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 SouthamptonThe 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.”
This image shows two of the Atacama Large Millimeter/submillimeter Array (ALMA) 12-metre antennas. ALMA has 66 antennas that work together as an interferometer. (Credit : Iztok Bonina/ESO)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.
This is a JWST image (not from this research) of an ancient galaxy merger from 13 billion years ago. The galaxy, named Gz9p3, has a double nucleus indicating that the merger is ongoing. While astronomers know that mergers are a critical part of galaxy growth and evolution, the role spheroids play has been difficult to discern. Image Credit: NASA/Boyett et al“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.
This figure from the research shows how the spheroids lose their shape after the intense period of star formation following a merger. (a) shows maps (2×2 kpc) of the central gas in three differentThese 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.”
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Imagine you’ve just gotten to Mars as part of the first contingent of settlers. Your first challenge: build a long-term habitat using local materials. Those might include water from the polar caps mixed with specific surface soils. They might even require some very personal contributions—your blood, sweat, and tears. Using such in situ materials is the challenge a team of Iranian engineers studied in a research project looking at local materials on Mars.
In situ resource utilization has always been part of Mars mission and colonization scenarios. It’s expensive to bring along habitat construction materials with you, and space will be limited onboard the ship. Once you settle on Mars, you can use your ship as a habitat until you build your new colony. But, what are you going to create new homes from?
Cement or concrete come to mind, made from whatever’s available on or just below the surface. The authors of the study, Omid Karimzade Soureshjani, Ali Massumi, and Gholmreza Nouri, focused on Martian cement. They assembled data sets about soil composition from Mars landers and orbiters and came up with a collection of concrete types that future colonists could use. Next, they applied structural engineering principles and suggested some options for onsite construction using what are called spider/radar diagrams and charts. These allow building planners to apply data for different concepts of Mars architecture.
A graph showing steps in the study of possible building materials on Mars. Courtesy: Soureshjani, et al. Click to enlarge. Building That Mars CityThe authors, like most of us, foresee permanent settlements in the next decades. They write, “The goal would be to establish a self-sustaining city (self-sufficient megabase) on the surface of Mars, accommodating at least a million people. However, constructing safe, stable, and sufficient buildings that can withstand the harsh Martian environment for such a population will be challenging. Due to the high costs associated with importing buildings, materials, and structural elements from Earth, it is necessary to construct all buildings on-site using local resources.”
Let’s look at the usability and cost-effectiveness of Martian soil (regolith). Chemically, it’s rich in the right amounts of elements to make different types of concrete. Of course, not all the regoliths are equally useful, so they propose surface scans to find the best surface materials mixes. Presumably, those scans will help future inhabitants find the best collections. Access to those raw materials from around the planet should make them cost-effective, eventually.
Challenges to Mars ConstructionOf course, there are other factors besides material availability at work in such a construction project. Here on Earth, we have centuries of experience building in this gravity well, with familiar materials. We know how to build things under this atmospheric pressure, and we don’t have to contend with the harsh conditions of a planet constantly bombarded by ultraviolet radiation. Mars presents the challenge of creating buildings that have to withstand that radiation, the lower atmospheric pressure, and water scarcity. That lower pressure and gravity on Mars could seriously affect the durability of a given concrete made from Martian materials.
In addition to planetary geology and surface conditions, it takes energy to collect, process, and create the building materials needed for long-term habitation. You need a simple, cost-effective energy source—particularly in the beginning. It’s not likely that nuclear power plants will be first on the list to build. Those require a tremendous number of resources. Perhaps later they can be built, but not in the first wave. Solar energy is going to be the “go-to” resource in the beginning. In addition, to make cement, you need water. And, water is a notably scarce resource on much of Mars, except at the poles. They could provide some water from the ice caps, but you’ll likely want to figure out a way to make good cement with the least amount of water.
Using Organic Binders for Mars Home Building BlocksInterestingly, the authors mention something called “blood concrete”, or its modern version: AstroCrete. It’s a concept based on ancient Roman practices of using organic additives to construction materials (think: animal blood, urine, etc.). Now, they aren’t suggesting that future Martians must “bleed for their art” but our bodies do make plasma rather easily. It could be a useful resource.
A substance called “human serum albumin” (HAS) is under study as a binder to mix with “AstroCrete” materials, along with sweat, tears, and urine. All those will be available in relative abundance in future Mars settlements. The AstroCrete made from Martian soils and human “contributions” is a strong building material you can rely on for strength (and you hope it won’t smell too bad). Essentially, AstroCrete is waterless cement.
Visible light images of the 3D-printed HSA-ERB based on Martian Global Simulant. (a) after fabrication, (b) during compression testing, and (c) after compression testing. Courtesy: Robertsad, et al. Exploring the PossibilitiesThe authors studied 11 types of cement, including geopolymer and magnesium silica mixtures, all of which require specific materials. They point out that sulfur concrete is probably going to be the most promising avenue for structures on Mars. Others will take more study and implementation to understand their usability in Martian conditions. In the long term, searching out and understanding the materials available on the Red Planet will help future colonists build the necessary habitats and cities. Finally, the authors point out that additional study of both materials and the Martian environment using data from current and future missions is necessary. Their paper is well worth reading in more detail.
For More InformationMartian Buildings: Feasible Cement/concrete for Onsite Sustainable Construction from the Structural Point of View
Martian Concrete Could be Tough Stuff
Blood, Sweat, and Tears: Extraterrestrial Regolith Biocomposites with in vivo Binders
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