Orphan cloud discovered in galaxy cluster

Abell 1367
Hi-res image

New observations made with ESA’s X-ray XMM Newton telescope have revealed an “orphan cloud” – an isolated cloud in a galaxy cluster that is the first discovery of its kind.

A lot goes on in a galaxy cluster. There can be anything from tens to thousands of galaxies bound together by gravity. The galaxies themselves have a range of different properties, but typically contain systems with stars and planets, along with the material in between the stars – the interstellar medium. In between the galaxies is more material – tenuous hot gas known as the intercluster medium. And sometimes in all the chaos, some of the interstellar medium can get ripped out of a galaxy and get stranded in an isolated region of the cluster, as this new study reveals.

Unexpected discovery

Abell 1367, also known as the Leo Cluster, is a young cluster that contains around 70 galaxies and is located around 300 million light-years from Earth. In 2017, a small warm gas cloud of unknown origin was discovered in A1367 by the Subaru telescope in Japan. A follow-up X-ray survey to study other aspects of A1367 unexpectedly discovered X-rays emanating from this cloud, revealing that the cloud is actually bigger than the Milky Way.

This is the first time an intercluster clump has been observed in both X-rays and the light that comes from the warm gas. Since the orphan cloud is isolated and not associated with any galaxy, it has likely been floating in the space between galaxies for a long time, making its mere survival surprising.

The discovery of this orphan cloud was made by Chong Ge at the University of Alabama in Huntsville, and colleagues, and the study has been published in Monthly Notices of the Royal Astronomical Society.

Along with data from XMM-Newton and Subaru, Chong and colleagues also used the Multi Unit Spectroscopic Explorer (MUSE) on the Very Large Telescope (VLT) to observe the cluster in visible light.

The orphan cloud is the blue umbrella-shaped part of the image. It has been colour-coded to show the X-ray part of the cloud in blue, the warm gas in red, and the visible region in white shows some of the galaxies in the cluster. The part of the cloud that had been discovered in 2017 (in red) overlaps with the X-ray at the bottom of the cloud.

How the cloud became an orphan

It was previously thought that the distribution of material between galaxies is smooth, however more recent X-ray studies have revealed the presence of clumps in clusters. It was theorised that clumps of gas in the clusters were originally the gas that exists between stars in individual galaxies. The intercluster gas acts as a wind that is strong enough to pull the interstellar gas out of the galaxy as the galaxy is moving through the cluster. However, observations showing that intercluster clumps are originally stripped interstellar material have never been made until now. The observation of the warm gas in the clump provides the evidence to show that this orphan cloud originated within a galaxy. Interstellar material is much cooler than intercluster material, and the temperature of the orphan cloud matches that of interstellar gas. The researchers were also able to determine why the orphan cloud has survived for as long as it has. An isolated cloud would be expected to be ripped apart by instabilities caused by velocity and density differences. However, they found that a magnetic field in the cloud would be able to suppress these instabilities.

Searching for the parent galaxy

It is likely that the parent galaxy of the orphan cloud is a massive one as the mass of the X-ray gas in the orphan is substantial. It is possible that the parent might one day be discovered with future observations by following some breadcrumbs. For example, there are traces of the warm gas that extend beyond the orphan cloud that could be used to identify the parent with more data. There are other unsolved mysteries regarding the cloud that could be deciphered with more observations, such as mysterious offset between the brightest X-rays and the brightest light from the warm gas.

A closer inspection of this orphan will also further our understanding of the evolution of stripped interstellar medium at such a great distance from its parent galaxy and will provide a rare laboratory to study other things such as turbulence and heat conduction. This study paves the way for research on intercluster clumps, as future warm gas surveys can now be targeted to search for other orphan clouds.

Source: ESA/Science Exploration

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Mind the Gap: Scientists Use Stellar Mass to Link Exoplanets to Planet-Forming Disks

Protoplanetary disks are classified into three main categories: transition, ring, or extended. These false-color images from the Atacama Large Millimeter/submillimeter Array (ALMA) show these classifications in stark contrast. On left: the ring disk of RU Lup is characterized by narrow gaps thought to be carved by giant planets with masses ranging between a Neptune mass and a Jupiter mass. Middle: the transition disk of J1604.3-2130 is characterized by a large inner cavity thought to be carved by planets more massive than Jupiter, also known as Super-Jovian planets. On right: the compact disk of Sz104 is believed not to contain giant planets, as it lacks the telltale gaps and cavities associated with the presence of giant planets.  Credit: ALMA (ESO/NAOJ/NRAO), S. Dagnello (NRAO). Hi-Res File

 

New survey reveals that the presence of gaps in planet-forming disks is more common to higher mass stars and to the development of large, gaseous exoplanets

 

Using data for more than 500 young stars observed with the Atacama Large Millimeter/Submillimeter Array (ALMA), scientists have uncovered a direct link between protoplanetary disk structures—the planet-forming disks that surround stars—and planet demographics. The survey proves that higher mass stars are more likely to be surrounded by disks with “gaps” in them and that these gaps directly correlate to the high occurrence of observed giant exoplanets around such stars. These results provide scientists with a window back through time, allowing them to predict what exoplanetary systems looked like through each stage of their formation.

"We found a strong correlation between gaps in protoplanetary disks and stellar mass, which can be linked to the presence of large, gaseous exoplanets,” said Nienke van der Marel, a Banting fellow in the Department of Physics and Astronomy at the University of Victoria in British Columbia, and the primary author on the research. “Higher mass stars have relatively more disks with gaps than lower mass stars, consistent with the already known correlations in exoplanets, where higher mass stars more often host gas-giant exoplanets. These correlations directly tell us that gaps in planet-forming disks are most likely caused by giant planets of Neptune mass and above.”

Gaps in protoplanetary disks have long been considered as overall evidence of planet formation. However, there has been some skepticism due to the observed orbital distance between exoplanets and their stars. “One of the primary reasons that scientists have been skeptical about the link between gaps and planets before is that exoplanets at wide orbits of tens of astronomical units are rare. However, exoplanets at smaller orbits, between one and ten astronomical units, are much more common,” said Gijs Mulders, assistant professor of astronomy at Universidad Adolfo Ibáñez in Santiago, Chile, and co-author on the research. “We believe that planets that clear the gaps will migrate inwards later on.” The new study is the first to show that the number of gapped disks in these regions matches the number of giant exoplanets in a star system. “Previous studies indicated that there were many more gapped disks than detected giant exoplanets,” said Mulders. “Our study shows that there are enough exoplanets to explain the observed frequency of the gapped disks at different stellar masses.”

The correlation also applies to star systems with low-mass stars, where scientists are more likely to find massive rocky exoplanets, also known as Super-Earths. Van der Marel, who will become an assistant professor at Leiden University in the Netherlands beginning September 2021 said, “Lower mass stars have more rocky Super-Earths—between an Earth mass and a Neptune mass. Disks without gaps, which are more compact, lead to the formation of Super-Earths.”

This link between stellar mass and planetary demographics could help scientists identify which stars to target in the search for rocky planets throughout the Milky Way. “This new understanding of stellar mass dependencies will help to guide the search for small, rocky planets like Earth in the solar neighborhood,” said Mulders, who is also a part of the NASA-funded Alien Earths team. “We can use the stellar mass to connect the planet-forming disks around young stars to exoplanets around mature stars. When an exoplanet is detected, the planet-forming material is usually gone. So the stellar mass is a ‘tag’ that tells us what the planet-forming environment might have looked like for these exoplanets.”

And what it all comes down to is dust. “An important element of planet formation is the influence of dust evolution,” said van der Marel. “Without giant planets, dust will always drift inwards, creating the optimal conditions for the formation of smaller, rocky planets close to the star.”

The current research was conducted using data for more than 500 objects observed in prior studies using ALMA’s high-resolution Band 6 and Band 7 antennas. At present, ALMA is the only telescope that can image the distribution of millimeter-dust at high enough angular resolution to resolve the dust disks and reveal its substructure, or lack thereof. “Over the past five years, ALMA has produced many snapshot surveys of nearby star-forming regions resulting in hundreds of measurements of disk dust mass, size, and morphology,” said van der Marel. “The large number of observed disk properties has allowed us to make a statistical comparison of protoplanetary disks to the thousands of discovered exoplanets. This is the first time that a stellar mass dependency of gapped disks and compact disks has been successfully demonstrated using the ALMA telescope.”

“Our new findings link the beautiful gap structures in disks observed with ALMA directly to the properties of the thousands of exoplanets detected by the NASA Kepler mission and other exoplanet surveys,” said Mulders. “Exoplanets and their formation help us place the origins of the Earth and the Solar System in the context of what we see happening around other stars.”

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Resource

“A stellar mass dependence of structured disks: A possible link with exoplanet demographics,” N. van der Marel and G. Mulders, ApJ, DOI: 10.3847/1538-3881/ac0255, preview [https://arxiv.org/pdf/2104.06838.pdf

About ALMA

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Organisation for Astronomical Research in the Southern Hemisphere (ESO), the U.S. National Science Foundation (NSF) and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the Ministry of Science and Technology (MOST) and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI).

ALMA construction and operations are led by ESO on behalf of its Member States; by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.

Media Contact:

>Amy C. Oliver
Public Information Officer, ALMA
Public Information & News Manager, NRAO
+1 434 242 9584
aoliver@nrao.edu

Source: National Radio Astronomy Observatory (NRAO)/News

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Giant Comet Found in Outer Solar System by Dark Energy Survey

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Illustration of Comet Bernardinelli-Bernstein

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Discovery image of Comet Bernardinelli-Bernstein (annotated) 

 

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Discovery image of Comet Bernardinelli-Bernstein (unannotated) 

 

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Discovery image of Comet Bernardinelli-Bernstein (wide field) 

 

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Videos

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Zooming on Comet Bernardinelli-Bernstein   Videos

 

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Estimated to be 100–200 kilometers across, the unusual wandering body will make its closest approach to the Sun in 2031

A giant comet from the outskirts of our Solar System has been discovered in 6 years of data from the Dark Energy Survey. Comet Bernardinelli-Bernstein is estimated to be about 1000 times more massive than a typical comet, making it arguably the largest comet discovered in modern times. It has an extremely elongated orbit, journeying inward from the distant Oort Cloud over millions of years. It is the most distant comet to be discovered on its incoming path, giving us years to watch it evolve as it approaches the Sun, though it's not predicted to become a naked-eye spectacle.

A giant comet has been discovered by two astronomers following a comprehensive search of data from Dark Energy Survey (DES). The comet, which is estimated to be 100–200 kilometers across, or about 10 times the diameter of most comets, is an icy relic flung out of the Solar System by the migrating giant planets in the early history of the Solar System. This comet is quite unlike any other seen before and the huge size estimate is based on how much sunlight it reflects. 

Pedro Bernardinelli and Gary Bernstein, of the University of Pennsylvania, found the comet — named Comet Bernardinelli-Bernstein (with the designation C/2014 UN₂₇₁) — hidden among data collected by the 570-megapixel Dark Energy Camera (DECam) mounted on the Víctor M. Blanco 4-meter Telescope at Cerro Tololo Inter-American Observatory (CTIO) in Chile. The analysis of data from the Dark Energy Survey is supported by the Department of Energy (DOE) and the National Science Foundation (NSF), and the DECam science archive is curated by the Community Science and Data Center (CSDC) at NSF’s NOIRLab. CTIO and CSDC are Programs of NOIRLab.

One of the highest-performance, wide-field CCD imagers in the world, DECam was designed specifically for the DES and operated by the DOE and NSF between 2013 and 2019. DECam was funded by the DOE and was built and tested at DOE's Fermilab. At present DECam is used for programs covering a huge range of science.

DES was tasked with mapping 300 million galaxies across a 5000-square-degree area of the night sky, but during its six years of observations it also observed many comets and trans-Neptunian objects passing through the surveyed field. A trans-Neptunian object, or TNO, is an icy body that resides in our Solar System beyond the orbit of Neptune.

Bernardinelli and Bernstein used 15–20 million CPU hours at the National Center for Supercomputing Applications and Fermilab, employing sophisticated identification and tracking algorithms to identify over 800 individual TNOs from among the more than 16 billion individual sources detected in 80,000 exposures taken as part of the DES. Thirty-two of those detections belonged to one object in particular — C/2014 UN₂₇₁.

Comets are icy bodies that evaporate as they approach the warmth of the Sun, growing their coma and tails. The DES images of the object in 2014–2018 did not show a typical comet tail, but within a day of the announcement of its discovery via the Minor Planet Center, astronomers using the Las Cumbres Observatory network took fresh images of Comet Bernardinelli-Bernstein which revealed that it has grown a coma in the past 3 years, making it officially a comet. 

Its current inward journey began at a distance of over 40,000 astronomical units (au) from the Sun — in other words 40,000 times farther from the Sun than Earth is, or 6 trillion kilometers away (3.7 trillion miles or 0.6 light-years — 1/7 of the distance to the nearest star). For comparison, Pluto is 39 au from the Sun, on average. This means that Comet Bernardinelli-Bernstein originated in the Oort Cloud of objects, ejected during early history of the Solar System. It could be the largest member of the Oort Cloud ever detected, and it is the first comet on an incoming path to be detected so far away. 

Comet Bernardinelli-Bernstein is currently much closer to the Sun. It was first seen by DES in 2014 at a distance of 29 au (4 billion kilometers or 2.5 billion miles, roughly the distance of Neptune), and as of June 2021, it was 20 au (3 billion kilometers or 1.8 billion miles, the distance of Uranus) from the Sun and currently shines at magnitude 20. The comet's orbit is perpendicular to the plane of the Solar System and it will reach its closest point to the Sun (known as perihelion) in 2031, when it will be around 11 au away (a bit more than Saturn’s distance from the Sun) — but it will get no closer. Despite the comet’s size, it is currently predicted that skywatchers will require a large amateur telescope to see it, even at its brightest. 

"We have the privilege of having discovered perhaps the largest comet ever seen — or at least larger than any well-studied one — and caught it early enough for people to watch it evolve as it approaches and warms up," said Gary Bernstein. "It has not visited the Solar System in more than 3 million years."

Comet Bernardinelli-Bernstein will be followed intensively by the astronomical community, including with NOIRLab facilities, to understand the composition and origin of this massive relic from the birth of our own planet. Astronomers suspect that there may be many more undiscovered comets of this size waiting in the Oort Cloud far beyond Pluto and the Kuiper Belt. These giant comets are thought to have been scattered to the far reaches of the Solar System by the migration of Jupiter, Saturn, Uranus and Neptune early in their history.

“This is a much needed anchor on the unknown population of large objects in the Oort Cloud and their connection with early migration of the ice/gas giants soon after the Solar System was formed,” said NOIRLab astronomer Tod Lauer. 

“These observations demonstrate the value of long-duration survey observations on national facilities like the Blanco telescope,” says Chris Davis, National Science Foundation Program Director for NOIRLab. “Finding huge objects like Comet Bernardinelli-Bernstein is crucial to our understanding of the early history of our Solar System.”

It is not yet known how active and bright it will become when it reaches perihelion. However, Bernardinelli says that Vera C. Rubin Observatory, a future Program of NOIRLab, “will continuously measure Comet Bernardinelli-Bernstein all the way to its perihelion in 2031, and probably find many, many others like it,” allowing astronomers to characterize objects from the Oort Cloud in much greater detail.

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More Information

This research was reported to the Minor Planet Center.

NSF’s NOIRLab (National Optical-Infrared Astronomy Research Laboratory), the US center for ground-based optical-infrared astronomy, operates the international Gemini Observatory (a facility of NSF, NRC–Canada, ANID–Chile, MCTIC–Brazil, MINCyT–Argentina, and KASI–Republic of Korea), Kitt Peak National Observatory (KPNO), Cerro Tololo Inter-American Observatory (CTIO), the Community Science and Data Center (CSDC), and Vera C. Rubin Observatory (operated in cooperation with the Department of Energy’s SLAC National Accelerator Laboratory). It is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with NSF and is headquartered in Tucson, Arizona. The astronomical community is honored to have the opportunity to conduct astronomical research on Iolkam Du’ag (Kitt Peak) in Arizona, on Maunakea in Hawai‘i, and on Cerro Tololo and Cerro Pachón in Chile. We recognize and acknowledge the very significant cultural role and reverence that these sites have to the Tohono O'odham Nation, to the Native Hawaiian community, and to the local communities in Chile, respectively.

The Dark Energy Survey (DES) is a collaboration of more than 400 scientists from 25 institutions in seven countries. Funding for the DES Projects has been provided by the US Department of Energy Office of Science, US National Science Foundation, Ministry of Science and Education of Spain, Science and Technology Facilities Council of the United Kingdom, Higher Education Funding Council for England, ETH Zurich for Switzerland, National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign, Kavli Institute of Cosmological Physics at the University of Chicago, Center for Cosmology and AstroParticle Physics at Ohio State University, Mitchell Institute for Fundamental Physics and Astronomy at Texas A&M University, Financiadora de Estudos e Projetos, Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro, Conselho Nacional de Desenvolvimento Científico e Tecnológico and Ministério da Ciência e Tecnologia, Deutsche Forschungsgemeinschaft, and the collaborating institutions in the Dark Energy Survey.

NCSA at the University of Illinois at Urbana-Champaign provides supercomputing and advanced digital resources for the nation’s science enterprise. At NCSA, University of Illinois faculty, staff, students and collaborators from around the globe use advanced digital resources to address research grand challenges for the benefit of science and society. NCSA has been advancing one third of the Fortune 50® for more than 30 years by bringing industry, researchers and students together to solve grand challenges at rapid speed and scale. For more information.

Fermilab is America’s premier national laboratory for particle physics and accelerator research. A US Department of Energy Office of Science laboratory, Fermilab is located near Chicago, Illinois, and operated under contract by the Fermi Research Alliance LLC. Follow Fermilab on Twitter at @Fermilab.

The DOE Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. 

Bernardinelli and Bernstein’s search was partially supported by a grant from the National Science Foundation.

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Links

• Minor Planet Center report
• Photos of DECam
• Photos of the Victor M. Blanco Telescope
• Videos of the Victor M. Blanco Telescope

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Contacts

Pedro Bernardinelli
University of Pennsylvania
Email: pedrobe@sas.upenn.edu

Gary Bernstein
University of Pennsylvania
Email: garyb@physics.upenn.edu

Amanda Kocz
Press and Internal Communications Officer
NSF’s NOIRLab
Cell: +1 626 524 5884
Email: amanda.kocz@noirlab.edu

Source: NOIRLab/News

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Hubble Images a Dazzling Dynamic Duo

IC 1623

Text credit: European Space Agency (ESA)

Image credit: ESA/Hubble & NASA, R. Chandar

A cataclysmic cosmic collision takes center stage in this image taken with the NASA/ESA Hubble Space Telescope. The image features the interacting galaxy pair IC 1623, which lies around 275 million light-years away in the constellation Cetus (the Whale). The two galaxies are in the final stages of merging, and astronomers expect a powerful inflow of gas to ignite a frenzied burst of star formation in the resulting compact starburst galaxy.

This interacting pair of galaxies is a familiar sight; Hubble captured IC 1623 in 2008 using two filters at optical and infrared wavelengths on the Advanced Camera for Surveys. This image incorporates data from Wide Field Camera 3, and combines observations taken in eight filters spanning infrared to ultraviolet wavelengths to reveal the finer details of IC 1623. Future observations of the galaxy pair with the NASA/ESA/CSA James Webb Space Telescope will shed more light on the processes powering extreme star formation in environments such as IC 1623.

Media Contact:

Claire Andreoli
NASA's Goddard Space Flight Center
301-286-1940

Editor: Lynn Jenner

Source: NASA/Galaxies

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NASA’s Webb Will Use Quasars to Unlock the Secrets of the Early Universe

This is an artist's concept of a galaxy with a brilliant quasar at its center. A quasar is a very bright, distant and active supermassive black hole that is millions to billions of times the mass of the Sun. Among the brightest objects in the universe, a quasar’s light outshines that of all the stars in its host galaxy combined. Quasars feed on infalling matter and unleash torrents of winds and radiation, shaping the galaxies in which they reside. Using the unique capabilities of Webb, scientists will study six of the most distant and luminous quasars in the universe. Credits: NASA, ESA and J. Olmsted (STScI). Hi-res image

Quasars are very bright, distant and active supermassive black holes that are millions to billions of times the mass of the Sun. Typically located at the centers of galaxies, they feed on infalling matter and unleash fantastic torrents of radiation. Among the brightest objects in the universe, a quasar’s light outshines that of all the stars in its host galaxy combined, and its jets and winds shape the galaxy in which it resides.

Shortly after its launch later this year, a team of scientists will train NASA’s James Webb Space Telescope on six of the most distant and luminous quasars. They will study the properties of these quasars and their host galaxies, and how they are interconnected during the first stages of galaxy evolution in the very early universe. The team will also use the quasars to examine the gas in the space between galaxies, particularly during the period of cosmic reionization, which ended when the universe was very young. They will accomplish this using Webb’s extreme sensitivity to low levels of light and its superb angular resolution.

Webb: Visiting the Young Universe

As Webb peers deep into the universe, it will actually look back in time. Light from these distant quasars began its journey to Webb when the universe was very young and took billions of years to arrive. We will see things as they were long ago, not as they are today.

“All these quasars we are studying existed very early, when the universe was less than 800 million years old, or less than 6 percent of its current age. So these observations give us the opportunity to study galaxy evolution and supermassive black hole formation and evolution at these very early times,” explained team member Santiago Arribas, a research professor at the Department of Astrophysics of the Center for Astrobiology in Madrid, Spain. Arribas is also a member of Webb’s Near-Infrared Spectrograph (NIRSpec) Instrument Science Team.

The light from these very distant objects has been stretched by the expansion of space. This is known as cosmological redshift. The farther the light has to travel, the more it is redshifted. In fact, the visible light emitted at the early universe is stretched so dramatically that it is shifted out into the infrared when it arrives to us. With its suite of infrared-tuned instruments, Webb is uniquely suited to studying this kind of light.

Studying Quasars, Their Host Galaxies and Environments, and Their Powerful Outflows

The quasars the team will study are not only among the most distant in the universe, but also among the brightest. These quasars typically have the highest black hole masses, and they also have the highest accretion rates — the rates at which material falls into the black holes.

“We’re interested in observing the most luminous quasars because the very high amount of energy that they’re generating down at their cores should lead to the largest impact on the host galaxy by the mechanisms such as quasar outflow and heating,” said Chris Willott, a research scientist at the Herzberg Astronomy and Astrophysics Research Centre of the National Research Council of Canada (NRC) in Victoria, British Columbia. Willott is also the Canadian Space Agency’s Webb project scientist. “We want to observe these quasars at the moment when they’re having the largest impact on their host galaxies.”

An enormous amount of energy is liberated when matter is accreted by the supermassive black hole. This energy heats and pushes the surrounding gas outward, generating strong outflows that tear across interstellar space like a tsunami, wreaking havoc on the host galaxy.

Outflows play an important role in galaxy evolution. Gas fuels the formation of stars, so when gas is removed due to outflows, the star-formation rate decreases. In some cases, outflows are so powerful and expel such large amounts of gas that they can completely halt star formation within the host galaxy. Scientists also think that outflows are the main mechanism by which gas, dust and elements are redistributed over large distances within the galaxy or can even be expelled into the space between galaxies – the intergalactic medium. This may provoke fundamental changes in the properties of both the host galaxy and the intergalactic medium.

Examining Properties of Intergalactic Space During the Era of Reionization

More than 13 billion years ago, when the universe was very young, the view was far from clear. Neutral gas between galaxies made the universe opaque to some types of light. Over hundreds of millions of years, the neutral gas in the intergalactic medium became charged or ionized, making it transparent to ultraviolet light. This period is called the Era of Reionization. But what led to the reionization that created the “clear” conditions detected in much of the universe today? Webb will peer deep into space to gather more information about this major transition in the history of the universe. The observations will help us understand the Era of Reionization, which is one of the key frontiers in astrophysics.

The team will use quasars as background light sources to study the gas between us and the quasar. That gas absorbs the quasar’s light at specific wavelengths. Through a technique called imaging spectroscopy, they will look for absorption lines in the intervening gas. The brighter the quasar is, the stronger those absorption line features will be in the spectrum. By determining whether the gas is neutral or ionized, scientists will learn how neutral the universe is and how much of this reionization process has occurred at that particular point in time.

“If you want to study the universe, you need very bright background sources. A quasar is the perfect object in the distant universe, because it’s luminous enough that we can see it very well,” said team member Camilla Pacifici, who is affiliated with the Canadian Space Agency but works as an instrument scientist at the Space Telescope Science Institute in Baltimore. “We want to study the early universe because the universe evolves, and we want to know how it got started.”

The team will analyze the light coming from the quasars with NIRSpec to look for what astronomers call “metals,” which are elements heavier than hydrogen and helium. These elements were formed in the first stars and the first galaxies and expelled by outflows. The gas moves out of the galaxies it was originally in and into the intergalactic medium. The team plans to measure the generation of these first “metals,” as well as the way they’re being pushed out into the intergalactic medium by these early outflows.

The Power of Webb

Webb is an extremely sensitive telescope able to detect very low levels of light. This is important, because even though the quasars are intrinsically very bright, the ones this team is going to observe are among the most distant objects in the universe. In fact, they are so distant that the signals Webb will receive are very, very low. Only with Webb’s exquisite sensitivity can this science be accomplished. Webb also provides excellent angular resolution, making it possible to disentangle the light of the quasar from its host galaxy.

The quasar programs described here are Guaranteed Time Observations involving the spectroscopic capabilities of NIRSpec.

The James Webb Space Telescope will be the world's premier space science observatory when it launches in 2021. Webb will solve mysteries in our solar system, look beyond to distant worlds around other stars, and probe the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency) and the Canadian Space Agency.

For more information about Webb, visit www.nasa.gov/webb.

Media contacts:

Ann Jenkins / Christine Pulliam
Space Telescope Science Institute, Baltimore, Maryland
410-338-4488 / 410-338-4366
jenkins@stsci.edu / cpulliam@stsci.edu

Laura Betz
NASA's Goddard Space Flight Center, Greenbelt, Md.
laura.e.betz@nasa.gov

Editor: Lynn Jenner

Source: NASA/Black Holes

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MSH 15-52: Cosmic Hand Hitting a Wall

MSH 15-52
Credit NASA/SAO/NCSU/Borkowski et al.

JPEG (252.1 kb) - Large JPEG (3.8 MB) - Tiff (49 MB) - More Images

Tour: Cosmic Hand Hitting a Wall - More Animations

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Motions of a remarkable cosmic structure have been measured for the first time, using NASA's Chandra X-ray Observatory. The blast wave and debris from an exploded star are seen moving away from the explosion site and colliding with a wall of surrounding gas.

Astronomers estimate that light from the supernova explosion reached Earth about 1,700 years ago, or when the Mayan empire was flourishing and the Jin dynasty ruled China. However, by cosmic standards the supernova remnant formed by the explosion, called MSH 15-52, is one of the youngest in the Milky Way galaxy. The explosion also created an ultra-dense, magnetized star called a pulsar, which then blew a bubble of energetic particles, an X-ray-emitting nebula.

Since the explosion the supernova remnant — made of debris from the shattered star, plus the explosion's blast wave — and the X-ray nebula have been changing as they expand outward into space. Notably, the supernova remnant and X-ray nebula now resemble the shape of fingers and a palm.

Previously, astronomers had released a full Chandra view of the "hand," as shown in the main graphic. A new study is now reporting how quickly the supernova remnant associated with the hand is moving, as it strikes a cloud of gas called RCW 89. The inner edge of this cloud forms a gas wall located about 35 light years from the center of the explosion.

To track the motion the team used Chandra data from 2004, 2008, and then a combined image from observations taken in late 2017 and early 2018. These three epochs are shown in the inset of the main graphic.

The rectangle (fixed in space) highlights the motion of the explosion's blast wave, which is located near one of the fingertips. This feature is moving at almost 9 million miles per hour. The fixed squares (seen in the images below) enclose clumps of magnesium and neon that likely formed in the star before it exploded and shot into space once the star blew up. Some of this explosion debris is moving at even faster speeds of more than 11 million miles per hour. A color version of the 2018 image shows the fingers in blue and green and the clumps of magnesium and neon in red and yellow.

Timelapse: 2004, 2008, 2018

The rectangle (fixed in space) highlights the motion of the explosion's blast wave, which is located near one of the fingertips. This feature is moving at almost 9 million miles per hour. The fixed squares (seen in the images below) enclose clumps of magnesium and neon that likely formed in the star before it exploded and shot into space once the star blew up. Some of this explosion debris is moving at even faster speeds of more than 11 million miles per hour. A color version of the 2018 image shows the fingers in blue and green and the clumps of magnesium and neon in red and yellow.

View by year - 2004, 2008 and 2018

While these are startling high speeds, they actually represent a slowing down of the remnant. Researchers estimate that to reach the farthest edge of RCW 89, material would have to travel on average at almost 30 million miles per hour. This estimate is based on the age of the supernova remnant and the distance between the center of the explosion and RCW 89. This difference in speed implies that the material has passed through a low-density cavity of gas and then been significantly decelerated by running into RCW 89.

The exploded star likely lost part or all of its outer layer of hydrogen gas in a wind, forming such a cavity, before exploding, as did the star that exploded to form the well-known supernova remnant Cassiopeia A (Cas A), which is much younger at an age of about 350 years. About 30% of massive stars that collapse to form supernovas are of this type. The clumps of debris seen in the 1,700-year-old supernova remnant could be older versions of those seen in Cas A at optical wavelengths in terms of their initial speeds and densities. This means that these two objects may have the same underlying source for their explosions, which is likely related to how stars with stripped hydrogen layers explode. However, astronomers do not understand the details of this yet and will continue to study this possibility.

A paper describing these results appeared in the June 1, 2020, issue of The Astrophysical Journal Letters, and a preprint is available online. The authors of the study are Kazimierz Borkowski, Stephen Reynolds, and William Miltich, all of North Carolina State University in Raleigh.

NASA's Marshall Space Flight Center manages the Chandra program. The Smithsonian Astrophysical Observatory's Chandra X-ray Center controls science from Cambridge Massachusetts and flight operations from Burlington, Massachusetts.

Quick Look: Cosmic Hand Hitting a Wall

Source: NASA's Chandra X-Ray Observatory

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Fast Facts for MSH 15-52:

 
Scale: Image is about 6.64 arcmin (33 light years) across.
Category: Supernovas & Supernova Remnants
Coordinates (J2000): RA 15h 13m 35.0s | Dec -59° 00´ 12"
Constellation: Circinus
Observation Date: 7 observations from Dec 31, 2004 through April 8, 2018
Observation Time: 75 hours 26 minutes (64 days 19 hours and 26 minutes)
Obs. ID: 5562, 9138, 19299, 19300, 20910, 20932, 20933
Instrument: ACIS
References: Borkowski, K, Reynolds, S, and Miltich, W, 2020, ApJL, 895, 32; arXiv:2005.07721
Color Code: Red: 0.2-2.0 keV; Yellow: 2.0-3.0 keV; Blue: 3.0-6.5 keVaa
Distance Estimate: About 17,000 light years

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Dark matter: ‘real stuff’ or gravity misunderstood?

In the centre of the image the elliptical galaxy NGC 5982, and to the right the spiral galaxy NGC 5985. These two types of galaxies turn out to behave very differently when it comes to the extra gravity – and therefore possibly the dark matter – in their outer regions. Images: Bart Delsaert (www.delsaert.com).

For many years now, astronomers and physicists have been in a conflict. Is the mysterious dark matter that we observe deep in the Universe real, or is what we see the result of subtle deviations from the laws of gravity as we know them? In 2016, Dutch physicist Erik Verlinde proposed a theory of the second kind: emergent gravity. New research, published in Astronomy & Astrophysics this week, pushes the limits of dark matter observations to the unknown outer regions of galaxies, and in doing so re-evaluates several dark matter models and alternative theories of gravity. Measurements of the gravity of 259,000 isolated galaxies show a very close relation between the contributions of dark matter and those of ordinary matter, as predicted in Verlinde’s theory of emergent gravity and an alternative model called Modified Newtonian Dynamics. However, the results also appear to agree with a computer simulation of the Universe that assumes that dark matter is ‘real stuff’.

The new research was carried out by an international team of astronomers, led by Margot Brouwer (RUG and UvA). Further important roles were played by Kyle Oman (RUG and Durham University) and Edwin Valentijn (RUG). In 2016, Brouwer also performed a first test of Verlinde’s ideas; this time, Verlinde himself also joined the research team.

Matter or gravity?

So far, dark matter has never been observed directly – hence the name. What astronomers observe in the night sky are the consequences of matter that is potentially present: bending of starlight, stars that move faster than expected, and even effects on the motion of entire galaxies. Without a doubt all of these effects are caused by gravity, but the question is: are we truly observing additional gravity, caused by invisible matter, or are the laws of gravity themselves the thing that we haven’t fully understood yet?

To answer this question, the new research uses a similar method to the one used in the original test in 2016. Brouwer and her colleagues make use of an ongoing series of photographic measurements that started ten years ago: the KiloDegree Survey (KiDS), performed using ESO’s VLT Survey Telescope in Chili. In these observations one measures how starlight from far away galaxies is bent by gravity on its way to our telescopes. Whereas in 2016 the measurements of such ‘lens effects’ only covered an area of about 180 square degrees on the night sky, in the mean time this has been extended to about 1000 square degrees – allowing the researchers to measure the distribution of gravity in around a million different galaxies.

Comparative testing

Brouwer and her colleagues selected over 259,000 isolated galaxies, for which they were able to measure the so-called ‘Radial Acceleration Relation’ (RAR). This RAR compares the amount of gravity expected based on the visible matter in the galaxy, to the amount of gravity that is actually present – in other words: the result shows how much ‘extra’ gravity there is, in addition to that due to normal matter. Until now, the amount of extra gravity had only been determined in the outer regions of galaxies by observing the motions of stars, and in a region about five times larger by measuring the rotational velocity of cold gas. Using the lensing effects of gravity, the researchers were now able to determine the RAR at gravitational strengths which were one hundred times smaller, allowing them to penetrate much deeper into the regions far outside the individual galaxies.

This made it possible to measure the extra gravity extremely precisely – but is this gravity the result of invisible dark matter, or do we need to improve our understanding of gravity itself? Author Kyle Oman indicates that the assumption of ‘real stuff’ at least partially appears to work: “In our research, we compare the measurements to four different theoretical models: two that assume the existence of dark matter and form the base of computer simulations of our universe, and two that modify the laws of gravity – Erik Verlinde’s model of emergent gravity and the so-called ‘Modified Newtonian Dynamics’ or MOND. One of the two dark matter simulations, MICE, makes predictions that match our measurements very nicely. It came as a surprise to us that the other simulation, BAHAMAS, led to very different predictions. That the predictions of the two models differed at all was already surprising, since the models are so similar. But moreover, we would have expected that if a difference would show up, BAHAMAS was going to perform best. BAHAMAS is a much more detailed model than MICE, approaching our current understanding of how galaxies form in a universe with dark matter much closer. Still, MICE performs better if we compare its predictions to our measurements. In the future, based on our findings, we want to further investigate what causes the differences between the simulations.”

Young and old galaxies

Thus it seems that, at least one dark matter model does appear to work. However, the alternative models of gravity also predict the measured RAR. A standoff, it seems – so how do we find out which model is correct? Margot Brouwer, who led the research team, continues: “Based on our tests, our original conclusion was that the two alternative gravity models and MICE matched the observations reasonably well. However, the most exciting part was yet to come: because we had access to over 259,000 galaxies, we could divide them into several types – relatively young, blue spiral galaxies versus relatively old, red elliptical galaxies.” Those two types of galaxies come about in very different ways: red elliptical galaxies form when different galaxies interact, for example when two blue spiral galaxies pass by each other closely, or even collide. As a result, the expectation within the particle theory of dark matter is that the ratio between regular and dark matter in the different types of galaxies can vary. 

Models such as Verlinde’s theory and MOND on the other hand do not make use of dark matter particles, and therefore predict a fixed ratio between the expected and measured gravity in the two types of galaxies – that is, independent of their type. Brouwer: “We discovered that the RARs for the two types of galaxies differed significantly. That would be a strong hint towards the existence of dark matter as a particle.”

A plot showing the Radial Acceleration Relation (RAR). The background is an image of the elliptical galaxy M87, showing the distance to the centre of the galaxy. The plot shows how the measurements range from high gravitational acceleration in the centre of the galaxy, to low gravitational acceleration in the far outer regions. Image: Chris Mihos (Case Western Reserve University) / ESO.

However, there is a caveat: gas. Many galaxies are probably surrounded by a diffuse cloud of hot gas, which is very difficult to observe. If it were the case that there is hardly any gas around young blue spiral galaxies, but that old red elliptical galaxies live in a large cloud of gas – of roughly the same mass as the stars themselves – then that could explain the difference in the RAR between the two types. To reach a final judgement on the measured difference, one would therefore also need to measure the amounts of diffuse gas – and this is exactly what is not possible using the KiDS telescopes. Other measurements have been done for a small group of around one hundred galaxies, and these measurements indeed found more gas around elliptical galaxies, but it is still unclear how representative those measurements are for the 259,000 galaxies that were studied in the current research.

Dark matter for the win?

If it turns out that extra gas cannot explain the difference between the two types of galaxies, then the results of the measurements are easier to understand in terms of dark matter particles than in terms of alternative models of gravity. But even then, the matter is not settled yet. While the measured differences are hard to explain using MOND, Erik Verlinde still sees a way out for his own model. Verlinde: “My current model only applies to static, isolated, spherical galaxies, so it cannot be expected to distinguish the different types of galaxies. I view these results as a challenge and inspiration to develop an asymmetric, dynamical version of my theory, in which galaxies with a different shape and history can have a different amount of ‘apparent dark matter’.”

Therefore, even after the new measurements, the dispute between dark matter and alternative gravity theories is not settled yet. Still, the new results are a major step forward: if the measured difference in gravity between the two types of galaxies is correct, then the ultimate model, whichever one that is, will have to be precise enough to explain this difference. This means in particular that many existing models can be discarded, which considerably thins out the landscape of possible explanations. On top of that, the new research shows that systematic measurements of the hot gas around galaxies are necessary. Edwin Valentijn formulates is as follows: “As observational astronomers, we have reached the point where we are able to measure the extra gravity around galaxies more precisely than we can measure the amount of visible matter. The counterintuitive conclusion is that we must first measure the presence of ordinary matter in the form of hot gas around galaxies, before future telescopes such as Euclid can finally solve the mystery of dark matter.”

Publication

“The weak lensing radial acceleration relation: Constraining modified gravity and cold dark matter theories with KiDS-1000”, M. Brouwer et al., Astronomy & Astrophysics 2021.

Source: University of Amesterdam/News

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The Future of the Search for a Gravitational-Wave Background

Artist's illustration of the Earth and surrounded pulsars embedded in spacetime that is deformed by an ocean of background gravitational waves. Credits: Tonia Klein / NANOGrav

Mrk 739 is an example of a galaxy merger where the two nuclei at the center of the newly-formed galaxy are still in the process of merging. Credit: SDSS

For decades, scientists have used networks of pulsars to search for a faint, background gravitational-wave signal that should pervade our universe. What have they found so far, and what can we expect in the future? A new publication details the possibilities.

Humming in the Background

In recent years, the LIGO and Virgo gravitational-wave detectors have clocked dozens of observations of stellar-mass black hole binary mergers. But what about larger black holes? When galaxies collide, the supermassive black holes at their centers should also form binaries, inspiral, and merge. The combination of all inspiraling supermassive black hole binaries across the universe should produce a deep background hum of gravitational waves — a signal that we could detect, with the right tool. Enter: pulsar timing arrays (PTAs).

Cosmic Clocks

PTAs rely on the remarkably consistent timing of flashes of light from a network of spinning neutron stars — pulsars — to measure the stretching of the spacetime in which these pulsars are embedded.

As gravitational waves pass through spacetime, signals from the various pulsars in the network have to travel longer or shorter distances to reach us. PTAs measure those timing differences to search for a stochastic background gravitational-wave signal.

An artist’s illustration showing how a network of pulsars could be used to search for the ripples in space-time

Credits: David Champion/NASA/JPL

A Hint of a Signal

How are PTAs doing so far? The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) has been searching for the gravitational-wave background for more than a decade — until now, without success. But the longer we observe the timings of a set of pulsars, the more subtle a signal we can detect.

The 12.5-year dataset released this year offers a first glimpse of hope: this most sensitive dataset yet shows signs of a signal consistent with the predicted gravitational-wave background. Definitive evidence of the background will require longer observations with NANOGrav to beat down the noise and reveal an expected set of correlations between pairs of pulsars.

So when can we hope to robustly detect this signal, and what will it tell us? In a new publication led by Nihan Pol (Vanderbilt University, West Virginia University), a team of scientists emulates and extends the NANOGrav dataset into the future and tries to recover injected gravitational-wave-background signals to learn what we can expect.

The gravitational wave spectrum and detectors. Here, frequency of the gravitational waves is plotted against strain (the fractional change in the separation between objects caused by the passage of the gravitational wave). Click to enlarge. [NANOGrav]

Milestones Ahead 

 Pol and collaborators identify three key milestones that we should soon achieve.

1. Robust evidence of the gravitational-wave background should be possible with 15–17 years of data — only another 2–5 years beyond the 12.5-year dataset already published.
2. The signal detected at this time will already contain enough information to identify whether the gravitational-wave background is caused by supermassive black hole binaries, as anticipated, or if it instead has more exotic origins, like primordial black holes or cosmic strings.
3. If the signal is caused by supermassive black holes, the initial detection will also be sufficient to distinguish between different population models for supermassive black hole binaries.

This work illustrates that NANOGrav has the potential to provide us with a wealth of information in the next few years! What’s more, those results will come even faster with the addition of new pulsars to NANOGrav’s network, or the combination of data from multiple PTAs. Gravitational-wave astronomy is truly only just getting started!

Top: Evolution of the signal-to-noise ratio as a function of time observing the pulsars. Middle: The predicted correlation signal between pulsars after 12 years, 15 years, and 20 years, compared to the model (dashed red line) showing the presence of a gravitational-wave background. Bottom: The signal-to-noise ratio of the correlation signal as a function of observing time. [Pol et al. 2021]

Citation

“Astrophysics Milestones for Pulsar Timing Array Gravitational-wave Detection,” Nihan S. Pol et al 2021 ApJL 911 L34. doi:10.3847/2041-8213/abf2c9

By Susanna Kohler

Source: American Astronomical Society (AAS-Nova)

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Probing Dust Close to Supermassive Black Holes

Artist's illustration of the surroundings of a supermassive black hole at the heart of an active galaxy
Credit:ESO/M. Kornmesser

What’s going on deep in the centers of active galaxies, close around the supermassive black holes feeding off of their surroundings? A new study uses infrared observations to explore this inner region in one active galaxy.

The geometric dependence of AGN types in the unified AGN model. Type 1 AGN are viewed from an angle where the central engine is visible. In Type 2 AGN, a dusty torus obscures the central engine from view. Credit: Urry & Padovani, 1995

A Unified Picture?

We know that active galactic nuclei (AGN) consist of a supermassive black hole accreting surrounding material and shining brightly across the electromagnetic spectrum. But the structure of the gas and dust close around the black hole, and the causes of the different emission we see, have remained a topic of debate.

Decades ago, scientists proposed that Type 1 and Type 2 AGN — two different categories of active galaxies with different observational properties — might be the same objects viewed from different angles. This unification scheme relies on the presence of a dusty torus — a puffed-up, donut-like dust structure close to the black hole. In this model, the torus obscures the inner, emission-line-producing gas from some viewing angles, changing the appearance of the AGN based on its orientation.

But recent infrared observations have challenged this view. With powerful mid-infrared telescopes, we’ve taken a closer look at the inner few hundred light-years of nearby active galaxies — and instead of revealing an obscuring torus of dust, these observations have shown polar dust structures.

The Very Large Telescope Interferometer (VLTI) in Chile
Credit: ESO/G. Hüdepohl

A Search for Distant Dust

How can we explain these observations? Theorists have a solution: in the disk–wind model, the dust close to the black hole is arranged in a hot, equatorial disk rather than a torus. Radiation pressure then blows some of this dust off into a cooler wind from the poles, producing the polar structures we’ve seen in mid-infrared observations. Obscuration comes from the disk and the launch region of the wind.

The equatorial disk in this model should lie on scales too small to have been previously observed in mid-infrared — but there’s a new tool on the scene! GRAVITY, an interferometric instrument on the Very Large Telescope Interferometer in Chile, operates in the near-infrared. This makes it the perfect instrument to search for the very hot dust that would lie in a disk at the heart of an AGN.

In a new study led by James Leftley (University of Southampton, UK; Côte d’Azur University, France; ESO, Chile), a team of scientists has now used GRAVITY to obtain near-infrared observations of the center of ESO 323-G77, a local active galactic nucleus.

The Gaussian fits to the near-infrared emission show an elongated structure (center) oriented along the AGN’s equator. This is in contrast to the polar-aligned mid-infrared emission (orange region) that may represent a disk wind. Credit: Adapted from Leftley et al. 2021

Getting to the Heart of the Matter

Through careful analysis and modeling, Leftley and collaborators interpret their observations on scales of less than a light-year (for an object that’s hundreds of millions of light-years away!). The result? The near-infrared observations are consistent with an extended, equatorially aligned hot dust disk. The scale of this disk neatly matches the size predicted in disk–wind models.

Though the data are still too sparse and noisy to rule out the torus model in favor of the disk–wind model, these observations represent an important step in understanding how dust may be distributed in the heart of active galaxies.

Citation

“Resolving the Hot Dust Disk of ESO323-G77,” James H. Leftley et al 2021 ApJ 912 96 6.
doi:10.3847/1538-4357/abee80

By Susanna Kohler

Source: American Astronomical Society (AAS/Nova)

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Mystery of Galaxy's Missing Dark Matter Deepens

This Hubble Space Telescope snapshot reveals an unusual "see-through" galaxy. The giant cosmic cotton ball is so diffuse and its ancient stars so spread out that distant galaxies in the background can be seen through it. Called an ultra-diffuse galaxy, this galactic oddball is almost as wide as the Milky Way, but it contains only 1/200th the number of stars as our galaxy. The ghostly galaxy doesn't appear to have a noticeable central region, spiral arms, or a disk. Researchers calculated a more accurate distance to the galaxy, named NGC 1052-DF2, or DF2, by using Hubble to observe about 5,400 aging red giant stars. Red giant stars all reach the same peak brightness, so they are reliable yardsticks to measure distances to galaxies. The research team estimates that DF2 is 72 million light-years from Earth. They say the distance measurement solidifies their claim that DF2 lacks dark matter, the invisible glue that makes up the bulk of the universe's contents. The galaxy contains at most 1/400th the amount of dark matter that the astronomers had expected. The observations were taken between December 2020 and March 2021 with Hubble's Advanced Camera for Surveys.  Credits: SCIENCE: NASA, ESA, STScI, Zili Shen (Yale), Pieter van Dokkum (Yale), Shany Danieli (IAS) IMAGE PROCESSING: Alyssa Pagan (STScI). Hi-res File

When astronomers using NASA's Hubble Space Telescope uncovered an oddball galaxy that looked like it didn't have much dark matter, some thought the finding was hard to believe and looked for a simpler explanation.

Dark matter, after all, is the invisible glue that makes up the bulk of the universe's matter. All galaxies appear to be dominated by it; in fact, galaxies are thought to form inside immense halos of dark matter.

 So, finding a galaxy lacking the invisible stuff is an extraordinary claim that challenges conventional wisdom. It would have the potential to upset theories of galaxy formation and evolution.

To bolster their original finding, first reported in 2018 (Dark Matter Goes Missing in Oddball Galaxy (hubblesite.org)), a team of scientists led by Pieter van Dokkum of Yale University in New Haven, Connecticut, followed up their initial study with a more robust Hubble look at the galaxy, named NGC 1052-DF2. Scientists refer to it simply as "DF2."

"We went out on a limb with our initial Hubble observations of this galaxy in 2018," van Dokkum said. "I think people were right to question it because it's such an unusual result. It would be nice if there were a simple explanation, like a wrong distance. But I think it's more fun and more interesting if it actually is a weird galaxy."

Determining the amount of the galaxy's dark matter hinges on accurate measurements of how far away it is from Earth.

If DF2 is as far from Earth as van Dokkum's team asserts, the galaxy's dark-matter content may only be a few percent. The team's conclusion is based on the motions of the stars within the galaxy; their velocities are influenced by the pull of gravity. The researchers found that the observed number of stars accounts for the galaxy's total mass, and there's not much room left for dark matter.

However, if DF2 were closer to Earth, as some astronomers claim, it would be intrinsically fainter and less massive. The galaxy, therefore, would need dark matter to account for the observed effects of the total mass.

This Hubble Space Telescope image offers a sampling of aging, red stars in the ultra-diffuse galaxy NGC 1052-DF2, or DF2. The galaxy continues to puzzle astronomers because it is lacking dark matter, an invisible form of matter that provides the gravitational glue to hold galaxies together. Precisely establishing the galaxy’s distance form Earth is a step toward solving the mystery. The close-up at right reveals the many aging red giant stars on the outskirts of the galaxy that are used as intergalactic milepost markers. Researchers calculated a more accurate distance to DF2 by using Hubble to observe about 5,400 red giants. These older stars all reach the same peak brightness, so they are reliable yardsticks to measure distances to galaxies. The research team estimates that DF2 is 72 million light-years from Earth. They say the distance measurement solidifies their claim that DF2 lacks dark matter. The galaxy contains at most 1/400th the amount of dark matter that the astronomers had expected, based on theory and observations of many other galaxies. Called an ultra-diffuse galaxy, the galactic oddball is almost as wide as the Milky Way, but it contains only 1/200th the number of stars as our galaxy. The ghostly galaxy doesn't appear to have a noticeable central region, spiral arms, or a disk. The observations were taken between December 2020 and March 2021 with Hubble's Advanced Camera for Surveys. Credits: SCIENCE: NASA, ESA, STScI, Zili Shen (Yale), Pieter van Dokkum (Yale), Shany Danieli (IAS) IMAGE PROCESSING: Alyssa Pagan (STScI). Hi-Res File

 A Better Yardstick

Team member Zili Shen, from Yale University, says that the new Hubble observations help them confirm that DF2 is not only farther from Earth than some astronomers suggest, but also slightly more distant than the team's original estimates.

The new distance estimate is that DF2 is 72 million light-years as opposed to 42 million light-years, as reported by other independent teams. This places the galaxy farther than the original Hubble 2018 estimate of 65 light-years distance.

The research team based its new result on long exposures with Hubble's Advanced Camera for Surveys, which provide a deeper view of the galaxy for finding a reliable yardstick to nail down the distance. They targeted aging red giant stars on the outskirts of the galaxy that all reach the same peak brightness in their evolution. Astronomers can use the stars' intrinsic brightness to calculate vast intergalactic distances. "Studying the brightest red giants is a well-established distance indicator for nearby galaxies," Shen explained.

The more accurate Hubble measurements solidify the researchers' initial conclusion of a galaxy deficient in dark matter, team members say. So the mystery of why DF2 is missing most of its dark matter still persists.

"For almost every galaxy we look at, we say that we can't see most of the mass because it's dark matter," van Dokkum explained. "What you see is only the tip of the iceberg with Hubble. But in this case, what you see is what you get. Hubble really shows the entire thing. That's it. It’s not just the tip of the iceberg, it's the whole iceberg."

The team's science paper has appeared in The Astrophysical Journal Letters.

 A Stealthy Galaxy

DF2 is a giant cosmic cotton ball that van Dokkum calls a "see-through galaxy," where the stars are spread out. The galactic oddball is almost as wide as the Milky Way, but it contains only 1/200th the number of stars as our galaxy.

The ghostly galaxy doesn't appear to have a noticeable central region, spiral arms, or a disk. The team estimates that DF2 contains at most 1/400th the amount of dark matter than astronomers had expected. How the galaxy formed remains a complete mystery based on the team's latest measurements.

When astronomers using NASA’s Hubble Space Telescope uncovered an oddball galaxy that looks like it doesn’t have much dark matter, some thought the finding was hard to believe and looked for a simpler explanation. Dark matter, after all, is the invisible glue that makes up the bulk of the universe’s contents. All galaxies are dominated by it; in fact, galaxies are thought to form inside immense halos of dark matter. So, finding a galaxy lacking the invisible stuff is an extraordinary claim that challenges conventional wisdom. It would have the potential to upset theories of galaxy formation and evolution. Credits: NASA's Goddard Space Flight Center. YouTube

DF2 isn't the only galaxy devoid of dark matter. Shany Danieli of the Institute for Advanced Study in Princeton, New Jersey, used Hubble in 2020 to obtain an accurate distance to another ghostly galaxy, called NGC 1052-DF4 (or simply DF4), which apparently lacks dark matter, too. In this case, however, some scientists suggest the dark matter may have been stripped out of the galaxy due to tidal forces from another galaxy.

The researchers think both DF2 and DF4 were members of a collection of galaxies. However, the new Hubble observations show that the two galaxies are 6.5 million light-years away from each other, farther apart than they first thought. It also appears that DF2 has drifted away from the grouping and is isolated in space.

Both galaxies were discovered with the Dragonfly Telephoto Array at the New Mexico Skies observatory.

"Both of them probably were in the same group and formed at the same time," Danieli said. "So maybe there was something special in the environment where they were formed."

The researchers are hunting for more of these oddball galaxies. Other teams of astronomers are searching, too. In 2020, a group of researchers uncovered 19 unusual dwarf galaxies they say are deficient in dark matter (Off the Baryonic Tully–Fisher Relation: A Population of Baryon-dominated Ultra-diffuse Galaxies – IOPscience). However, it will take uncovering many more dark matter-less galaxies to resolve the mystery.

Nevertheless, van Dokkum thinks finding a galaxy lacking dark matter tells astronomers something about the invisible substance. "In our 2018 paper, we suggested that if you have a galaxy without dark matter, and other similar galaxies seem to have it, that means that dark matter is actually real and it exists," van Dokkum said. "It's not a mirage."

The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.

Release Credit: NASA, ESA, STScI

Media Contact:

Claire Andreoli
NASA's Goddard Space Flight Center, Greenbelt, Maryland

Donna Weaver
Space Telescope Science Institute, Baltimore, Maryland

Ray Villard
Space Telescope Science Institute, Baltimore, Maryland

Science Contact:

Zili Shen
Yale University, New Haven, Connecticut

Shany Danieli
Institute for Advanced Study, Princeton, New Jersey

Pieter van Dokkum
Yale University, New Haven, Connecticut

Editor: Lynn Jenner

 Source: NASA/Galaxies

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