
This study has provided the first strong observational evidence that spheroids can form directly through intense star formation, fueled by the accretion of cold gas and simultaneous galactic interactions in the cores of galaxies. This process, apparently very widespread in the distant universe, marks a turning point in our understanding of the formation and evolution of the bulges of spiral galaxies, and perhaps also of giant elliptical galaxies, such as M87 in the constellation Virgo, whose birth sites have been sought for decades.
Figure 3 - The JWST has recently revealed the true nature of the neighboring galaxy M104, known as the Sombrero Galaxy. Thanks to its infrared vision, the telescope was able to observe through the dust and gas, which gave the illusion of spiral arms. The new data confirm that the Sombrero is actually an elliptical galaxy surrounded by a ring, with very low star formation (less than one solar mass per year).
Credit: © (NASA, ESA, CSA, STScI)
New ALMA observations, benefiting from increased resolution and sensitivity, combined with archival data, will allow for detailed exploration of the distribution and kinematics of cold gas—the raw material of star formation—within these galaxies through statistical studies. Moreover, the capabilities of the JWST, Euclid, and the Chinese Space Station Telescope (CSST) to map the stellar components of galaxies will complement this approach, offering a more comprehensive view of their evolution (cf. Figure 3). Together, these tools promise to revolutionize our understanding of galaxy formation in the early universe.