Astronomers have spent decades trying to understand how galaxies grow so large. One piece of the puzzle is spheroids, also known as galactic bulges. Spiral galaxies and elliptical galaxies have different morphologies, but they both have spheroids. This is where most of their stars are and, in fact, where most stars in the Universe reside. Since most stars reside in spheroids, understanding them is critical to understanding how galaxies grow and evolve.
New research focused on spheroids has brought them closer than ever to understanding how galaxies become so massive.
Elliptical galaxies have no flat disk component. They’re smooth and featureless and contain comparatively little gas and dust compared to spirals. Without gas and dust, new stars seldom form, so ellipticals are populated with older stars.
Astronomers don’t know how these ancient, bulging galaxies formed and evolved. However, a new research letter in Nature may finally have the answer. It’s titled “In situ spheroid formation in distant submillimetre-bright galaxies.” The lead author is Qing-Hua Tan from the Purple Mountain Observatory, Chinese Academy of Sciences, China. Dr. Annagrazia Puglisi from the University of Southampton co-authored the research.
“Our findings take us closer to solving a long-standing mystery in astronomy that will redefine our understanding of how galaxies were created in the early universe.”
Dr. Annagrazia Puglisi, University of Southampton
The international team of researchers used the Atacama Large Millimetre/sub-millimetre Array (ALMA) to examine highly luminous starburst galaxies in the distant Universe. Sub-millimetre means it observes electromagnetic energy between far-infrared and microwave. Astronomers have suspected for a long time that these galaxies are connected to spheroids, but observing them is challenging.
“Infrared/submillimetre-bright galaxies at high redshifts have long been suspected to be related to spheroid formation,” the authors write. “Proving this connection has been hampered so far by heavy dust obscuration when focusing on their stellar emission or by methodologies and limited signal-to-noise ratios when looking at submillimetre wavelengths.”
The researchers used ALMA to analyze more than 100 of these ancient galaxies with a new technique that measures their distribution of light. These brightness profiles show that the majority of the galaxies have tri-axial shapes rather than flat disks, indicating that something in their history made them misshapen.
Two important concepts underpin the team’s results: The Sersic index and the Spergel index.
The Sersic index is a fundamental concept in describing the brightness profiles of galaxies. It characterizes the radial distribution of light coming from galaxies and basically describes how light is concentrated in a galaxy.
The Spergel index is less commonly used. It’s based on the distribution of dark matter in galaxies. Rather than light, it helps astronomers understand how matter is concentrated. Together, both indices help astronomers characterize the complex structure of galaxies.
These indices, along with the new ALMA observations, led to new insights into how spheroids formed through mergers and the resulting influx of cold, star-forming gas.
It all starts with a galaxy collision or merger, which sends large flows of cold gas into the galactic centre.
“Two disk galaxies smashing together caused gas—the fuel from which stars are formed—to sink towards their centre, generating trillions of new stars,” said co-author Puglisi. “These cosmic collisions happened some eight to 12 billion years ago when the universe was in a much more active phase of its evolution.”
“This is the first real evidence that spheroids form directly through intense episodes of star formation located in the cores of distant galaxies,” Puglisi said. “These galaxies form quickly—gas is sucked inwards to feed black holes and triggers bursts of stars, which are created at rates ten to 100 times faster than our Milky Way.”
The researchers compared their observations to hydro-simulations of galaxy mergers. The results show that the spheroids can maintain their shape for up to approximately 50 million years after the merger. “This is compatible with the inferred timescales for the submillimeter-bright bursts based on observations,” the authors write. After this intense period of star formation in the spheroid, the gas is used up, and things die down. No more energy is injected into the system, and the residual gas flattens out into a disk.
mergers, showing the flattest projection for these systems observed at 12 Myr from coalescence; that is, these systems are 3D spheroidal structures, not face-on disks. (b) shows the star-formation rate peaking and then dimishining over time. (c) shows C/A, which quantifies the relative system thickness encompassing all galactic components, including disks, bars, and bulges. It’s a ratio between C, the shortest axis, and A, the longest axis in a triaxial ellipsoid. Image Credit: Tan et al. 2024.
These types of galaxies were more plentiful in the early Universe than they are now. The researchers’ results show that these galaxies used up their fuel quickly, forming the spheroids that are now populated by old stars.
This isn’t the first time that astronomers have investigated the potential link between spheroids and distant submillimeter-bright galaxies. Previous research that found evidence for tri-axiality also found heavy ellipticity and other evidence showing that submillimeter-bright galaxies are disks with bars in the submillimeter. However, this new research relied on observations with a higher signal-to-noise ratio than previous research.
“Astrophysicists have sought to understand this process for decades,” Puglisi said. “Our findings take us closer to solving a long-standing mystery in astronomy that will redefine our understanding of how galaxies were created in the early universe.”
“This will give us a more complete picture of early galaxy formation and deepen our understanding of how the universe has evolved since the beginning of time.”