Drawing on technical and observational advances, an international team led by researchers from the Department of Astrophysics of the IRFU (CEA Paris-Saclay) has elucidated the mystery of the formation of spheroids, which are found in bulbs spiral galaxies and in giant elliptical galaxies.
These structures, long considered primarily the product of galactic mergers late in cosmic history, could 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.
The color images were reconstructed by combining three filters: F444W (red), F227W (green) and F150W (blue). The region outlined by cyan dashed lines 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 the evolution of galaxies, impacting current models which will also benefit from high-resolution observations using the latest generation telescopes (JWST, Euclid, etc.).
This research was presented in a paper titled “In situ spheroid formation in distant submillimetre-bright Galaxies,” published in the journal Nature.
Technical and observational limits finally lifted
The galaxies of the Universe are divided into two main morphological categories. On the one hand, spiral galaxies, shaped like discs, like ours Milky Way. They are young, rich in gas and continue to form stars. On the other hand, spheroidal galaxies, which include elliptical galaxies and the bulges of spiral galaxies. They are devoid of gas, composed of very old stars and almost no longer form stars; they are as if “dead”. If 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 remained 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, to the era of the “Cosmic Midday”, when the Universe was between 1.6 and 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 region of the spectrum, has therefore opened up the possibility of studying galactic bulges. These observations are supplemented by the vision infrared of the powerful Space Telescope James Webb (JWST) which provides a global view of the galaxies (see Figure 1).
On the far left, the infrared images captured by the JWST (see Fig. 1 legend) are followed by a zoom on their central submillimeter regions, obtained using ALMA. The scheme 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 compact submillimeter galaxies (orange ellipse) and a subsample of extended submillimeter galaxies (blue ellipse). These parameters are compared to those of local early-type (red ellipse) and late-type (represented by purple and cyan spiral shapes) galaxies.
Credit: Tan et al. 2024.
This research was also made possible thanks to a significant technical advance. In a previous publication (Tan et al. 2024, A&A), 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 in-depth analysis of spheroidal systems difficult.
New insights into the formation of giant elliptical galaxies in the early Universe
This study is based on ALMA observations collected over the years by various projects. Thanks to the A3COSMOS and A3GOODS archival projects, researchers were able to constitute a sample of more than a hundred galaxies with intense star formation, very bright in the submillimeter domain, with a high signal-to-noise ratio (S/N > 50). These galaxies come from the early Universe, then only 1.6 to 4.3 billion years old (redshift between z = 1.5 and 4). Such a wealth of data would have been impossible to obtain within the framework of a traditional request for research time.observationemphasizing the importance of using 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 places 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 from these galaxies is very compact, with surface brightness profiles deviating significantly 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 (see Figure 2).
The second revelation of this study concerns the mechanism of formation of spheroidal galaxies. It has long been thought that spheroids formed late in the history of the Universe, mainly by coalescence, that is to say by the merger of two galaxies after collision. However, this study provides a new perspective: spheroids have been observed to form directly from star flares, probably due to the simultaneous action of cold gas accretion and interactions between galaxies, without requiring merger. These processes lead to intense star formation concentrated in the three-dimensional hearts of these galaxies, from the earliest epochs of cosmic history.
Possible access to the birthplaces of large elliptical galaxies
This study provided the first strong observational evidence that spheroids can form directly through intense star formation, fueled by cold gas accretion and simultaneous galactic interactions in galaxy cores. This process, apparently very widespread in the distant Universe, constitutes 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 of the Virgin, whose birth sites had been sought for decades.
Credit: © (NASA, ESA, CSA, STScI)
New ALMA observations, benefiting from increased resolution and sensitivity, combined with archival data, will allow us to explore in detail the distribution and kinematics of cold gas — the raw material for star formation — within these galaxies through statistical studies. Furthermore, the capabilities of the JWST, Euclid telescopes and the space telescope of the Space Station Chinese (CSST) to map the stellar components of galaxies will complement this approach, offering a more complete vision of their evolution (see Figure 3). Together, these tools promise to revolutionize our understanding of galaxy formation in the early Universe.
