ST/ Dark matter in dwarf galaxy tracked using stellar motions
Space biweekly vol.100, 19th July — 2nd August
TL;DR
- The qualities and behavior of dark matter, the invisible ‘glue’ of the universe, continue to be shrouded in mystery. Though galaxies are mostly made of dark matter, understanding how it is distributed within a galaxy offers clues to what this substance is, and how it’s relevant to a galaxy’s evolution.
- Up to 60% of near-Earth objects could be dark comets, mysterious asteroids that orbit the sun in our solar system that likely contain or previously contained ice and could have been one route for delivering water to Earth, according to a new study.
- Most known black holes are either extremely massive, like the supermassive black holes that lie at the cores of large galaxies, or relatively lightweight, with a mass of under 100 times that of the Sun. Intermediate-mass black holes (IMBHs) are scarce, however, and are considered rare ‘missing links’ in black hole evolution.
- An exoplanet infamous for its deadly weather has been hiding another bizarre feature — it reeks of rotten eggs, according to a new study of data from the James Webb Space Telescope.
- Astronomers have discovered the first millisecond pulsar in the stellar cluster Glimpse-CO1.
- And more!
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Latest research
HSTPROMO Internal Proper-motion Kinematics of Dwarf Spheroidal Galaxies. I. Velocity Anisotropy and Dark Matter Cusp Slope of Draco
by Eduardo Vitral, Roeland P. van der Marel, Sangmo Tony Sohn, Mattia Libralato, Andrés del Pino, Laura L. Watkins, Andrea Bellini, Matthew G. Walker, Gurtina Besla, Marcel S. Pawlowski, Gary A. Mamon in The Astrophysical Journal
The qualities and behavior of dark matter, the invisible “glue” of the universe, continue to be shrouded in mystery. Though galaxies are mostly made of dark matter, understanding how it is distributed within a galaxy offers clues to what this substance is, and how it’s relevant to a galaxy’s evolution.
While computer simulations suggest dark matter should pile up in a galaxy’s center, called a density cusp, many previous telescopic observations have indicated that it is instead more evenly dispersed throughout a galaxy. The reason for this tension between model and observation continues to puzzle astronomers, reinforcing the mystery of dark matter.
A team of astronomers has turned toward NASA’s Hubble Space Telescope to try and clarify this debate by measuring the dynamic motions of stars within the Draco dwarf galaxy, a system located roughly 250,000 light-years from Earth. Using observations that spanned 18 years, they succeeded in building the most accurate three-dimensional understanding of stars’ movements within the diminutive galaxy. This required scouring nearly two decades of Hubble archival observations of the Draco galaxy.
“Our models tend to agree more with a cusp-like structure, which aligns with cosmological models,” said Eduardo Vitral of the Space Telescope Science Institute (STScI) in Baltimore and lead author of the study. “While we cannot definitively say all galaxies contain a cusp-like dark matter distribution, it’s exciting to have such well measured data that surpasses anything we’ve had before.”
To learn about dark matter within a galaxy, scientists can look to its stars and their movements that are dominated by the pull of dark matter. A common approach to measure the speed of objects moving in space is by the Doppler Effect — an observed change of the wavelength of light if a star is approaching or receding from Earth. Although this line-of-sight velocity can provide valuable insight, only so much can be gleaned from this one-dimensional source of information.
Besides moving closer or further away from us, stars also move across the sky, measured as their proper motion. By combining line-of-sight velocity with proper motions, the team created an unprecedented analysis of the stars’ 3D movements.
“Improvements in data and improvements in modeling usually go hand in hand,” explained Roeland van der Marel of STScI, a co-author of the paper who initiated the study more than 10 years ago. “If you don’t have very sophisticated data or only one-dimensional data, then relatively straightforward models can often fit. The more dimensions and complexity of data you gather, the more complex your models need to be to truly capture all the subtleties of the data.”
Since dwarf galaxies are known to have a higher proportion of dark matter content than other types of galaxies, the team honed in on the Draco dwarf galaxy, which is a relatively small and spheroidal nearby satellite of the Milky Way galaxy.
“When measuring proper motions, you note the position of a star at one epoch and then many years later measure the position of that same star. You measure the displacement to determine how much it moved,” explained Sangmo Tony Sohn of STScI, another co-author of the paper and the principal investigator of the latest observational program. “For this kind of observation, the longer you wait, the better you can measure the stars shifting.”
The team analyzed a series of epochs spanning from 2004 to 2022, an extensive baseline that only Hubble could offer, due to the combination of its sharp stable vision and record time in operation. The telescope’s rich data archive helped decrease the level of uncertainty in the measurement of the stars’ proper motions. The precision is equivalent to measuring an annual shift a little less than the width of a golf ball as seen on the Moon from Earth.
With three dimensions of data, the team reduced the amount of assumptions applied in previous studies and considered characteristics specific to the galaxy — such as its rotation, and distribution of its stars and dark matter — in their own modeling efforts.
The methodologies and models developed for the Draco dwarf galaxy can be applied to other galaxies in the future. The team is already analyzing Hubble observations of the Sculptor dwarf galaxy and the Ursa Minor dwarf galaxy. Studying dark matter requires observing different galactic environments, and also entails collaboration across different space telescope missions. For example, NASA’s upcoming Nancy Grace Roman Space Telescope will help reveal new details of dark matter’s properties among different galaxies thanks to its ability to survey large swaths of the sky.
“This kind of study is a long-term investment and requires a lot of patience,” reflected Vitral. “We’re able to do this science because of all the planning that was done throughout the years to actually gather these data. The insights we’ve collected are the result of a larger group of researchers that has been working on these things for many years.”
The dynamical origins of the dark comets and a proposed evolutionary track
by Aster G. Taylor, Jordan K. Steckloff, Darryl Z. Seligman, Davide Farnocchia, Luke Dones, David Vokrouhlický, David Nesvorný, Marco Micheli in Icarus
Up to 60% of near-Earth objects could be dark comets, mysterious asteroids that orbit the sun in our solar system that likely contain or previously contained ice and could have been one route for delivering water to Earth, according to a University of Michigan study.
The findings suggest that asteroids in the asteroid belt, a region of the solar system roughly between Jupiter and Mars that contains much of the system’s rocky asteroids, have subsurface ice, something that has been suspected since the 1980s, according to Aster Taylor, a U-M graduate student in astronomy and lead author of the study. The study also shows a potential pathway for delivering ice into the near-Earth solar system, according to Taylor. How Earth got its water is a longstanding question.
“We don’t know if these dark comets delivered water to Earth. We can’t say that. But we can say that there is still debate over how exactly the Earth’s water got here,” Taylor said. “The work we’ve done has shown that this is another pathway to get ice from somewhere in the rest of the solar system to the Earth’s environment.”
The research further suggests that one large object may come from the Jupiter-family comets, comets whose orbits take them close to the planet Jupiter. The team’s results are published in the journal Icarus. Dark comets are a bit of a mystery because they combine characteristics of both asteroids and comets. Asteroids are rocky bodies with no ice that orbit closer to the sun, typically within what’s called the ice line. This means they are close enough to the sun for any ice the asteroid may have been carrying to sublimate, or change from solid ice directly into gas.
Comets are icy bodies that show a fuzzy coma, a cloud that often surrounds a comet. Sublimating ice carries dust along with it, creating the cloud. Additionally, comets typically have slight accelerations propelled not by gravity, but by the sublimation of ice, called nongravitational accelerations.
The study examined seven dark comets and estimates that between 0.5 and 60% of all near-Earth objects could be dark comets, which do not have comae but do have nongravitational accelerations. The researchers also suggest that these dark comets likely come from the asteroid belt, and because these dark comets have nongravitational accelerations, the study findings suggest asteroids in the asteroid belt contain ice.
“We think these objects came from the inner and/or outer main asteroid belt, and the implication of that is that this is another mechanism for getting some ice into the inner solar system,” Taylor said. “There may be more ice in the inner main belt than we thought. There may be more objects like this out there. This could be a significant fraction of the nearest population. We don’t really know, but we have many more questions because of these findings.”
In previous work, a team of researchers including Taylor identified nongravitational accelerations on a set of near-Earth objects, naming them “dark comets.” They determined that the dark comets’ nongravitational accelerations are likely the result of small amounts of sublimating ice. In the current work, Taylor and their colleagues wanted to discover where the dark comets came from.
“Near-Earth objects don’t stay on their current orbits very long because the near-Earth environment is messy,” they said. “They only stay in the near-Earth environment for around 10 million years. Because the solar system is much older than that, that means near-Earth objects are coming from somewhere — that we’re constantly being fed near-Earth objects from another, much larger source.”
To determine the origin of this dark comet population, Taylor and their coauthors created dynamical models that assigned nongravitational accelerations to objects from different populations. Then, they modeled a path these objects would follow given the assigned nongravitational accelerations over a period of 100,000 years. The researchers observed that many of these objects ended up where dark comets are today, and found that out of all potential sources, the main asteroid belt is the most likely place of origin.
One of the dark comets called 2003 RM, which passes in an elliptical orbit close to Earth, then out to Jupiter and back past Earth, follows the same path that would be expected from a Jupiter family comet, Taylor says — that is, its position is consistent with a comet that was knocked inward from its orbit.
Meanwhile, the study finds that the rest of the dark comets likely came from the inner band of the asteroid belt. Since the dark comets likely have ice, this shows that ice are present in the inner main belt. Then, the researchers applied a previously suggested theory to their population of dark comets to determine why the objects are so small and quickly rotating. Comets are rocky structures bound together by ice — picture a dirty ice cube, Taylor says. Once they get bumped within the solar system’s ice line, that ice starts to off gas. This causes the object’s acceleration, but it can also cause the object to spin quite fast — fast enough for the object to break apart.
“These pieces will also have ice on them, so they will also spin out faster and faster until they break into more pieces,” Taylor said. “You can just keep doing this as you get smaller and smaller and smaller. What we suggest is that the way you get these small, fast rotating objects is you take a few bigger objects and break them into pieces.”
As this happens, the objects continue to lose their ice, get even smaller, and rotate even more rapidly.
The researchers believe that while the larger dark comet, 2003 RM, was likely a larger object that got kicked out of the outer main belt of the asteroid belt, the six other objects they were examining likely came from the inner main belt and were made by an object that had gotten knocked inward and then broke apart.
Fast-moving stars around an intermediate-mass black hole in ω Centauri
by Maximilian Häberle, Nadine Neumayer, Anil Seth, Andrea Bellini, Mattia Libralato, Holger Baumgardt, Matthew Whitaker, Antoine Dumont, Mayte Alfaro-Cuello, Jay Anderson, Callie Clontz, Nikolay Kacharov, Sebastian Kamann, Anja Feldmeier-Krause, Antonino Milone, Maria Selina Nitschai, Renuka Pechetti, Glenn van de Ven in Nature
Most known black holes are either extremely massive, like the supermassive black holes that lie at the cores of large galaxies, or relatively lightweight, with a mass of under 100 times that of the Sun. Intermediate-mass black holes (IMBHs) are scarce, however, and are considered rare “missing links” in black hole evolution.
Now, an international team of astronomers has used more than 500 images from NASA’s Hubble Space Telescope — spanning two decades of observations — to search for evidence of an intermediate-mass black hole by following the motion of seven fast-moving stars in the innermost region of the globular star cluster Omega Centauri.
These stars provide new compelling evidence for the presence of the gravitational pull from an intermediate-mass black hole tugging on them. Only a few other IMBH candidates have been found to date.
Omega Centauri consists of roughly 10 million stars that are gravitationally bound. The cluster is about 10 times as massive as other big globular clusters — almost as massive as a small galaxy.
Among the many questions scientists want to answer: Are there any IMBHs, and if so, how common are they? Does a supermassive black hole grow from an IMBH? How do IMBHs themselves form? Are dense star clusters their favored home? The astronomers have now created an enormous catalog for the motions of these stars, measuring the velocities for 1.4 million stars gleaned from the Hubble images of the cluster. Most of these observations were intended to calibrate Hubble’s instruments rather than for scientific use, but they turned out to be an ideal database for the team’s research efforts.
“We discovered seven stars that should not be there,” explained Maximilian Häberle of the Max Planck Institute for Astronomy in Germany, who led this investigation. “They are moving so fast that they would escape the cluster and never come back. The most likely explanation is that a very massive object is gravitationally pulling on these stars and keeping them close to the center. The only object that can be so massive is a black hole, with a mass at least 8,200 times that of our Sun.”
Several studies have suggested the presence of an IMBH in Omega Centauri. However, other studies proposed the mass could be contributed by a central cluster of stellar-mass black holes, and had suggested the lack of fast-moving stars above the necessary escape velocity made an IMBH less likely in comparison.
“This discovery is the most direct evidence so far of an IMBH in Omega Centauri,” added team lead Nadine Neumayer of the Max Planck Institute for Astronomy in Germany, who initiated the study, together with Anil Seth from the University of Utah, Salt Lake City. “This is exciting because there are only very few other black holes known with a similar mass. The black hole in Omega Centauri may be the best example of an IMBH in our cosmic neighborhood.”
If confirmed, at a distance of 17,700 light-years the candidate black hole resides closer to Earth than the 4.3-million-solar-mass black hole in the center of the Milky Way, located 26,000 light-years away.
Omega Centauri is visible from Earth with the naked eye and is one of the favorite celestial objects for stargazers living in the southern hemisphere. Located just above the plane of the Milky Way, the cluster appears almost as large as the full Moon when seen from a dark rural area. It was first listed in Ptolemy’s catalog nearly 2,000 years ago as a single star. Edmond Halley reported it as a nebula in 1677. In the 1830s the English astronomer John Herschel was the first to recognize it as a globular cluster.
Hydrogen sulfide and metal-enriched atmosphere for a Jupiter-mass exoplanet
by Guangwei Fu, Luis Welbanks, Drake Deming, Julie Inglis, Michael Zhang, Joshua Lothringer, Jegug Ih, Julianne I. Moses, Everett Schlawin, Heather A. Knutson, Gregory Henry, Thomas Greene, David K. Sing, Arjun B. Savel, Eliza M.-R. Kempton, Dana R. Louie, Michael Line, Matt Nixon in Nature
An exoplanet infamous for its deadly weather has been hiding another bizarre feature — it reeks of rotten eggs, according to a new Johns Hopkins University study of data from the James Webb Space Telescope.
The atmosphere of HD 189733 b, a Jupiter-sized gas giant, has trace amounts of hydrogen sulfide, a molecule that not only gives off a stench but also offers scientists new clues about how sulfur, a building block of planets, might influence the insides and atmospheres of gas worlds beyond the solar system.
“Hydrogen sulfide is a major molecule that we didn’t know was there. We predicted it would be, and we know it’s in Jupiter, but we hadn’t really detected it outside the solar system,” said Guangwei Fu, an astrophysicist at Johns Hopkins who led the research. “We’re not looking for life on this planet because it’s way too hot, but finding hydrogen sulfide is a stepping stone for finding this molecule on other planets and gaining more understanding of how different types of planets form.”
In addition to detecting hydrogen sulfide and measuring overall sulfur in HD 189733 b’s atmosphere, Fu’s team precisely measured the main sources of the planet’s oxygen and carbon — water, carbon dioxide, and carbon monoxide.
“Sulfur is a vital element for building more complex molecules, and — like carbon, nitrogen, oxygen, and phosphate — scientists need to study it more to fully understand how planets are made and what they’re made of,” Fu said.
At only 64 light-years from Earth, HD 189733 b is the nearest “hot Jupiter” astronomers can observe passing in front of its star, making it a benchmark planet for detailed studies of exoplanetary atmospheres since its discovery in 2005, Fu said.
The planet is about 13 times closer to its star than Mercury is to the sun and takes only about two Earth days to complete an orbit. It has scorching temperatures of 1,700 degrees Fahrenheit and is notorious for vicious weather, including raining glass that blows sideways on winds of 5,000 mph.
As it did by detecting water, carbon dioxide, methane, and other critical molecules in other exoplanets, Webb gives scientists yet another new tool to track hydrogen sulfide and measure sulfur in gas planets outside the solar system.
“Say we study another 100 hot Jupiters and they’re all sulfur enhanced. What does that mean about how they were born and how they form differently compared to our own Jupiter?” Fu said.
The new data also ruled out the presence of methane in HD 189733 b with unprecedented precision and infrared wavelength observations from the Webb telescope, countering previous claims about that molecule’s abundance in the atmosphere.
“We had been thinking this planet was too hot to have high concentrations of methane, and now we know that it doesn’t,” Fu said.
The team also measured levels of heavy metals like those on Jupiter, a finding that could help scientists answer questions about how a planet’s metallicity correlates to its mass, Fu said.
Less-massive giant icy planets like Neptune and Uranus contain more metals than those found in gas giants like Jupiter and Saturn, the largest planets in the solar system. The higher metallicities suggest Neptune and Uranus accumulated more ice, rock, and other heavy elements relative to gases like hydrogen and helium during early periods of formation. Scientists are testing whether that correlation also holds true for exoplanets, Fu said.
“This Jupiter-mass planet is very close to Earth and has been very well studied. Now we have this new measurement to show that indeed the metal concentrations it has provide a very important anchor point to this study of how a planet’s composition varies with its mass and radius,” Fu said. “The findings support our understanding of how planets form through creating more solid material after initial core formation and then are naturally enhanced with heavy metals.”
In coming months, Fu’s team plans to track sulfur in more exoplanets and figure out how high levels of that compound might influence how close they form near their parent stars.
“We want to know how these kinds of planets got there, and understanding their atmospheric composition will help us answer that question,” Fu said.
A VLITE Search for Millisecond Pulsars in Globular Clusters: Discovery of a Pulsar in GLIMPSE-C01
by Amaris V. McCarver, Thomas J. Maccarone, Scott M. Ransom, Tracy E. Clarke, Simona Giacintucci, Wendy M. Peters, Emil Polisensky, Kristina Nyland, Tasha Gautam, Paulo C. C. Freire, Blagoy Rangelov in The Astrophysical Journal
U.S. Naval Research Laboratory (NRL) Remote Sensing Division intern, Amaris McCarver, along with a team of astronomers, discovered the first millisecond pulsar in the stellar cluster Glimpse-CO1.
Pulsars are natural laboratories for studying the behavior of matter under extreme gravitational and magnetic fields — conditions difficult or impossible to replicate on Earth. They also function as natural timekeepers. Precise timing of the observed pulses from an array of pulsars offers a means to detect gravitational waves propagating through our galaxy from the merging supermassive black holes that result from galaxy collisions. Some pulsars are observed to have an accuracy and stability comparable to our most precise atomic clocks. These pulsars hold the potential to establish a “celestial GPS” system for satellite navigation in space.
McCarver’s team used images from the Karl G. Jansky Very Large Array (VLA) Low-band Ionosphere and Transient Experiment (VLITE) to search for new pulsars in 97 stellar clusters.
“It was exciting so early in my career to see a speculative project work out so successfully,” said McCarver. Her new approach of using VLITE images coupled with images from several radio surveys at different frequencies identified multiple candidate pulsars, with the strongest candidate residing in a system known as GLIMPSE-C01.
“This type of scientific discovery is only possible thanks to the collaboration between NRL and the National Radio Astronomy Observatory that enabled this continual dual-frequency capability on the VLA,” said Tracy E. Clarke, Ph.D., NRL Remote Sensing Division astronomer. “This research highlights how we can use measures of radio brightness at different frequencies to find new pulsars efficiently, and that available sky surveys combined with the mountain of VLITE data mean those measurements are essentially always available. This opens the door to a new era of searches for highly dispersed and highly accelerated pulsars.”
The presence of a millisecond pulsar, designated GLIMPSE-C01A, was confirmed through re-processing of archival data from the Robert C. Byrd Green Bank Telescope. Millisecond pulsars, such as GLIMPSE-C01A, are born in supernova explosions and are spun up by consuming material from a companion star.
“Millisecond pulsars, or MSP, offer a promising method for autonomously navigating spacecraft from low Earth orbit to interstellar space, independent of ground contact and GPS availability,” said Emil Polisensky, Ph.D., an NRL Remote Sensing Division astronomer. “The confirmation of a new MSP identified by Amaris highlights the exciting potential for discovery with NRL’s VLITE data and the key role student interns play in cutting edge research.”
The NRL Remote Sensing Division conducts a program of basic research, science, and applications aimed at the development of new concepts for sensors and imaging systems for objects and targets on the Earth, in the near-Earth environment, and in deep space. The research, both theoretical and experimental, deals with discovering and understanding the basic physical principles and mechanisms that give rise to target and background emission, and to absorption and emission by the intervening medium.
The research includes theory, laboratory, and field experiments leading to ground-based, airborne, or space systems for use in such areas as remote sensing, astrometry, astrophysics, surveillance, non-acoustic anti-submarine warfare, and improved meteorological support systems for the operational Navy.
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