Galaxies in dense environments tend to be larger, settling one cosmic question and raising others

For decades, scientists have known that some galaxies reside in dense environments with lots of other galaxies nearby. Others drift through the cosmos essentially alone, with few or no other galaxies in their corner of the universe.

A new study has found a major difference between galaxies in these divergent settings: Galaxies with more neighbors tend to be larger than their counterparts, which have a similar shape and mass, but reside in less dense environments. In a paper published Aug. 14 in the Astrophysical Journal, researchers at the University of Washington, Yale University, the Leibniz Institute for Astrophysics Potsdam in Germany and Waseda University in Japan report that galaxies found in denser regions of the universe are as much as 25% larger than isolated galaxies.

The research, which used a new machine-learning tool to analyze millions of galaxies, helps resolve a long-standing debate among astrophysicists over the relationship between a galaxy's size and its environment. The findings also raise new questions about how galaxies form and evolve over billions of years.

"Current theories of galaxy formation and evolution cannot adequately explain the finding that clustered galaxies are larger than their identical counterparts in less dense regions of the universe," said lead author Aritra Ghosh, a UW postdoctoral researcher in astronomy and an LSST-DA Catalyst Fellow with the UW's DiRAC Institute. "That's one of the most interesting things about astrophysics. Sometimes what the theories predict we should find and what a survey actually finds are not in agreement, and so we go back and try to modify existing theories to better explain the observations."

Past studies that looked into the relationship between galaxy size and environment came up with contradictory results. Some determined that galaxies in clusters were smaller than isolated galaxies. Others came to the opposite conclusion. The studies were generally much smaller in scope, based on observations of hundreds or thousands of galaxies.

In this new study, Ghosh and his colleagues utilized a survey of millions of galaxies conducted using the Subaru Telescope in Hawaii. This endeavor, known as the Hyper Suprime-Cam Subaru Strategic Program, took high-quality images of each galaxy. The team selected approximately 3 million galaxies with the highest-quality data and used a machine learning algorithm to determine the size of each one. Next, the researchers essentially placed a circle -- one with a radius of 30 million light years -- around each galaxy. The circle represents the galaxy's immediate vicinity. They then asked a simple question: How many neighboring galaxies lie within that circle?

The answer showed a clear general trend: Galaxies with more neighbors were also on average larger.

There could be many reasons why. Perhaps densely clustered galaxies are simply larger when they first form, or are more likely to undergo efficient mergers with close neighbors. Perhaps dark matter -- that mysterious substance that makes up most of the matter in the universe, yet cannot be detected directly by any current means -- plays a role. After all, galaxies form within individual "halos" of dark matter and the gravitational pull from those halos plays a critical role in how galaxies evolve.

"Theoretical astrophysicists will have to perform more comprehensive studies using simulations to conclusively establish why galaxies with more neighbors tend to be larger," said Ghosh. "For now, the best we can say is that we're confident that this relationship between galaxy environment and galaxy size exists."

Utilizing an incredibly large dataset like the Hyper Suprime-Cam Subaru Strategic Program helped the team reach a clear conclusion. But that's only part of the story. The novel machine learning tool they used to help determine the size of each individual galaxy also accounted for inherent uncertainties in the measurements of galaxy size.

"One important lesson we had learned prior to this study is that settling this question doesn't just require surveying large numbers of galaxies," said Ghosh. "You also need careful statistical analysis. A part of that comes from machine learning tools that can accurately quantify the degree of uncertainty in our measurements of galaxy properties."

The machine learning tool that they used is called GaMPEN -- or Galaxy Morphology Posterior Estimation Network. As a doctoral student at Yale, Ghosh led development of GaMPEN, which was unveiled in papers published in 2022 and 2023 in the Astrophysical Journal. The tool is freely available online and could be adapted to analyze other large surveys, said Ghosh.

Though this new study focuses on galaxies, it also forecasts the types of research -- centered on complex analyses of incredibly large datasets -- that will soon take astronomy by storm. When a generation of new telescopes with powerful cameras, including the Vera C. Rubin Observatory in Chile, come online, they will collect massive amounts of data on the cosmos every night. In anticipation, scientists have been developing new tools like GaMPEN that can utilize these large datasets to answer pressing questions in astrophysics.

"Very soon, large datasets will be the norm in astronomy," said Ghosh. "This study is a perfect demonstration of what you can do with them -- when you have the right tools."

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Size doesn't matter for mammals with more complex brains

In many mammal species, the males can be bigger than the females (or vice versa), a trait called sexual size dimorphism (SSD). For example, male elephant seals are around three times bigger than females. In contrast, dolphins have no difference in sizes between the sexes. Humans are somewhere in between, with the average male being larger than the average female, but across the population there is an overlap.

To understand how this trait is associated with genome evolution, scientists from the Milner Centre for Evolution at the University of Bath in the UK looked at similarities between the genomes of 124 species of mammals.

They grouped the genes into families of similar functions and measured the size of these gene families.

They found that those species with a big difference in size between the sexes had bigger gene families linked to olfactory functions (sense of smell) and smaller gene families associated with brain development.

Therefore, this could also mean that those species with very little difference in sizes between males and females (termed monomorphic) had bigger gene families associated with brain development.

Publishing in Nature Communications, the authors suggest that in species with a large SSD, traits such as the sense of smell could be important for identifying mates and territories.

In contrast, mammals with a smaller SSD are potentially investing in their brain development and tend to have more complex social structures.

This means they compete for mates using other means than simply using size to select who to breed with.

Dr Benjamin Padilla-Morales, from the Milner Centre for Evolution at the University of Bath led the research.

He said: "We were surprised to see such a strong statistical link between a large SSD and expanded gene families for olfactory function. Even more interestingly, the gene families under contraction were linked with brain development.

"This could mean that those species with a small SSD have bigger gene families associated with brain function and tend to show more complex behaviours such as biparental care and monogamous breeding systems.

"It shows that while size in some species is an important sexual selection pressure for evolution, for others it doesn't matter so much.

"It makes us ask the question how traits like SSD are shaping the evolution of our brains and genomes."

In future work, the researchers want to investigate how testes size impacts the evolution of mammals' genomes.

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Scientists assess how large dinosaurs could really get

A new study published today in the scientific journal Ecology and Evoiution looks at the maximum possible sizes of dinosaurs, using the carnivore, Tyrannosaurus rex, as an example. Using computer modelling, Dr. Jordan Mallon of the Canadian Museum of Nature and Dr. David Hone of Queen Mary University of London, produced estimates that T. Rex might have been 70% heavier than what the fossil evidence suggests.

The researchers assert that the huge sizes attained by many dinosaurs make them a source of endless fascination, raising the question as to how these animals evolved to be so big. There are perennial claims and counter-claims about which dinosaur species was the largest of its group or even the largest ever.

Most dinosaur species are known from only one or a handful of specimens, so it's extraordinarily unlikely that their size ranges will include the largest individuals that ever existed. The question remains: how big were the largest individuals, and are we likely to find them?

To address this question, Mallon and Hone used computer modelling to assess a population of T. rex. They factored in variables such as population size, growth rate, lifespan, the incompleteness of the fossil record, and more.

T. rex was chosen for the model because it is a familiar dinosaur for which many of these details are already well estimated. Body-size variance at adulthood, which is still poorly known in T. rex, was modelled with and without sex differences, and is based on examples of living alligators, chosen for their large size and close kinship with the dinosaurs.

The palaeontologists found that the largest known T. rex fossils probably fall in the 99th percentile, representing the top 1% of body size, but to find an animal in the top 99.99% (a one-in-ten-thousand individual) scientists would need to excavate fossils at the current rate for another 1,000 years.

The computer models suggest that the largest individual that could have existed (one in 2.5 billion animals) may have been 70% more massive than the current largest-known T. rex specimens (an estimated 15 tonnes vs 8.8 tonnes) and 25% longer (15 metres vs 12 metres).

The values are estimates based on the model, but patterns of discovery of giants of modern species tell us there must have been larger dinosaurs out there that have not yet been found. "Some isolated bones and pieces certainly hint at still larger individuals than for which we currently have skeletons," says Hone.

This study adds to the debates about the largest fossil animals. Many of the largest dinosaurs in various groups are known from a single good specimen, so it's impossible to know if that one animal was a big or small example of the species. An apparently large species might be based on a single giant individual, and a small species based on a particularly tiny individual -- neither of which reflect the average size of their respective species.

The chances that palaeontologists will find the largest ever individuals for a given species are incredibly small. So, despite the giant skeletons that can be seen in museums around the world, the very largest individuals of these species were likely even larger than those on display.

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Size does matter: Group size and mating preferences drive deeper male voices

Deeper male voices in primates, including humans, offer more than sex appeal -- they may have evolved as another way for males to drive off competitors in large groups that favored polygyny, or mating systems where a male has multiple mates, according to researchers. The research is the most comprehensive investigation of differences in vocal pitch between sexes to date and has the potential to help to shed light on social behavior in humans and their closest living relatives.

The average speaking pitch of an adult male human is about half the average pitch, an octave lower, than that of an adult female human, said David Puts, professor of anthropology at Penn State and study co-author.

"It's a sex difference that emerges at sexual maturity across species and it probably influences mating success through attracting mates or by intimidating competitors," he said. "I thought it has to be a trait that's been subjected to sexual selection, in which mating opportunities influence which traits are passed down to offspring. Humans and many other primates are highly communicative, especially through vocal communication. So it seems like a really relevant trait for thinking about social behavior in humans and primates in general."

The researchers used specialized computer software to visualize vocalizations and measure voice pitch in recordings from 37 anthropoid primate species, or those most closely related to humans, including gorillas, chimpanzees and recordings of 60 humans evenly divided by sex. Samples for each species included at least two male and two female vocal recordings, for a total of 1,914 vocalizations. The team then calculated average male and female vocal fundamental frequency for each species to see how pronounced the difference was between sexes.

The scientists collected additional information for each species to help identify correlations between male versus female voice pitch and factors that could have contributed to the trait's evolution. Additional variables included body size and body mass differences between males and females, habitat type, adult sex ratios, mating competition intensity and testes size. They also categorized each species by mating system -- monogamous, in which males and females have one mate at a time; polygynandrous, in which males and females have multiple mating partners; and polygynous, in which some males have several mates.

The researchers used these data to test five hypotheses simultaneously to identify which factors may have played the strongest roles in driving sex differences in vocal pitch. The hypotheses were: intensity of mating competition, large group size, multilevel social organization, trade-off against the intensity of sperm competition, and poor acoustic habitats. Previous research has looked at one or two of these hypotheses at a time. The current study is the first to test multiple hypotheses simultaneously for vocal pitch differences using a robust dataset, ensuring data consistency and garnering convincing results, according to Puts.

The team found that fundamental frequency differences by sex increased in larger groups and those with polygynous mating systems, especially in groups with a higher female-to-male ratio. They reported their findings today (July 10) in Nature Communications.

"Our findings highlight the important role of sexual selection and offer possible evolutionary explanations for why males and females differ in voice pitch across primates," said Toe Aung, first author and assistant professor of psychology and counseling at Immaculata University, who worked on the study as part of his doctoral dissertation at Penn State. "This research also provides insight into sex differences in voice pitch in our common ancestors who lived millions of years ago."

Deeper male voices may act as an additional way to fend off mating competitors without having to engage in costly fighting by making males sound bigger, in addition to other physical traits like height and muscle size, according to the researchers. In adult humans, for instance, males vocalize at an average of 120 hertz whereas females vocalize at an average of about 220 hertz, putting humans right in the middle of polygynous societies, the researchers reported.

"Although social monogamy is really common in humans, mating and reproduction in our ancestors was substantially polygynous," Puts said. "Our findings help us to understand why male and female voices of our species differ so drastically. It may be a product of our evolutionary history, particularly our history of living in large groups in which some males reproduced with multiple females."

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A new model offers an explanation for the huge variety of sizes of DNA in nature

A new model developed at Tel Aviv University offers a possible solution to the scientific question of why neutral sequences, sometimes referred to as "junk DNA," are not eliminated from the genome of living creatures in nature and continue to exist within it even millions of years later.

According to the researchers, the explanation is that junk DNA is often located in the vicinity of functional DNA. Deletion events around the borders between junk and functional DNA are likely to damage the functional regions and so evolution rejects them. The model contributes to the understanding of the huge variety of genome sizes observed in nature.

The phenomenon that the new model describes is called by the team of researchers "border induced selection." It was developed under the leadership of the PhD student Gil Loewenthal in the laboratory of Prof. Tal Pupko from the Shmunis School of Biomedicine and Cancer Research, Faculty of Life Sciences and in collaboration with Prof. Itay Mayrose (Faculty of Life Sciences, Tel Aviv University). The study was published in the journal Open Biology.

The researchers explain that throughout evolution, the size of the genome in living creatures in nature changes. For example, some salamander species have a genome ten times larger than the human genome.

Prof. Pupko explains: "The rate of deletions and short insertions, which are termed in short as 'indels', is usually measured by examining pseudogenes. Pseudogenes are genes that have lost their function, and in which there are frequent mutations, including deletions and insertions of DNA segments. In previous studies that characterized the indels, it was found that the rate of deletions is greater than the rate of additions in a variety of creatures including bacteria, insects, and even mammals such as humans. The question we tried to answer is how the genomes are not deleted when the probability of DNA deletion events is significantly greater than DNA addition events."

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Researchers measure size-luminosity relation of galaxies less than a billion years after Big Bang

An international team of researchers including the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) has studied the relation between galaxy size and luminosity of some of the earliest galaxies in the universe taken by the brand-new James Webb Space Telescope (JWST), less than a billion years after the Big Bang, reports a new study in The Astrophysical Journal Letters.

The result is part of the Grim Lens-Amplified Survey from Space (GLASS) Early-Release Science Program, led by University of California, Los Angeles, Professor Tommaso Treu. It is aimed at studying the early universe when the first stars/galaxies ignited, which ionized the neutral gas in the universe at the time and allowed light to shine through. This is called the epoch of reionization.

However, details of reionization have remained unknown because telescopes until today have not been capable of observing galaxies in this period of the universe's history in detail. Finding out more about the epoch of reionization would help researchers understand how stars and galaxies have evolved to create today's universe as we see it.

One study, led by Kavli IPMU JSPS Fellow Lilan Yang, and including Project Researcher Xuheng Ding, used multiband NIRCAM imaging data from the GLASS-JWST program to measure galaxy size and luminosity to figure out the morphology and the size-luminosity relation from rest-frame optical to UV.

"It's the first time that we can study the galaxy's properties in rest-frame optical at redshift larger than 7 with JWST, and the size-luminosity is important for determining the shape of luminosity function which indicates the primary sources responsible for the cosmic reionization, i.e., numerous faint galaxies or relatively less bright galaxies.

"The original wavelength of light will shift to longer wavelength when it travels from the early universe to us. Thus, the rest-frame wavelength is used to clarify their intrinsic wavelength, rather than observed wavelength.

Previously, with Hubble Space Telescope, we know the properties of galaxies only in rest-frame UV band. Now, with JWST, we can measure longer wavelength than UV," said first author Yang.

The researchers found the first rest-frame optical size-luminosity relation of galaxies at redshift larger than 7, or roughly 800 million years after the Big Bang, allowing them to study the size as function of wavelength. They found the median size at the reference luminosity is roughly 450-600 parsecs and decreased slightly from rest-frame optical to UV. But was this expected?

"The answer is we don't know what's to expect. Previous simulation studies give a range of predictions," said Yang.

The team also found the slope of the size-luminosity relationship was somewhat steeper in the shortest wavelength band when allowing the slope to vary.

"That would suggest higher surface brightness density at shorter wavelength, hence less observational incompleteness correction when estimating luminosity function, but the result is not conclusive. We don't want to over-interpret here," said Yang.

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In visual memory, size matters

Every day we encounter images on the wall, in newspapers, books, and electronic devices. Some become etched in our memory and some don't. The elements influencing whether we remember one image and not the other aren't yet known, but researchers have assumed that image size and memory aren't connected to one another, since we usually understand what appears in an image, whether it is large or small.

A new study led by Dr. Sharon Gilaie-Dotan, of Bar-Ilan University's School of Optometry and Vision Science and Gonda (Goldschmied) Multidisciplinary Brain Research Center, sought to determine whether large images are better remembered than small ones during natural daily behavior. Her assumption was based on the fact that large images require the visual system to utilize greater resources for processing them.

The results of the study, just published in the journal Proceedings of the National Academy of Sciences, show for the first time that in natural vision, visual memory of images is affected by the size of the image on the retina. These findings can have many implications, including on the use of different types of electronic screens and the quality of information processing when we rely on large vs. small screens.

Shaimaa Masarwa and Olga Kreichman, PhD students in Dr. Gilaie-Dotan's lab, examined what happens to visual memory when participants were asked to look at pictures without knowing anything about a memory task to come. Each participant was shown different pictures in different sizes, each presented to them just once.

One hundred eighty-two subjects participated in seven different experiments. Time and time again the researchers found that the large images were better remembered (1.5 times more) than the small images. This phenomenon was not dependent on specific stimuli, the order in which the images appeared, their resolution, or the amount of information they contained.

To understand whether this result was determined by size rather than amount of detail, the researchers also examined whether large, blurred images are better etched in memory than clear, small images, where the large images contained the same details as the small images. To their surprise, they found that even in this case, the participants remembered the large, blurry images better than the small, clear images.

They also found that most images were better remembered when they were presented as bigger relative to when they were presented as smaller.

"In areas of the brain that represent the retinal image, more resources will be directed to processing large images than to processing small images because the processing is determined by the area of the retina that the image stimulates," says Dr. Gilaie-Dotan. She points out that additional factors may contribute to remembering large images, such as different eye movements, and more attention and interest that large images elicit.

The study was conducted on young adults aged 18-40, ages in which vision is completely developed but has not yet begun to age. Different ages may be affected by the size of the stimuli in a different way, since both age and experience with screens are quite different between young and old.

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Crushed resistance: Tectonic plate sinking into a subduction zone

The Earth's surface consists of a few large plates and numerous smaller ones that are continuously moving either away from or towards each other at an extremely slow pace. At the boundaries of two plates, the heavier oceanic plate sinks below the lighter continental plate in a process that experts call subduction. For a long time, though, those experts have been puzzling over what happens to the plate margin that dives into the Earth's mantle, known as the subducting slab. Some scientists assumed that the slab remains as rigid and strong as the plate itself and simply bends due to the gravity force and mechanical interaction with the Earth's mantle.

Heavily deformed plate margin

However, models of the Earth's interior constructed by scientists using seismic tomography revealed contradictory results: in the western United States, for example, the researchers observed anomalies at different depths on their tomographic images. These indicated that the slabs submerged beneath the Americas may be segmented. The scientists therefore concluded that the slabs in the mantle must be strongly deformed and are by no means rigid and immobile.

With the aid of computer models, other researchers, including ETH Professor Paul Tackley, confirmed that subducted slabs are indeed weak and deformable. And they formulated the subduction dichotomy hypothesis that can be expressed in simple terms: plates on the surface are rigid and strong (read: non-deformable), while the slabs in the mantle are soft and weak.

Seeking a plausible mechanism

"Until now, however, research has lacked a plausible mechanism to explain how this bending occurs and why sinking plate margins (slabs) become soft and weak," says Taras Gerya, Professor of Geophysics at ETH Zurich.

Observations revealed that numerous faults are found on the upper surface of a sinking plate where it meets the other plate. Seawater penetrates the plate through these faults and is in fact literally sucked in by suction forces. This weakens the plate on its upper side.

Yet this alone is not sufficient to explain the segmentation of the slab -- the anomalies observed on tomographic images. Another mechanism must also be at work to weaken the underside of the margin enough for segmentation to occur.

Gerya and his American colleagues David Bercovici and Thorsten Becker therefore suspected that compression of the underside of the plate at the point where it bends downward was "crushing" large and strong, millimetres size olivine crystals in the plate by forcing them to recrystallise into much weaker, micrometres size granular aggregate -- thereby reducing the plate's resistance and allowing it to bend.

Sinking plate margin divided into segments

Using a new two-dimensional computer model that integrated this grain reduction as a central mechanism, the three researchers then studied the process in silico. Their study was recently published in the journal Nature.

And indeed, the simulations revealed that sinking plates deform due to the massive reduction of olivine grains on their undersides, splitting into individual segments over time. These segments are rigid and stiff, but remain connected to each other by weak hinges made of ground grains.

In the simulations, parallel cracks appear at the segment boundaries on the plate's upper surface. Below these cracks are the zones with "crushed" mineral grains.

"Just imagine you're breaking a bar of chocolate," Gerya says with a grin. A bar of chocolate, too, can be divided into segments only along the specified weak points. The squares of chocolate are rigid, but the connecting pieces between them are weak. "That's why a sinking plate isn't uniformly bent or deformed, but segmented."

And here's how it might play out in reality: The heavier plate sinks under the lighter one. A weak spot with smaller mineral grains within the sinking plate allows it to bend. The bending stress causes the minerals to crumble in more places on the underside. The resulting weakness leads to a fracture, and a segment forms. As the plate margin sinks deeper and deeper into the mantle, it causes further segments to form at the bend. As a result, the slab eventually resembles a chain with rigid links and bendable connectors. At a depth of about 600 kilometres, the segmented plate margin slides onto what is known as the 670 km discontinuity in the Earth's mantle, from which point it moves horizontally.

Clues from nature support simulation

"The results of our simulations are consistent with observations in nature," Gerya explains. A great deal of research has been done on the natural situation along the Japan Trench, where the Pacific plate sinks below the Okhotsk plate. The pattern of faults found here is an exact match for the pattern produced in the simulations.

Researchers have also studied the seismic velocity structure of subducting Japan slab thoroughly using its recently produced high-resolution seismic tomography model. They found that the velocity of the seismic waves sent out by earthquakes was reduced at some nodes inside the slab. The pattern with which these nodes occur in reality coincides with that of the segment boundaries from the simulations. And both in nature and in the computer model, it is zones with very small crystals only micrometres across that are responsible for reducing the velocity of the seismic waves.

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Diet restricted size of hunter-gatherer societies

Short growing seasons limited the possible size of hunter-gatherer societies by forcing people to rely on meat, according to a recent study by a team of international reseachers including McGill University professor Eric Galbraith.

After looking at population size for the roughly 300 hunter-gatherer societies which existed until quite recently, the researchers found that many of these groups were much smaller than might have been expected from the local ecosystem productivity. In regions with short growing seasons, hunter-gatherer groups had smaller populations per square kilometre than groups who depended on abundant plant foods throughout the year.

Need for meat limited population size

"Basically, if people had to live through long dry or cold seasons when plant food was scarce, in order to survive they had to depend on hunting a very limited number of animals," explains Galbraith, a professor in McGill's Department of Earth and Planetary Sciences and at the ICTA-UAB (Institut de Ciència i Tecnologia Ambientals of the Autonomous University of Barcelona), and a senior author on the paper published recently in the journal Nature Ecology & Evolution.

"This led to a seasonal bottleneck in the amount of food available, which then set the overall limit on the population size, no matter how much food there was during the plentiful times."

The team developed a mathematical model that simulates daily human foraging activities (gathering and hunting) and the resultant carbon (energy) flows between vegetation, animals, and hunter-gatherers in a realistic global environment.

"We were struck by the fact that -- despite a long list of unknowns -- a very strong result emerged from the model equations," says Galbraith. "Wherever growing seasons were short, hunter gatherers required meat to make up a high percentage of their diets. And -- just as in the modern world -- it took much more land to produce the same amount of meat as plant-based food."

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