ST/ Stars are more massive than we thought

Paradigm

Follow

Published in

Paradigm

39 min readJun 8, 2022

--

Space biweekly vol.53, 25th May — 8th June

TL;DR

A team of astrophysicists has arrived at a major result regarding star populations beyond the Milky Way. The result could change our understanding of a wide range of astronomical phenomena, including the formation of black holes, supernovae and why galaxies die.
Astronomers may now understand why the similar planets Uranus and Neptune are different colors. Researchers have now developed a single atmospheric model that matches observations of both planets. The model reveals that excess haze on Uranus builds up in the planet’s stagnant, sluggish atmosphere and makes it appear a lighter tone than Neptune.
Using NASA’s SOFIA observatory and other data resources, an international team of astronomers has uncovered evidence of metals in local galaxies — found to be deficient in earlier studies — by analyzing infrared data gathered during a multiyear campaign.
NASA scientists and engineers give new details about the Deep Atmosphere Venus Investigation of Noble gases, Chemistry, and Imaging (DAVINCI) mission, which will descend through the layered Venus atmosphere to the surface of the planet in mid-2031. DAVINCI is the first mission to study Venus using both spacecraft flybys and a descent probe.
Researchers demonstrate that under low fluid shear force conditions that simulate those found in microgravity culture during spaceflight, the foodborne pathogen Salmonella infects 3-D models of human intestinal tissue at much higher levels, and induces unique alterations in gene expression.
NASA’s Hubble Space Telescope has calibrated more than 40 ‘milepost markers’ of space and time to help scientists precisely measure the expansion rate of the universe — a quest with a plot twist.
A weather satellite accidentally caught Betelgeuse dimming .
As a result of achieving high imaging dynamic range, a team of astronomers in Japan has discovered for the first time a faint radio emission covering a giant galaxy with an energetic black hole at its center. The radio emission is released from gas created directly by the central black hole. The team expects to understand how a black hole interacts with its host galaxy by applying the same technique to other quasars.
A team of astronomers studied two nearby globular clusters, 47 Tucanae and Omega Centauri, searching for signals produced by annihilating dark matter. Though the searches turned up empty, they weren’t a failure. The lack of a detection placed strict upper limits on the mass of the hypothetical dark matter particle.
Scientists who study the cosmos have a favorite philosophy known as the “mediocrity principle,” which, in essence, suggests that there’s really nothing special about Earth, the sun or the Milky Way galaxy compared to the rest of the universe. Now, new research fadds yet another piece of evidence to the case for mediocrity: Galaxies are, on average, at rest with respect to the early universe.
Upcoming industry events. And more!

Space industry in numbers

The global smart space market size is projected to grow from USD 9.4 billion in 2020 to USD 15.3 billion by 2025, at a Compound Annual Growth Rate (CAGR) of 10.2% during the forecast period. The increasing venture capital funding and growing investments in smart space technology to drive market growth.

Analysts at Morgan Stanley and Goldman Sachs have predicted that economic activity in space will become a multi-trillion-dollar market in the coming decades. Morgan Stanley’s Space Team estimates that the roughly USD 350 billion global space industry could surge to over USD 1 trillion by 2040.

Source: Satellite Industry Association, Morgan Stanley Research, Thomson Reuters. *2040 estimates

Space industry news

Latest research

Implications of a Temperature-dependent Initial Mass Function. I. Photometric Template Fitting

by Albert Sneppen, Charles L. Steinhardt, Hagan Hensley, Adam S. Jermyn, Basel Mostafa, John R. Weaver in The Astrophysical Journal

A team of University of Copenhagen astrophysicists has arrived at a major result regarding star populations beyond the Milky Way. The result could change our understanding of a wide range of astronomical phenomena, including the formation of black holes, supernovae and why galaxies die.

For as long as humans have studied the heavens, how stars look in distant galaxies has been a mystery. In a study, a team of researchers at the University of Copenhagen’s Niels Bohr Institute is doing away with previous understandings of stars beyond our own galaxy.

Since 1955, it has been assumed that the composition of stars in the universe’s other galaxies is similar to that of the hundreds of billions of stars within our own — a mixture of massive, medium mass and low mass stars. But with the help of observations from 140,000 galaxies across the universe and a wide range of advanced models, the team has tested whether the same distribution of stars apparent in the Milky Way applies elsewhere. The answer is no. Stars in distant galaxies are typically more massive than those in our “local neighborhood.” The finding has a major impact on what we think we know about the universe.

“The mass of stars tells us astronomers a lot. If you change mass, you also change the number of supernovae and black holes that arise out of massive stars. As such, our result means that we’ll have to revise many of the things we once presumed, because distant galaxies look quite different from our own,” says Albert Sneppen, a graduate student at the Niels Bohr Institute and first author of the study.

Median difference in χ between the best-fit templates at TIMF and 20 K for objects at 1.2 < z < 1.5 with best-fit TIMF from 30 to 35 K as a function of rest-frame wavelength (orange). Shading indicates lower and upper quartiles. The difference is shown as a running boxcar with width constant in log-space and set equal to the wavelength. The primary improvement in fit lies in the UV and NIR, likely due to a difference in shape between (power-law) extinction and the more complex effects of a change in TIMF. The fit residuals between a TIMF = 32 K FSPS template and a best-fit reconstructed spectrum at TIMF = 20 K are shown for comparison (blue), exhibiting a similar shape.

Researchers assumed that the size and weight of stars in other galaxies was similar to our own for more than fifty years, for the simple reason that they were unable to observe them through a telescope, as they could with the stars of our own galaxy.

Distant galaxies are billions of light-years away. As a result, only light from their most powerful stars ever reaches Earth. This has been a headache for researchers around the world for years, as they could never accurately clarify how stars in other galaxies were distributed, an uncertainty that forced them to believe that they were distributed much like the stars in our Milky Way.

“We’ve only been able to see the tip of the iceberg and known for a long time that expecting other galaxies to look like our own was not a particularly good assumption to make. However, no one has ever been able to prove that other galaxies form different populations of stars. This study has allowed us to do just that, which may open the door for a deeper understanding of galaxy formation and evolution,” says Associate Professor Charles Steinhardt, a co-author of the study.

In the study, the researchers analyzed light from 140,000 galaxies using the COSMOS catalog, a large international database of more than one million observations of light from other galaxies. These galaxies are distributed from the nearest to farthest reaches of the universe, from which light has traveled a full twelve billion years before being observable on Earth.

Left: best-fit temperature from 10 to 50 K vs. lookback time from a sample of 139,535 COSMOS2015 galaxies with S/N > 10 in the V band (Laigle et al. 2016). At each redshift, the distribution is individually normalized in order to emphasize the temperature distribution at all redshifts. With increased redshift, fewer galaxies are fit at lower temperatures. Right: boxcar-smoothed mean with standard deviation of best-fit gas temperature at different lookback times (with mean determined from objects in 2 Gyr width age bins and not including galaxies fit at the bounds of temperature range). The mean temperature increases from ∼28 to ∼36 K from present to 12 Gyr, while the spread decreases.

According to the researchers, the new discovery will have a wide range of implications. For example, it remains unresolved why galaxies die and stop forming new stars. The new result suggests that this might be explained by a simple trend.

“Now that we are better able to decode the mass of stars, we can see a new pattern; the least massive galaxies continue to form stars, while the more massive galaxies stop birthing new stars,. This suggests a remarkably universal trend in the death of galaxies,” concludes Albert Sneppen.

Best-fit stellar mass (M) and SFR from EAZY template fitting for individual galaxies as a function of TIMF. The dashed line is the median with shading indicating the first and third quartiles. Any observed galaxy will only have a single best-fit IMF, but the stellar mass and SFR may be biased by an erroneous choice of IMF. Stellar mass decreases monotonically with increasing temperature as we would expect from a top-heavier IMF, while SFR increases slightly beyond 40 K.

The research was conducted at the Cosmic Dawn Center (DAWN), an international basic research center for astronomy. DAWN is a collaboration between the Niels Bohr Institute at the University of Copenhagen and DTU Space at the Technical University of Denmark. The center is dedicated to understanding when and how the first galaxies, stars and black holes formed and evolved in the early universe, through observations using the largest telescopes along with theoretical work and simulations.

The empirical function used to describe the distribution of masses for a population of stars is known as the IMF — Initial Mass Function. It covers a distribution of low mass, medium mass and massive stars that astronomers have observed across the Milky Way. Historically, researchers have worked under the assumption that the IMF is universal and applies to other galaxies in the universe as well.

Composite SED normalized at 5600 Å for galaxies with best-fit temperature in the 10–20 K range (red) and 35–45 K range (blue) with standard deviation in shading. The central line indicates the median SED with shading indicating 16th and 84th percentiles. The cooler galaxies display systematically less flux than the hotter galaxies at all wavelengths of the ultraviolet (λ > 4000 Å). Cooler galaxies look more quiescent than hotter galaxies, which share spectral features with more star-forming galaxies.

In their analysis of galaxies, the researchers looked at how much light galaxies emit at various wavelengths. Large massive stars are bluish, while small and low mass stars are more yellow or red in color. This means that by comparing the distribution of blue versus red colors in a galaxy, one can measure the distribution of large versus small stars. The results demonstrate that stars in distant galaxies are typically more massive than those in our local neighborhoods, and that the farther away the researchers look, the more massive the average stars become.

Hazy blue worlds: A holistic aerosol model for Uranus and Neptune, including Dark Spots

by P.G.J. Irwin, N.A. Teanby, L.N. Fletcher, D. Toledo, G.S. Orton, M.H. Wong, M.T. Roman, S. Pérez‐Hoyos, A. James, J. Dobinson in Journal of Geophysical Research: Planets

Astronomers may now understand why the similar planets Uranus and Neptune are different colors. Using observations from the Gemini North telescope, the NASA Infrared Telescope Facility, and the Hubble Space Telescope, researchers have developed a single atmospheric model that matches observations of both planets. The model reveals that excess haze on Uranus builds up in the planet’s stagnant, sluggish atmosphere and makes it appear a lighter tone than Neptune.

Neptune and Uranus have much in common — they have similar masses, sizes, and atmospheric compositions — yet their appearances are notably different. At visible wavelengths Neptune has a distinctly bluer color whereas Uranus is a pale shade of cyan. Astronomers now have an explanation for why the two planets are different colors.

New research suggests that a layer of concentrated haze that exists on both planets is thicker on Uranus than a similar layer on Neptune and ‘whitens’ Uranus’s appearance more than Neptune’s. If there were no haze in the atmospheres of Neptune and Uranus, both would appear almost equally blue.

This diagram shows three layers of aerosols in the atmospheres of Uranus and Neptune, as modeled by a team of scientists led by Patrick Irwin. The height scale on the diagram represents the pressure above 10 bar. **Credit:** International Gemini Observatory/NOIRLab/NSF/AURA, J. da Silva/NASA /JPL-Caltech /B. Jónsson

This conclusion comes from a model that an international team led by Patrick Irwin, Professor of Planetary Physics at Oxford University, developed to describe aerosol layers in the atmospheres of Neptune and Uranus. Previous investigations of these planets’ upper atmospheres had focused on the appearance of the atmosphere at only specific wavelengths. However, this new model, consisting of multiple atmospheric layers, matches observations from both planets across a wide range of wavelengths. The new model also includes haze particles within deeper layers that had previously been thought to contain only clouds of methane and hydrogen sulfide ices.

“This is the first model to simultaneously fit observations of reflected sunlight from ultraviolet to near-infrared wavelengths,” explained Irwin, who is the lead author of a paper. “It’s also the first to explain the difference in visible color between Uranus and Neptune.”

The team’s model consists of three layers of aerosols at different heights. The key layer that affects the colors is the middle layer, which is a layer of haze particles (referred to in the paper as the Aerosol-2 layer) that is thicker on Uranus than on Neptune. The team suspects that, on both planets, methane ice condenses onto the particles in this layer, pulling the particles deeper into the atmosphere in a shower of methane snow. Because Neptune has a more active, turbulent atmosphere than Uranus does, the team believes Neptune’s atmosphere is more efficient at churning up methane particles into the haze layer and producing this snow. This removes more of the haze and keeps Neptune’s haze layer thinner than it is on Uranus, meaning the blue color of Neptune looks stronger.

“We hoped that developing this model would help us understand clouds and hazes in the ice giant atmospheres,” commented Mike Wong, an astronomer at the University of California, Berkeley, and a member of the team behind this result. “Explaining the difference in color between Uranus and Neptune was an unexpected bonus!”

To create this model, Irwin’s team analyzed a set of observations of the planets encompassing ultraviolet, visible, and near-infrared wavelengths (from 0.3 to 2.5 micrometers) taken with the Near-Infrared Integral Field Spectrometer (NIFS) on the Gemini North telescope near the summit of Maunakea in Hawai’i — which is part of the international Gemini Observatory, a Program of NSF’s NOIRLab — as well as archival data from the NASA Infrared Telescope Facility, also located in Hawai’i, and the NASA/ESA Hubble Space Telescope.

The NIFS instrument on Gemini North was particularly important to this result as it is able to provide spectra — measurements of how bright an object is at different wavelengths — for every point in its field of view. This provided the team with detailed measurements of how reflective both planets’ atmospheres are across both the full disk of the planet and across a range of near-infrared wavelengths.

Two-way transmission from space, for a vertical path, to different levels in our standard Uranus and Neptune atmospheres for aerosol-free conditions.

“The Gemini observatories continue to deliver new insights into the nature of our planetary neighbors,” said Martin Still, Gemini Program Officer at the National Science Foundation. “In this experiment, Gemini North provided a component within a suite of ground- and space-based facilities critical to the detection and characterization of atmospheric hazes.”

The model also helps explain the dark spots that are occasionally visible on Neptune and less commonly detected on Uranus. While astronomers were already aware of the presence of dark spots in the atmospheres of both planets, they didn’t know which aerosol layer was causing these dark spots or why the aerosols at those layers were less reflective. The team’s research sheds light on these questions by showing that a darkening of the deepest layer of their model would produce dark spots similar to those seen on Neptune and perhaps Uranus.

Low gas-phase metallicities of ultraluminous infrared galaxies are a result of dust obscuration

by Nima Chartab, Asantha Cooray, Jingzhe Ma, Hooshang Nayyeri, Preston Zilliot, Jonathan Lopez, Dario Fadda, Rodrigo Herrera-Camus, Matthew Malkan, Dimitra Rigopoulou, Kartik Sheth, Julie Wardlow in Nature Astronomy

A thorough understanding of galaxy evolution depends in part on an accurate measurement of the abundance of metals in the intergalactic medium — the space between stars — but dust can impede observations in optical wavelengths. An international team of astronomers at the University of California, Irvine, Oxford University in England, and other institutions uncovered evidence of heavier elements in local galaxies — found to be deficient in earlier studies — by analyzing infrared data gathered during a multiyear campaign.

For a paper the researchers examined five galaxies that are dim in visible wavelengths but trillions of times more luminous than the sun in the infrared. Interactions between these galaxies and neighboring star systems cause gas to shift around and collapse, setting up conditions for prodigious star formation.

“Studying the gas content of these galaxies with optical instruments, astronomers were convinced that they were significantly metal-poor when compared with other galaxies of similar mass,” said lead author Nima Chartab, UCI postdoctoral scholar in physics & astronomy. “But when we observed emission lines of these dusty galaxies in infrared wavelengths, we were afforded a clear view of them and found no significant metal deficiency.”

Metal abundances of galaxies using the oxygen and nitrogen spectral emission lines.

To determine the abundance of gas-phase metals in the intergalactic medium, the astronomers sought to acquire data on the ratios of proxies, oxygen and nitrogen, because infrared emissions from these elements are less obscured by galactic dust.

“We are looking for evidence of baryon cycling in which stars process elements like hydrogen and helium to produce carbon, nitrogen and oxygen,” said co-author Asantha Cooray, UCI professor of physics & astronomy. “The stars eventually go supernovae and blow up and then all of that gas in the outskirts of the stars gets turned into clouds that get thrown around. The material in them is loose and diffuse but eventually through gravitational perturbations caused by other stars moving around, the gas will start to clump and collapse, leading to the formation of new stars.”

Observing this process in infrared wavelengths is a challenge for astronomers because water vapor in Earth’s atmosphere blocks radiation on this part of the electromagnetic spectrum, making measurements from even the highest-altitude ground telescopes — like those at the Keck Observatory in Hawaii — insufficient.

Part of the dataset used by the team came from the now-retired Herschel Space Telescope, but Herschel was not equipped with a spectrometer capable of reading a specific emission line that the UCI-led team needed for its study. The researchers’ solution was to take to the skies — reaching more than 45,000 feet above sea level — in the Stratospheric Observatory for Infrared Astronomy, NASA’s Boeing 747 equipped with a 2.5-meter telescope.

“It took us nearly three years to collect all the data in using NASA’s SOFIA observatory, because these flights don’t last all night; they’re more in the range of 45 minutes of observing time, so the study took a lot of flight planning and coordination,” said Cooray.

Line maps and spectra of our sample targeted by SOFIA/FIFI-LS.

By analyzing infrared emissions, the researchers were able to compare the metallicity of their target ultraluminous infrared galaxies with less dusty galaxies with similar mass and star formation rates. Chartab explained that these new data show that ultraluminous infrared galaxies are in line with the fundamental metallicity relation determined by stellar mass, metal abundance and star formation rate. The new data further show that the underabundance of metals derived from optical emission lines is likely due to “heavy dust obscuration associated with starburst,” according to the paper.

“This study is one example where it was critical for us to use this infrared wavelength to get a full understanding of what’s going on in some of these galaxies,” said Cooray. “When the optical observations initially came out suggesting that these galaxies had low metals, theorists went and wrote papers, there were a lot of simulations trying to explain what was going on. People thought, ‘Maybe they really are low-metal galaxies,’ but we found that not to be the case. Having a full view of the universe across the whole electromagnetic spectrum is really crucial, I think.”

Revealing the Mysteries of Venus: The DAVINCI Mission

by James B. Garvin, Stephanie A. Getty, Giada N. Arney, Natasha M. Johnson, et al in The Planetary Science Journal

In a recently published paper, NASA scientists and engineers give new details about the agency’s Deep Atmosphere Venus Investigation of Noble gases, Chemistry, and Imaging (DAVINCI) mission, which will descend through the layered Venus atmosphere to the surface of the planet in mid-2031. DAVINCI is the first mission to study Venus using both spacecraft flybys and a descent probe.

DAVINCI, a flying analytical chemistry laboratory, will measure critical aspects of Venus’ massive atmosphere-climate system for the first time, many of which have been measurement goals for Venus since the early 1980s. It will also provide the first descent imaging of the mountainous highlands of Venus while mapping their rock composition and surface relief at scales not possible from orbit. The mission supports measurements of undiscovered gases present in small amounts and the deepest atmosphere, including the key ratio of hydrogen isotopes — components of water that help reveal the history of water, either as liquid water oceans or steam within the early atmosphere.

Context for the DAVINCI mission within a framework of past orbital missions and with connections to future missions. Key science themes are highlighted with connections to questions left over from past missions, and with new connections to contemporary and future missions and the era of exoplanet science. DAVINCI is viewed as a gateway for Venus as a future astrobiology target in the context of how habitability is both established and lost in our solar system and beyond (e.g., Limaye et al. 2021).

The mission’s carrier, relay and imaging spacecraft (CRIS) has two onboard instruments that will study the planet’s clouds and map its highland areas during flybys of Venus and will also drop a small descent probe with five instruments that will provide a medley of new measurements at very high precision during its descent to the hellish Venus surface.

“This ensemble of chemistry, environmental, and descent imaging data will paint a picture of the layered Venus atmosphere and how it interacts with the surface in the mountains of Alpha Regio, which is twice the size of Texas,” said Jim Garvin, lead author of the paper and DAVINCI principal investigator from NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “These measurements will allow us to evaluate historical aspects of the atmosphere as well as detect special rock types at the surface such as granites while also looking for tell-tale landscape features that could tell us about erosion or other formational processes.”

DAVINCI will make use of three Venus gravity assists, which save fuel by using the planet’s gravity to change the speed and/or direction of the CRIS flight system. The first two gravity assists will set CRIS up for a Venus flyby to perform remote sensing in the ultraviolet and the near infrared light, acquiring over 60 gigabits of new data about the atmosphere and surface. The third Venus gravity assist will set up the spacecraft to release the probe for entry, descent, science, and touchdown, plus follow-on transmission to Earth.

A possible history of water on Venus (e.g., Way et al. 2016; Way & Del Genio 2020), compared to Earth’s history. Venus’s epoch of surface liquid water may have persisted for over 2 billion years, and DAVINCI measurements can help constrain this hypothesis. Evidence also suggests volcanic activity on Venus persists to this day (e.g., Smrekar et al. 2010), and DAVINCI noble gas measurements will help constrain the history of Venus’s volcanism.

The first flyby of Venus will be six and half months after launch and it will take two years to get the probe into position for entry into the atmosphere over Alpha Regio under ideal lighting at “high noon,” with the goal of measuring the landscapes of Venus at scales ranging from 328 feet (100 meters) down to finer than one meter. Such scales enable lander style geologic studies in the mountains of Venus without requiring landing.

Once the CRIS system is about two days away from Venus, the probe flight system will be released along with the titanium three foot (one meter) diameter probe safely encased inside. The probe will begin to interact with the Venus upper atmosphere at about 75 miles (120 kilometers) above the surface. The science probe will commence science observations after jettisoning its heat shield around 42 miles (67 kilometers) above the surface. With the heatshield jettisoned, the probe’s inlets will ingest atmospheric gas samples for detailed chemistry measurements of the sort that have been made on Mars with the Curiosity rover. During its hour-long descent to the surface, the probe will also acquire hundreds of images as soon as it emerges under the clouds at around 100,000 feet (30,500 meters) above the local surface.

DAVINCI’s descent sphere protects the instruments inside from the harsh Venusian ambient environment.

“The probe will touch-down in the Alpha Regio mountains but is not required to operate once it lands, as all of the required science data will be taken before reaching the surface.” said Stephanie Getty, deputy principal investigator from Goddard. “If we survive the touchdown at about 25 miles per hour (12 meters/second), we could have up to 17–18 minutes of operations on the surface under ideal conditions.”

DAVINCI is tentatively scheduled to launch June 2029 and enter the Venusian atmosphere in June 2031.

“No previous mission within the Venus atmosphere has measured the chemistry or environments at the detail that DAVINCI’s probe can do,” said Garvin. “Furthermore, no previous Venus mission has descended over the tesserae highlands of Venus, and none have conducted descent imaging of the Venus surface. DAVINCI will build on what Huygens probe did at Titan and improve on what previous in situ Venus missions have done, but with 21st century capabilities and sensors.”

Spaceflight Analogue Culture Enhances the Host-Pathogen Interaction Between Salmonella and a 3-D Biomimetic Intestinal Co-Culture Model

by Jennifer Barrila et al. in Frontiers in Cellular and Infection Microbiology

Infectious microbes have evolved sophisticated means to invade host cells, outwit the body’s defenses and cause disease. While researchers have tried to puzzle out the complicated interactions between microorganisms and the host cells they infect, one facet of the disease process has often been overlooked — the physical forces that impact host-pathogen interactions and disease outcomes.

In a new study, corresponding authors Cheryl Nickerson, Jennifer Barrila and their colleagues demonstrate that under low fluid shear force conditions that simulate those found in microgravity culture during spaceflight, the foodborne pathogen Salmonella infects 3-D models of human intestinal tissue at much higher levels, and induces unique alterations in gene expression.

This study advances previous work by the same team showing that physical forces of fluid shear acting on both the pathogen and host can transform the landscape of infection. Understanding this subtle interplay of host and pathogen during infection is critical to ensuring astronaut health, particularly on extended space missions. Such research also sheds new light on the still largely mysterious processes of infection on earth, as low fluid shear forces are also found in certain tissues in our bodies that pathogens infect, including the intestinal tract.

While the team has extensively characterized the interaction between conventionally grown shake flask cultures of Salmonella Typhimurium and 3-D intestinal models, this study marks the first time that S. Typhimurium has been grown under the low fluid shear conditions of simulated microgravity and then used to infect a 3-D model of human intestinal epithelium co-cultured with macrophage immune cells, key cell types targeted by Salmonella during infection. The 3-D co-culture intestinal model used in this study more faithfully replicates the structure and behavior of the same tissue within the human body and is more predictive of responses to infection, as compared with conventional laboratory cell cultures.

Results showed dramatic changes in gene expression of 3-D intestinal cells following infection with both wild-type and mutant S. Typhimurium strains grown under simulated microgravity conditions. Many of these changes occurred in genes known to be intimately involved with S. Typhimurium’s prodigious ability to invade and colonize host cells and escape surveillance and destruction by the host’s immune system. Nickerson, who co-directed the new study with Jennifer Barrila, is a researcher in the Biodesign Center for Fundamental and Applied Microbiomics and is also a professor with ASU’s School of Life Sciences.

“A major challenge limiting human exploration of space is the lack of a comprehensive understanding of the impact of space travel on crew health,” Nickerson says. “This challenge will negatively impact both deep space exploration by professional astronauts, as well as civilians participating in the rapidly expanding commercial space market in low Earth orbit. Since microbes accompany humans wherever they travel and are essential for controlling the balance between health and disease, understanding the relationship between spaceflight, immune cell function, and microorganisms will be essential to understand infectious disease risk for humans.”

Life on earth has diversified into an almost incomprehensibly vast array of forms, evolving under wildly dissimilar environmental conditions. Yet one parameter has remained constant. Throughout the 3.7-billion-year history of life on earth, all living organisms evolved under, and respond to, the pull of Earth’s gravity.

For more than 20 years, Nickerson has been a pioneer in exploring the effects of the reduced microgravity environment of spaceflight on a range of pathogenic microbes and the impact on interactions with human cells and animals they infect. She and her colleagues have doggedly pursued this research in both land-based and spaceflight settings, the results of which helped lay the foundation for the rapidly growing research field, mechanobiology of infectious disease, the study of how physical forces impact infection and disease outcomes.

Among their important findings is that the low fluid shear conditions associated with the reduced gravity environment of spaceflight and spaceflight analog culture are similar to those encountered by pathogens inside the infected host, and that these conditions can induce unique changes in the ability of pathogenic microbes like Salmonella to aggressively infect host cells and exacerbate disease, a property known as virulence.

The infectious agent explored in the new study, Salmonella Typhimurium, is a bacterial pathogen responsible for gastrointestinal disease in humans and animals. Salmonella is the leading cause of death from food-borne illness in the United States. According to the CDC, Salmonella bacteria cause about 1.35 million infections, 26,500 hospitalizations, and 420 deaths in the United States each year. Foods contaminated by the bacteria are the primary source for most of these illnesses.

Salmonella infection typically causes diarrhea, fever, and stomach cramps, beginning 6 hours to 6 days after infection. Illness from the disease usually lasts 4 to 7 days. In severe cases, hospitalization may be required.

Δhfq mutant construction and bacterial growth curves.

Cells in mammalian organisms, including humans, as well as the bacterial cells that infect them, are exposed to extracellular fluid flowing over their outer surfaces. Just as a gentle downstream current will affect the pebbles in the underlying streambed differently than a raging torrent, so the force of fluid gliding over cell surfaces can cause changes to affected cells. This liquid abrasion of cell surfaces is known as fluid shear.

Since spaceflight experiments are rare and access to the space research platform is currently limited, researchers often simulate the low fluid shear conditions that microbes encounter during culture in spaceflight by growing cells in liquid growth media within a device known as a rotating wall vessel bioreactor or RWV. As the cylindrical reactor rotates, cells are maintained in suspension, gently and continuously tumbling in their surrounding culture medium. This process mimics the low fluid shear conditions of microgravity that cells experience during culture in spaceflight.

The team has also shown that this fluid shear level is relevant to conditions that microbial cells encounter in the human intestine and other tissues during infection, triggering changes in gene expression that can help some pathogens better colonize host cells and evade the immune system’s efforts to destroy them.

Response of 3-D co-culture model to infection with LSMMG- or control-cultured S. Typhimurium.

The study found significant changes in both gene expression and ability to infect 3-D intestinal models by Salmonella bacteria cultured in the RWV bioreactor. These experiments involved two S. Typhimurium strains, one unaltered or wild type strain and one mutant strain.

The mutant strain was otherwise identical to the wild type but lacked an important protein known as Hfq, a major stress response regulator in Salmonella. In earlier research, Nickerson and her team discovered that Hfq acts as a master regulator of Salmonella’s infection process in both spaceflight and spaceflight analog culture. They later discovered additional pathogens that also use Hfq to regulate their responses to these same conditions.

Unexpectedly, in the current study, the hfq mutant strain was still able to attach, invade into, and survive within 3-D tissue models at levels comparable to the wild type strain. In agreement with this finding, many genes responsible for Salmonella’s ability to colonize human cells, including those associated with cell adherence, motility, and invasion were still activated in the mutant strain under simulated microgravity conditions, despite the removal of Hfq.

From the host perspective, the 3-D intestinal co-culture model responded to Salmonella infection by upregulating genes involved in inflammation, tissue remodeling, and wound healing at higher levels when the bacteria were grown under simulated microgravity conditions prior to use in infection studies. This was observed for both wild type and hfq mutant strains of the pathogen.

Data from this new spaceflight analog study reinforces previous findings from the team’s 2006, 2008 and 2010 Space Shuttle experiments. In particular, the 2010 flight experiment conducted aboard Space Shuttle Discovery, called STL-IMMUNE, used the same wild type strain of S. Typhimurium to infect a 3-D model of human intestinal tissue made from the same epithelial cells used in the new study.

Several commonalities were observed between host cell responses to infection in the new spaceflight analog study and those previously reported when infections took place in true spaceflight during the STL-IMMUNE experiment. These results further reinforce the RWV as a predictive ground-based spaceflight analogue culture system that mimics key aspects of microbial responses to true spaceflight culture.

“During STL-IMMUNE, we discovered that infection of a human 3-D intestinal epithelial model by Salmonella during spaceflight induced key transcriptional and proteomic biosignatures that were consistent with enhanced infection by the pathogen,” Barrila says. “However, due to the technical challenges of performing in-flight infections, we could not quantify whether the bacteria were actually attaching and invading into the tissue at higher levels. The use of the RWV bioreactor as a spaceflight analog culture system in our current study has been a powerful tool which allowed us to explore this experimental question at a deeper level.”

Astronauts face a double risk from infectious disease during their missions far from earth. The combined rigors of spaceflight act to weaken their immune systems. At the same time, some pathogens like Salmonella may be triggered by low fluid shear conditions induced by microgravity to become more effective infectious agents.

With longer spaceflight missions in the advanced planning stages and the advent of civilian space travel rapidly emerging, safeguarding space travelers from infectious disease is vital. Studies like the current one are also helping to pull back the curtain on the infection process, revealing foundational details with broad relevance for the battle against diseases, on Earth and beyond.

A Comprehensive Measurement of the Local Value of the Hubble Constant with 1 km/s/Mpc Uncertainty from the Hubble Space Telescope and the SH0ES Team

by Adam G. Riess, Wenlong Yuan, Lucas M. Macri, Dan Scolnic, et al in Astrophysical Journal

Completing a nearly 30-year marathon, NASA’s Hubble Space Telescope has calibrated more than 40 “milepost markers” of space and time to help scientists precisely measure the expansion rate of the universe — a quest with a plot twist.

Pursuit of the universe’s expansion rate began in the 1920s with measurements by astronomers Edwin P. Hubble and Georges Lemaître. In 1998, this led to the discovery of “dark energy,” a mysterious repulsive force accelerating the universe’s expansion. In recent years, thanks to data from Hubble and other telescopes, astronomers found another twist: a discrepancy between the expansion rate as measured in the local universe compared to independent observations from right after the big bang, which predict a different expansion value. The cause of this discrepancy remains a mystery. But Hubble data, encompassing a variety of cosmic objects that serve as distance markers, support the idea that something weird is going on, possibly involving brand new physics.

“You are getting the most precise measure of the expansion rate for the universe from the gold standard of telescopes and cosmic mile markers,” said Nobel Laureate Adam Riess of the Space Telescope Science Institute (STScI) and the Johns Hopkins University in Baltimore, Maryland.

Riess leads a scientific collaboration investigating the universe’s expansion rate called SH0ES, which stands for Supernova, H0, for the Equation of State of Dark Energy. “This is what the Hubble Space Telescope was built to do, using the best techniques we know to do it. This is likely Hubble’s magnum opus, because it would take another 30 years of Hubble’s life to even double this sample size,” Riess said.

Riess’s team’s paper, reports on completing the biggest and likely last major update on the Hubble constant. The new results more than double the prior sample of cosmic distance markers. His team also reanalyzed all of the prior data, with the whole dataset now including over 1,000 Hubble orbits.

Pseudo-color images of all Cepheid-bearing galaxies analyzed in this work.

When NASA conceived of a large space telescope in the 1970s, one of the primary justifications for the expense and extraordinary technical effort was to be able to resolve Cepheids, stars that brighten and dim periodically, seen inside our Milky Way and external galaxies. Cepheids have long been the gold standard of cosmic mile markers since their utility was discovered by astronomer Henrietta Swan Leavitt in 1912. To calculate much greater distances, astronomers use exploding stars called Type Ia supernovae.

Combined, these objects built a “cosmic distance ladder” across the universe and are essential to measuring the expansion rate of the universe, called the Hubble constant after Edwin Hubble. That value is critical to estimating the age of the universe and provides a basic test of our understanding of the universe.

Starting right after Hubble’s launch in 1990, the first set of observations of Cepheid stars to refine the Hubble constant was undertaken by two teams: the HST Key Project led by Wendy Freedman, Robert Kennicutt, Jeremy Mould, and Marc Aaronson, and another by Allan Sandage and collaborators, that used Cepheids as milepost markers to refine the distance measurement to nearby galaxies. By the early 2000s the teams declared “mission accomplished” by reaching an accuracy of 10 percent for the Hubble constant, 72 plus or minus 8 kilometers per second per megaparsec.

In 2005 and again in 2009, the addition of powerful new cameras onboard the Hubble telescope launched “Generation 2” of the Hubble constant research as teams set out to refine the value to an accuracy of just one percent. This was inaugurated by the SH0ES program. Several teams of astronomers using Hubble, including SH0ES, have converged on a Hubble constant value of 73 plus or minus 1 kilometer per second per megaparsec. While other approaches have been used to investigate the Hubble constant question, different teams have come up with values close to the same number.

The SH0ES team includes long-time leaders Dr. Wenlong Yuan of Johns Hopkins University, Dr. Lucas Macri of Texas A&M University, Dr. Stefano Casertano of STScI, and Dr. Dan Scolnic of Duke University. The project was designed to bracket the universe by matching the precision of the Hubble constant inferred from studying the cosmic microwave background radiation leftover from the dawn of the universe.

“The Hubble constant is a very special number. It can be used to thread a needle from the past to the present for an end-to-end test of our understanding of the universe. This took a phenomenal amount of detailed work,” said Dr. Licia Verde, a cosmologist at ICREA and the ICC-University of Barcelona, speaking about the SH0ES team’s work.

The team measured 42 of the supernova milepost markers with Hubble. Because they are seen exploding at a rate of about one per year, Hubble has, for all practical purposes, logged as many supernovae as possible for measuring the universe’s expansion. Riess said, “We have a complete sample of all the supernovae accessible to the Hubble telescope seen in the last 40 years.” Like the lyrics from the song “Kansas City,” from the Broadway musical Oklahoma, Hubble has “gone about as fur as it c’n go!”

Comparison of Cepheids measured in a dense (inner) eld (in red) and sparse (outer) eld (in blue) of NGC4258.

The expansion rate of the universe was predicted to be slower than what Hubble actually sees. By combining the Standard Cosmological Model of the Universe and measurements by the European Space Agency’s Planck mission (which observed the relic cosmic microwave background from 13.8 billion years ago), astronomers predict a lower value for the Hubble constant: 67.5 plus or minus 0.5 kilometers per second per megaparsec, compared to the SH0ES team’s estimate of 73.

Given the large Hubble sample size, there is only a one-in-a-million chance astronomers are wrong due to an unlucky draw, said Riess, a common threshold for taking a problem seriously in physics. This finding is untangling what was becoming a nice and tidy picture of the universe’s dynamical evolution. Astronomers are at a loss for an explanation of the disconnect between the expansion rate of the local universe versus the primeval universe, but the answer might involve additional physics of the universe.

Such confounding findings have made life more exciting for cosmologists like Riess. Thirty years ago they started out to measure the Hubble constant to benchmark the universe, but now it has become something even more interesting. “Actually, I don’t care what the expansion value is specifically, but I like to use it to learn about the universe,” Riess added.

The Great Dimming of Betelgeuse seen by the Himawari-8 meteorological satellite

by Daisuke Taniguchi, Kazuya Yamazaki & in Shinsuke Uno in Nature Astronomy

A weather satellite accidentally caught Betelgeuse dimming . The weather imagery is surprisingly useful as astronomical data.

When Betelgeuse mysteriously dimmed in late 2019 by over a magnitude, astronomers around the world rushed to turn their telescopes to the red giant star on Orion’s shoulder. Little did they know that they were being joined by a Japanese weather satellite named Himawari-8.

From its geostationary orbit, Himawari-8 takes high-resolution images of Earth, capturing clouds, changes in vegetation — and occasionally, photobombing astronomical objects, appearing just off Earth’s limb. Inspired by a tweet of a Himawari-8 image that had serendipitously captured the Moon, two graduate students at the University of Tokyo, Daisuke Taniguchi and Shinsuke Uno, wondered if they could harness the weather satellite’s archives for astronomical research.

About every couple of days, Betelgeuse appears in images captured by the Japanese weather satellite Himawari-8.T

They found that Betelgeuse appeared in Himawari-8’s images roughly once every 1.72 days — including before, during, and after its enigmatic period of dimming from 2019 to 2020. What’s more, Himawari-8’s camera operates at mid-infrared wavelengths, where it can see temperature differences between clouds and the ground. That wavelength range is also good for observing astronomical dust, like the dust that shrouds young stars — or the dust that some astronomers think temporarily blocked Betelgeuse’s light and caused it to dim.

To help analyze Himawari-8’s data, Taniguchi and Uno recruited Kazuya Yamazaki, a graduate student in meteorology. From the satellite’s data, they were able to estimate the amount of dust around Betelgeuse. They found that the dimming is most likely due to a combination of dust and also a cooling of the star’s temperature by about 250 degrees Fahrenheit (140 degrees Celsius). This is consistent with one of the most popular theories — that Betelgeuse expelled a hot clump of gas that condensed into dust when exposed to a cool region on the star’s surface.

The team think weather satellites have a lot of unrealized potential as astronomical telescopes. Mid-infrared radiation is blocked by Earth’s atmosphere, rendering it invisible to ground-based telescopes. There’s also a dearth of space-based observatories currently operating in the mid-IR. The Spitzer Space Telescope has been decommissioned since January 2020, and the SOFIA airborne observatory will also be shut down later this year. While the upcoming James Webb Space Telescope will be able to observe in the mid-IR, its time is so precious it won’t be able to look at one object repeatedly for years on end as weather satellites can.

Taniguchi says he and his colleagues have already begun other projects with Himawari-8 data, including making a catalog of tens of other giant stars and searching for transient infrared objects. “I hope some other astronomers in the world will start their own projects using Himawari-8 or other meteorological satellites,” he told.

The Universe is Brighter in the Direction of Our Motion: Galaxy Counts and Fluxes are Consistent with the CMB Dipole

by Jeremy Darling in The Astrophysical Journal Letters

Scientists who study the cosmos have a favorite philosophy known as the “mediocrity principle,” which, in essence, suggests that there’s really nothing special about Earth, the sun or the Milky Way galaxy compared to the rest of the universe. Now, new research from CU Boulder adds yet another piece of evidence to the case for mediocrity: Galaxies are, on average, at rest with respect to the early universe. Jeremy Darling, a CU Boulder astrophysics professor, recently published this new cosmological finding.

“What this research is telling us is that we have a funny motion, but that funny motion is consistent with everything we know about the universe — there’s nothing special going on here,” said Darling. “We’re not special as a galaxy or as observers.”

Roughly 35 years ago, researchers discovered the cosmic microwave background, which is electromagnetic radiation left over from the universe’s formation during the Big Bang. The cosmic microwave background appears warmer in the direction of our motion and cooler away from the direction of our motion.

From this glow of the early universe, scientists can infer that the sun — and the Earth orbiting around it — is moving in a certain direction, at a certain speed. Researchers find that our inferred velocity is a fraction of a percent of the speed of light — small, but not zero.

VLASS (left) and RACS (right) catalog sources in galactic coordinates. Darker color indicates fewer sources. ∣b∣ < 5° was excluded from the VLASS.

Scientists can independently test this inference by counting the galaxies that are visible from Earth or adding up their brightness. They can do this thanks largely to Albert Einstein’s 1905 theory of special relativity, which explains how speed affects time and space. In this application, a person on Earth looking out into the universe in one direction — the same direction that the sun and the Earth are moving — should see galaxies that are brighter, bluer and more concentrated. Similarly, by looking the other direction, the person should see galaxies that are darker, redder and spaced farther apart.

But when investigators have tried to count galaxies in recent years — a process that’s difficult to do accurately — they’ve come up with numbers that suggest the sun is moving much faster than previously thought, which is at odds with standard cosmology.

“It’s hard to count galaxies over the whole sky — you’re usually stuck with a hemisphere or less,” said Darling. “And, on top of that, our own galaxy gets in the way. It has dust that will cause you to find fewer galaxies and will make them look dimmer as you get closer to our galaxy.”

Darling was intrigued and perplexed by this cosmological puzzle, so he decided to investigate for himself. He also knew there were two recently released surveys that could help improve the accuracy of a galaxy count — and shed light on the velocity mystery: one called the Very Large Array Sky Survey (VLASS) in New Mexico, and the other called the Rapid Australian Square Kilometer Array Pathfinder Continuum Survey (RACS) in Australia.

Together, these surveys allowed Darling to study the entire sky by patching together views from the northern and southern hemispheres. Importantly, the new surveys also used radio waves, which made it easier to “see” through the dust of the Milky Way, thus improving the view of the universe.

Left: dipole apex measured from radio source number counts (top) and fluxes (bottom) using the methods listed in Table 1. The star indicates the CMB dipole apex, and error bars are 1σ uncertainties. Right: dipole velocity measured from radio source number counts (top) and fluxes (bottom). The vertical line indicates the observed CMB dipole. The envelopes show the error distributions in dipole amplitudes, and the vertical tick marks show 1σ and 2σ uncertainties.

When Darling analyzed the surveys, he found that the number of galaxies and their brightness was in perfect agreement with the velocity researchers had previously inferred from the cosmic microwave background.

“We find a bright direction and a dim direction — we find a direction where there are more galaxies and a direction where there are fewer galaxies,” he said. “The big difference is that it lines up with the early universe from the cosmic microwave background and it has the right speed. Our cosmology is just fine.”

Because Darling’s findings differ from past results, his paper will likely prompt various follow-up studies to confirm or dispute his results. But in addition to pushing the field of cosmology forward, the findings are a good real-world example of Einstein’s special relativity theory — and they demonstrate how researchers are still putting the theory into practice, more than 100 years after the famed physicist first proposed it.

“I love the idea that this basic principle that Einstein told us about a long time ago is something you can see,” Darling said. “It’s a really esoteric thing that seems super weird, but if you go out and count galaxies, you could see this neat effect. It’s not quite as esoteric or weird as you might think.”

Detection of Extended Millimeter Emission in the Host Galaxy of 3C 273 and Its Implications for QSO Feedback via High Dynamic Range ALMA Imaging

by Shinya Komugi et al in The Astrophysical Journal

As a result of achieving high imaging dynamic range, a team of astronomers in Japan has discovered for the first time a faint radio emission covering a giant galaxy with an energetic black hole at its center. The radio emission is released from gas created directly by the central black hole. The team expects to understand how a black hole interacts with its host galaxy by applying the same technique to other quasars.

3C273, which lies at a distance of 2.4 billion light-years from Earth, is a quasar. A quasar is the nucleus of a galaxy believed to house a massive black hole at its center, which swallows its surrounding material, giving off enormous radiation. Contrary to its bland name, 3C273 is the first quasar ever discovered, the brightest, and the best studied. It is one of the most frequently observed sources with telescopes because it can be used as a standard of position in the sky: in other words, 3C273 is a radio lighthouse.

When you see a car’s headlight, the dazzling brightness makes it challenging to see the darker surroundings. The same thing happens to telescopes when you observe bright objects. Dynamic range is the contrast between the most brilliant and darkest tones in an image. You need a high dynamic range to reveal both the bright and dark parts in a telescope’s single shot. ALMA can regularly attain imaging dynamic ranges up to around 100, but commercially available digital cameras would typically have a dynamic range of several thousands. Radio telescopes aren’t very good at seeing objects with significant contrast.

Quasar 3C273 observed by the Hubble Space Telescope (HST) (left). The exceeding brightness results in radial leaks of light created by light scattered by the telescope. At the lower right is a high-energy jet released by the gas around the central black hole. Radio image of 3C273 observed by ALMA, showing the faint and extended radio emission (in blue-white color) around the nucleus (right). The bright central source has been subtracted from the image. The same jet as the image on the left can be seen in orange. Credit: Komugi et al., NASA/ESA Hubble Space Telescope

3C273 has been known for decades as the most famous quasar, but knowledge has been concentrated on its bright central nuclei, where most radio waves come from. However, much less has been known about its host galaxy itself because the combination of the faint and diffuse galaxy with the 3C273 nucleus required such high dynamic ranges to detect. The research team used a technique called self-calibration to reduce the leakage of radio waves from 3C273 to the galaxy, which used 3C273 itself to correct for the effects of Earth’s atmospheric fluctuations on the telescope system. They reached an imaging dynamic range of 85000, an ALMA record for extragalactic objects.

HST/WFPC2 F606W image1 of 3C 273. The host elliptical is seen as a diffuse structure lopsided to the northeast. The jet structure extends to the southwest.

As a result of achieving high imaging dynamic range, the team discovered the faint radio emission extending for tens of thousands of light-years over the host galaxy of 3C273. Radio emission around quasars typically suggests synchrotron emission, which comes from highly energetic events like bursts of star formation or ultra-fast jets emanating from the central nucleus. A synchrotron jet exists in 3C273 as well, seen in the lower right of the images. An essential characteristic of synchrotron emission is its brightness changes with frequency, but the faint radio emission discovered by the team had constant brightness irrespective of the radio frequency. After considering alternative mechanisms, the team found that this faint and extended radio emission came from hydrogen gas in the galaxy energized directly by the 3C273 nucleus. This is the first time that radio waves from such a mechanism are found to extend for tens of thousands of light-years in the host galaxy of a quasar. Astronomers had overlooked this phenomenon for decades in this iconic cosmic lighthouse.

So why is this discovery so important? It has been a big mystery in galactic astronomy whether the energy from a quasar nucleus can be strong enough to deprive the galaxy’s ability to form stars. The faint radio emission may help to solve it. Hydrogen gas is an essential ingredient in creating stars, but if such an intense light shines on it that the gas is disassembled (ionized), no stars can be born. To study whether this process is happening around quasars, astronomers have used optical light emitted by ionized gas. The problem working with optical light is that cosmic dust absorbs the light along the way to the telescope, so it is difficult to know how much light the gas gives off.

Moreover, the mechanism responsible for giving off optical light is complex, forcing astronomers to make a lot of assumptions. The radio waves discovered in this study come from the same gas due to simple processes and are not absorbed by dust. Using radio waves makes measuring ionized gas created by 3C273’s nucleus much easier. In this study, the astronomers found that at least 7% of the light from 3C273 was absorbed by gas in the host galaxy, creating ionized gas amounting to 10–100 billion times the sun’s mass. However, 3C273 had a lot of gas just before the formation of stars, so as a whole, it didn’t look like star formation was strongly suppressed by the nucleus.

“This discovery provides a new avenue to studying problems previously tackled using observations by optical light,” says Shinya Komugi, an associate professor at Kogakuin University and lead author of the study. “By applying the same technique to other quasars, we expect to understand how a galaxy evolves through its interaction with the central nucleus.”

A Search for Annihilating Dark Matter in 47 Tucanae and Omega Centauri

by Lister Staveley-Smith, Emma Bond, Kenji Bekki, Tobias Westmeier in arXiv

A team of astronomers studied two nearby globular clusters, 47 Tucanae and Omega Centauri, searching for signals produced by annihilating dark matter. Though the searches turned up empty, they weren’t a failure. The lack of a detection placed strict upper limits on the mass of the hypothetical dark matter particle.

Dark matter makes up around 80% of all the mass in the universe, although it’s completely invisible. It simply doesn’t interact with the electromagnetic force, and so it doesn’t glow or reflect or absorb or anything. So far, the only evidence we have for its existence is through its gravitational effects on the rest of the universe. Because of this, astronomers aren’t exactly sure about what dark matter is, although many physicists believe that it’s some new kind of particle, previously unknown to the standard model of particle physics.

One possibility is that dark matter is made of some ultra-light particle, like an axion. And while these particles wouldn’t interact with normal matter, they might very rarely interact with themselves, colliding together and annihilating. If the energy of the collision is high enough, it can result in the production of a gamma ray, which then splits off to become an electron and positron. Those electrons and positrons can glue together to form bound states, called positronium. However, the positronium atoms aren’t stable, and they eventually decay, leaving behind a flash of radio emission. So even though dark matter doesn’t interact with electromagnetism directly, there’s still the possibility of us seeing the radio emission from the collision and decay of dark matter particles.

The globular cluster Messier 54. Credit: NASA

To make this work you need a lot of dark matter. If the dark matter particles collided easily enough, we would’ve seen it already. So the collisions must be rare. The density of dark matter in our galactic neighborhood is far too low to make detectable emission, but the dense cores of galaxies may offer better access.

The natural place to look is our galactic core, but that place is swamped with all kinds of radio emission, so it’s difficult to tell if a particular signal is coming from annihilating dark matter or something more mundane. So that’s why a team of astronomers looked to two nearby globular clusters, as reported in a paper.

The two clusters, 47 Tucanae and Omega Centauri, are only a few thousand light-years away, making them relatively easy to observe. And astronomers believe that they are the remnants of dwarf galaxies, the bulk of their stars stripped away from them through interactions with the Milky Way.

This makes the clusters ideal laboratories, because they are essentially balls of dense dark matter with very little contamination. The team of astronomers went looking for the unique radio signal of decaying positronium using the Parkes observatory in Australia.

They didn’t find anything, which isn’t necessarily a bad thing. Based on their observations, they were able to place the best upper limits yet on the mass and cross-section (a measure of how frequently the particles interact) of these light dark matter models. Sure, it would have been awesome to see a confirmed signal and finally put this dark matter mystery to rest, but new knowledge in any direction is always welcome and always helpful.