VLA Reveals Dramatic New Evidence About Star, Planet Formation

In this artist's conception, a widely-separated pair of young, still-forming stars is in the background, forming by fragmentation of the material in the larger cloud in which they are born. In the foreground, companions in a multiple-star system are forming through fragmentation of a dusty disk that surrounds the original young star. Credit: Bill Saxton, NRAO/AUI/NSF. Hi-res image

A young double-star system in the Perseus Molecular Cloud, imaged with the VLA. This pair would fit within the orbit of Neptune in our Solar System. 

Credit: Tobin, et al., NRAO/AUI/NSF. Hi-res image

A young triple-star system in the Perseus Molecular Cloud, imaged with the VLA. 

Credit: Tobin et al., NRAO/AUI/NSF. Hi-res image

Disks of material surrounding young stars in the Perseus Molecular Cloud, imaged with the VLA. Arrows indicate the direction of outflows from the young systems. Credit: Segura-Cox, et al., NRAO/AUI/NSF. Hi-res image

A detailed study of young stars and their surroundings has produced dramatic new evidence about how multiple-star systems form and how the dusty disks that are the raw material for planets grow around young stars. Teams of scientists used the National Science Foundation's Karl G. Jansky Very Large Array (VLA) radio telescope to study nearly 100 newborn stars in a cloud of gas and dust about 750 light-years from Earth, in which new stars are forming.

Images made from the study showed unprecedented detail of a number of the young stars, and are helping astronomers resolve important questions about how stars, binary stars, and planets get their starts. The astronomers presented their results to the American Astronomical Society's meeting in Kissimmee, Florida.

Looking at young multiple-star systems, one team concluded that two different formation mechanisms may be at work to produce such systems. They noted that the systems they studied fall into two distinct types, based on the distance between the stars in the system. The closer systems have stars separated by about 75 times the Sun-Earth distance, and another group has its stars separated by about 3,000 times the Sun-Earth distance. They also found that more than half of the youngest stars they studied are in multiple systems, suggesting that star formation tends to produce multiples rather than single stars.

"Several different processes have been suggested for how multiple-star systems form, and our results indicate that the separation between stars may tell us which of these processes is responsible for a particular system," said John Tobin, of Leiden Observatory in the Netherlands.

Stars form in giant clouds of gas and dust, when tenuous material in such clouds collapses gravitationally into cores that then begin to draw additional material inward. Infalling material forms a rotating disk around the young star. Eventually, the young star gathers enough mass to create the temperatures and pressures at its center that will trigger thermonuclear reactions. The rotating disk around the star provides the material from which planets may form.

The researchers concluded that the more widely-separated multiple-star systems form through turbulent fragmentation of the larger cloud, while the closer systems are the result of fragmentation within the disk of material orbiting the original protostar. They also found that somewhat older systems have fewer widely-separated companions than the youngest group of protostars. This, they said, suggests that perhaps some young stars that form as widely-separated systems are not gravitationally bound and simply drift apart over time.

Another team, led by Dominique Segura-Cox, of the University of Illinois, found that the dusty disks around some of the protostars are larger than some theoretical models predict. These disks are essential to the formation of planets, some binary companions, and the young star's ability to draw in additional material. Despite their central role in these processes, however, their formation mechanisms have been debated among astronomers.

As material falls inward toward a young star, it pulls magnetic fields along with it. Theorists suggested that these fields, which become stronger as they are concentrated closer to the star, could be aligned so that they drastically slow the disk's rotation, limiting the size of the disk. Theoretical models predicted that this effect, called magnetic braking, would limit the disks to a radius about 10 times the Earth-Sun distance, or slightly more than the distance from the Sun to Saturn.

"We found disks with radii that are at least 15-30 times the Earth-Sun distance, significantly larger than the magnetic-braking model would allow," Segura-Cox said. "This is a lower limit, and the disks may actually be larger. Studies of other systems have indicated that disks are larger when observed at radio frequencies different than the ones we used in this project," she added.

One explanation for the larger disk sizes may be that, in some systems, the magnetic field and the rotation axis of the star are misaligned, a configuration that reduces the magnetic-braking effect. Evidence for this has been seen in some objects, the researchers said.

In another study published last December, a team using data from the same project found that the material falling toward one protostar is twisting the magnetic field lines and changing their configuration as it drags them inward. That study, which measured the magnetic-field alignments near the star, indicates one mechanism for minimizing the magnetic-braking effect.

"These observations of disks around such young stars suggests that all the elements needed for planet formation are present very early in the life of a star. Plus, it is probable that there are already centimeter-sized particles in these young disks, meaning that the growth of solids progresses rapidly," Tobin said.

The images for this work came from a project called the VLA Nascent Disk and Multiplicity (VANDAM) Survey. This survey used 264 hours of VLA observing time from 2013 to 2015 to study protostars in the Perseus Molecular Cloud, about 750 light-years distant. The Perseus Molecular Cloud, containing as much material as 10,000 suns, is one of the closest regions where low- to intermediate-mass stars are actively forming, and thus serves as a valuable "laboratory" for astronomers seeking to understand star formation.

"This survey sampled the largest number of young stars, and revealed fainter objects than we could study previously, and did so in greater detail. The information it provided has dramatically improved our knowledge," Tobin said.

"The disks we studied are difficult to observe as they are obscured by the cloud in which they are forming, but these new VLA observations reveal the disks and provide critical data into their formation mechanism,” Segura-Cox said.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

*  *  *

Media Contact: 

Dave Finley, Public Information Officer
(575) 835-7302
dfinley@nrao.edu
  

Source: National Radio Astronomy Observatory (NRAO)

Dwarf Galaxy Sends Ripples Through Milky Way’s Suburbs: A New Way to Explore Dark Matter

Computer simulation of gas distribution (left) and stars (right) after the Milky Way is perturbed by the dwarf satellite. 

Download the entire animation here.

Ripples in gas at the outer disk of our galaxy have puzzled astronomers since they were first revealed by radio observations a decade ago. Now, astronomers believe they have found the culprit – a dwarf galaxy, containing dark, unseen material, which skimmed the outskirts of our galaxy a few hundred million years ago. 

The research, led by Sukanya Chakrabarti of the Rochester Institute of Technology, presents the first plausible explanation for the galactic ripples. “It’s a bit like throwing a stone into a pond and making ripples,” said Chakrabarti at today’s press conference at the 227th meeting of the American Astronomical Society in Kissimmee, Florida. 

“Of course we aren’t talking about a pond, but our galaxy, which is tens of thousands of light years across, and made of stars and gas, but the result is the same – ripples!” Chakrabarti adds that this work is part of a new discipline called galactoseismology, “This is really the first non-theoretical application of this field, where we can infer things about the unseen composition of galaxies from analyzing galactic-quakes.” 

To reach their conclusion the research team studied a trio of stars, called Cepheid variables, which are part of the likely dwarf galaxy now estimated to lie about 300,000 light years away from our galaxy in the direction of the constellation Norma. “We have a pretty good idea of the distance to these stars because the intrinsic brightness of Cepheid variable stars depends on their period of pulsation, which we can measure,” says Chakrabarti. “What I wanted to know was how fast this speeding bullet was going when it passed by our galaxy – with that information we can begin to understand the dynamics, and ultimately how much unseen dark matter is there.” 

To do that, Chakrabarti and her team focused on three Cepheids in the tiny galaxy. Using spectroscopic observations obtained at the Gemini Observatory (as well as the Magellan Telescope, and the WiFeS spectrograph) the researchers found that the stars are all speeding away at similar velocities – about 450,000 mph (~ 200 kilometers/second). “This really implicates these stars as being part of an organized, fast-moving system which we believe is a dwarf galaxy. It’s also very likely that this dwarf satellite brushed our galaxy millions of years ago and left ripples in its wake,” said Chakrabarti. 

“This new, potentially powerful way to study how stars, gas and dust are distributed in galaxies is really quite exciting,” said Chris Davis, program director at the U.S. National Science Foundation that funds roughly 65% of Gemini as part of its international partnership, as well as this research program. "Known as galactoseismology, it can trace both visible and invisible materials, including the elusive dark matter. It’s a great way to better understand how galaxies and neighboring satellite dwarf galaxies interact as well." 

Gemini Observatory astronomer Rodolfo Angeloni oversaw the observations at the Gemini South telescope in Chile. He adds that Gemini South is uniquely well-equipped to make these types of observations. “The combination of Gemini’s silver-coated mirror and the versatility of the infrared spectrograph Flamingos-2 really made this work possible.” However, he continues, “These were especially faint and remote targets – we really had to push the limits." 

The team plans to continue this work by looking for more Cepheid variable stars in our galaxy’s halo. “There could be a population of yet undiscovered Cepheid variables that formed from a gas-rich dwarf galaxy falling into our galaxy’s halo,” said Chakrabarti. “With the capabilities of today’s telescopes and instruments we should be able to sample enough of the Milky Way’s halo to make reasonable estimates on dark matter content - one of the greatest mysteries in astronomy today!” 

The international research team includes Rodolfo Angeloni, Ken Freeman, Leo Blitz, among others, and RIT research scientist Benjamin Sargent and Andrew Lipnicky, a graduate student in the astrophysical sciences and technology program. The Gemini observations were made possible by an award of Director’s Discretionary Time, and the research was funded by NSF research grant #1517488. 

Additional background on this research on TED_x talk by Principal Investigator at: https://www.youtube.com/watch?v=I9tel-ZCswM. 

Science Contacts:

• Sukanya Chakrabarti
  Rochester Institute of Technology
  chakrabarti@astro.rit.edu


• Rodolfo Angeloni
  Gemini Observatory
  Gemini South Telescope, Chile
  rangelon@gemini.edu

Media Contacts:

• Peter Michaud
  Gemini Observatory
  Desk: +1 (808) 974-2510
  Cell: +1 (808) 936-6643
  pmichaud@gemini.edu


• Alexis-Ann Acohido
  Media Relations Intern
  Gemini Observatory
  Desk: +1 (808) 974-2528
  aacohido@gemini.edu

Source: Gemini Observatory

NuSTAR Finds Cosmic Clumpy Doughnut Around Black Hole

Galaxy NGC 1068 can be seen in close-up in this view from NASA's Hubble Space Telescope. NuSTAR's high-energy X-rays eyes were able to obtain the best view yet into the hidden lair of the galaxy's central, supermassive black hole. Image credit: NASA/JPL-Caltech.  › Full image and caption 

The most massive black holes in the universe are often encircled by thick, doughnut-shaped disks of gas and dust. This deep-space doughnut material ultimately feeds and nourishes the growing black holes tucked inside.

Until recently, telescopes weren't able to penetrate some of these doughnuts, also known as tori.

"Originally, we thought that some black holes were hidden behind walls or screens of material that could not be seen through," said Andrea Marinucci of the Roma Tre University in Italy, lead author of a new Monthly Notices of the Royal Astronomical Society study describing results from NASA's Nuclear Spectroscopic Telescope Array, or NuSTAR, and the European Space Agency's XMM-Newton space observatory.

With its X-ray vision, NuSTAR recently peered inside one of the densest of these doughnuts known to surround a supermassive black hole. This black hole lies at the center of a well-studied spiral galaxy called NGC 1068, located 47 million light-years away in the Cetus constellation.

The observations revealed a clumpy, cosmic doughnut.

"The rotating material is not a simple, rounded doughnut as originally thought, but clumpy," said Marinucci.

Doughnut-shaped disks of gas and dust around supermassive black holes were first proposed in the mid-1980s to explain why some black holes are hidden behind gas and dust, while others are not. The idea is that the orientation of the doughnut relative to Earth affects the way we perceive a black hole and its intense radiation. If the doughnut is viewed edge-on, the black hole is blocked. If the doughnut is viewed face-on, the black hole and its surrounding, blazing materials can be detected. This idea is referred to as the unified model because it neatly joins together the different black hole types, based solely upon orientation.

In the past decade, astronomers have been finding hints that these doughnuts aren't as smoothly shaped as once thought. They are more like defective, lumpy doughnuts that a doughnut shop might throw away.

The new discovery is the first time this clumpiness has been observed in an ultra-thick doughnut, and supports the idea that this phenomenon may be common. The research is important for understanding the growth and evolution of massive black holes and their host galaxies.

"We don't fully understand why some supermassive black holes are so heavily obscured, or why the surrounding material is clumpy," said co-author Poshak Gandhi of the University of Southampton in the United Kingdom. "This is a subject of hot research."

Both NuSTAR and XMM-Newton observed the supermassive black hole in NGC 1068 simultaneously on two occasions between 2014 to 2015. On one of those occasions, in August 2014, NuSTAR observed a spike in brightness. NuSTAR observes X-rays in a higher-energy range than XMM-Newton, and those high-energy X-rays can uniquely pierce thick clouds around the black hole. The scientists say the spike in high-energy X-rays was due to a clearing in the thickness of the material entombing the supermassive black hole.

"It's like a cloudy day, when the clouds partially move away from the sun to let more light shine through," said Marinucci.

NGC 1068 is well known to astronomers as the first black hole to give birth to the unification idea. "But it is only with NuSTAR that we now have a direct glimpse of its black hole through such clouds, albeit fleeting, allowing a better test of the unification concept," said Marinucci.

The team says that future research will address the question of what causes the unevenness in doughnuts. The answer could come in many flavors. It's possible that a black hole generates turbulence as it chomps on nearby material. Or, the energy given off by young stars could stir up turbulence, which would then percolate outward through the doughnut. Another possibility is that the clumps may come from material falling onto the doughnut. As galaxies form, material migrates toward the center, where the density and gravity is greatest. The material tends to fall in clumps, almost like a falling stream of water condensing into droplets as it hits the ground.

"We'd like to figure out if the unevenness of the material is being generated from outside the doughnut, or within it," said Gandhi.

"These coordinated observations with NuSTAR and XMM-Newton show yet again the exciting science possible when these satellites work together," said Daniel Stern, NuSTAR project scientist at NASA's Jet Propulsion Laboratory in Pasadena, California.

For more information on NuSTAR, visit:  http://www.nasa.gov/nustar  - http://www.nustar.caltech.edu/

Media Contact

Whitney Clavin

Jet Propulsion Laboratory, Pasadena, California

818-354-4673

whitney.clavin@jpl.nasa.gov 

Source: JPL-Caltech

NASA's Fermi Space Telescope Sharpens its High-energy Vision

Major improvements to methods used to process observations from NASA's Fermi Gamma-ray Space Telescope have yielded an expanded, higher-quality set of data that allows astronomers to produce the most detailed census of the sky yet made at extreme energies. A new sky map reveals hundreds of these sources, including 12 that produce gamma rays with energies exceeding a trillion times the energy of visible light. The survey also discovered four dozen new sources that remain undetected at any other wavelength.

This image, constructed from more than six years of observations by NASA's Fermi Gamma-ray Space Telescope, is the first to show how the entire sky appears at energies between 50 billion (GeV) and 2 trillion electron volts (TeV). For comparison, the energy of visible light falls between about 2 and 3 electron volts. A diffuse glow fills the sky and is brightest in the middle of the map, along the central plane of our galaxy. The famous Fermi Bubbles, first detected in 2010, appear as red extensions north and south of the galactic center and are much more pronounced at these energies. Discrete gamma-ray sources include pulsar wind nebulae and supernova remnants within our galaxy, as well as distant galaxies called blazars powered by supermassive black holes. Labels show the highest-energy sources, all located within our galaxy and emitting gamma rays exceeding 1 TeV. Credits: NASA/DOE/Fermi LAT Collaboration. unlabeled image , labeled image

Watch Fermi scientists explain why they're so excited about Pass 8, a complete reprocessing of all data collected by the mission's Large Area Telescope. This analysis increased the LAT's sensitivity, widened its energy range, and effectively sharpened its view through improved backtracking of incoming gamma rays. Credits: NASA's Goddard Space Flight Center. Download the video in ultra-HD at NASA's Scientific Visualization Studio

Using 61,000 Pass 8 gamma rays collected over 80 months, Ajello and his colleagues constructed a map of the entire sky at energies ranging from 50 billion (GeV) to 2 trillion electron volts (TeV). For comparison, the energy of visible light ranges from about 2 to 3 electron volts.

Tour the best view of the high-energy gamma-ray sky yet seen. This video highlights the plane of our galaxy and identifies objects producing gamma rays with energies greater than 1 TeV. Credits: NASA's Goddard Space Flight Center. Download the video in ultra-HD at NASA's Scientific Visualization Studio

"Of the 360 sources we cataloged, about 75 percent are blazars, which are distant galaxies sporting jets powered by supermassive black holes," said co-investigator Alberto Domínguez at the Complutense University in Madrid. "The highest-energy sources, all located in our galaxy, are mostly remnants of supernova explosions and pulsar wind nebulae, places where rapidly rotating neutron stars accelerate particles to near the speed of light." One famous example, the Crab Nebula, tops the list of the highest-energy Fermi sources, producing a steady drizzle of gamma rays exceeding 1 TeV.

Astronomers think these very high-energy gamma rays are produced when lower-energy light collides with accelerated particles. This results in a small energy loss for the particle and a big gain for the light, transforming it into a gamma ray.

Gamma-ray emission from the highest-energy sources detected by Fermi is likely produced by what scientists call the inverse Compton process. When an electron moving near the speed of light strikes a low-energy photon, the collision slightly slows the electron and boosts the light's energy into the gamma-ray regime. Credits: NASA's Goddard Space Flight Center

For the first time, Fermi data now extend to energies previously seen only by ground-based detectors. Because ground-based telescopes have much smaller fields of view than the LAT, which scans the whole sky every three hours, they have detected only about a quarter of the objects in the catalog. This study provides ground facilities with more than 280 new targets for follow-up observations.

"An exciting aspect of this catalog is that we find many new sources that emit gamma rays over a comparatively large patch of the sky," explained Jamie Cohen, a University of Maryland graduate student working with the Fermi team at NASA's Goddard Space Flight Center in Greenbelt. "Finding more of these objects enables us to probe their structures as well as better understand mechanisms that accelerate the subatomic particles that ultimately produce gamma-ray emission." The new catalog identifies 25 of these extended objects, including three new pulsar wind nebulae and two new supernova remnants.

Ajello presented the findings Thursday at the 227th meeting of the American Astronomical Society in Kissimmee, Florida. A paper describing the catalog has been accepted for publication in The Astrophysical Journal Supplement.

NASA's Fermi Gamma-ray Space Telescope is an astrophysics and particle physics partnership, developed in collaboration with the U.S. Department of Energy and with important contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden and the United States.

For more information about NASA's Fermi, visit:  www.nasa.gov/fermi

Francis Reddy
NASA's Goddard Space Flight Center, Greenbelt, Maryland

Source: NASA/Fermi Space Telescope

Globular Clusters Could Nurture Interstellar Civilizations

Globular star clusters like this one, 47 Tucanae, might be excellent places to search for interstellar civilizations. Their crowded nature means intelligent life at our stage of technological advancement could send probes to the nearest stars. Credit: NASA, ESA, and the Hubble Heritage Team.  High Resolution (jpg) - Low Resolution (jpg)

Kissimmee, FL - Globular star clusters are extraordinary in almost every way. They're densely packed, holding a million stars in a ball only about 100 light-years across on average. They're old, dating back almost to the birth of the Milky Way. And according to new research, they also could be extraordinarily good places to look for space-faring civilizations.

"A globular cluster might be the first place in which intelligent life is identified in our galaxy," says lead author Rosanne DiStefano of the Harvard-Smithsonian Center for Astrophysics (CfA).

DiStefano presented this research today in a press conference at a meeting of the American Astronomical Society.

Our Milky Way galaxy hosts about 150 globular clusters, most of them orbiting in the galactic outskirts. They formed about 10 billion years ago on average. As a result, their stars contain fewer of the heavy elements needed to construct planets, since those elements (like iron and silicon) must be created in earlier generations of stars. Some scientists have argued that this makes globular cluster stars less likely to host planets. In fact, only one planet has been found in a globular cluster to date.

However, DiStefano and her colleague Alak Ray (Tata Institute of Fundamental Research, Mumbai) argue that this view is too pessimistic. Exoplanets have been found around stars only one-tenth as metal-rich as our Sun. And while Jupiter-sized planets are found preferentially around stars containing higher levels of heavy elements, research finds that smaller, Earth-sized planets show no such preference.

"It's premature to say there are no planets in globular clusters," states Ray.

Another concern is that a globular cluster's crowded environment would threaten any planets that do form. A neighboring star could wander too close and gravitationally disrupt a planetary system, flinging worlds into icy interstellar space.

However, a star's habitable zone - the distance at which a planet would be warm enough for liquid water - varies depending on the star. While brighter stars have more distant habitable zones, planets orbiting dimmer stars would have to huddle much closer. Brighter stars also live shorter lives, and since globular clusters are old, those stars have died out. The predominant stars in globular clusters are faint, long-lived red dwarfs. 

Any potentially habitable planets they host would orbit nearby and be relatively safe from stellar interactions.

"Once planets form, they can survive for long periods of time, even longer than the current age of the universe," explains DiStefano.

So if habitable planets can form in globular clusters and survive for billions of years, what are the consequences for life should it evolve? Life would have ample time to become increasingly complex, and even potentially develop intelligence. 

Such a civilization would enjoy a very different environment than our own. The nearest star to our solar system is four light-years, or 24 trillion miles, away. In contrast, the nearest star within a globular cluster could be about 20 times closer - just one trillion miles away. This would make interstellar communication and exploration significantly easier.

"We call it the 'globular cluster opportunity,'" says DiStefano. "Sending a broadcast between the stars wouldn't take any longer than a letter from the U.S. to Europe in the 18th century."

"Interstellar travel would take less time too. The Voyager probes are 100 billion miles from Earth, or one-tenth as far as it would take to reach the closest star if we lived in a globular cluster. That means sending an interstellar probe is something a civilization at our technological level could do in a globular cluster," she adds.

The closest globular cluster to Earth is still several thousand light-years away, making it difficult to find planets, particularly in a cluster’s crowded core. But it could be possible to detect transiting planets on the outskirts of globular clusters. Astronomers might even spot free-floating planets through gravitational lensing, in which the planet’s gravity magnifies light from a background star.

A more intriguing idea might be to target globular clusters with SETI search methods, looking for radio or laser broadcasts. The concept has a long history: In 1974 astronomer Frank Drake used the Arecibo radio telescope to broadcast the first deliberate message from Earth to outer space. It was directed at the globular cluster Messier 13 (M13).

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

For more information, contact:

Christine Pulliam
Media Relations Manager
Harvard-Smithsonian Center for Astrophysics
617-495-7463
cpulliam@cfa.harvard.edu

Source: Harvard-Smithsonian Center for Astrophysics (CfA)

Supermassive and super-hungry

NGC 4845

Credit: ESA/Hubble & NASA and S. Smartt (Queen's University Belfast)

This NASA/ESA Hubble Space Telescope image shows the spiral galaxy NGC 4845, located over 65 million light-years away in the constellation of Virgo (The Virgin). The galaxy’s orientation clearly reveals the galaxy’s striking spiral structure: a flat and dust-mottled disc surrounding a bright galactic bulge.

NGC 4845’s glowing centre hosts a gigantic version of a black hole, known as a supermassive black hole. The presence of a black hole in a distant galaxy like NGC 4845 can be inferred from its effect on the galaxy’s innermost stars; these stars experience a strong gravitational pull from the black hole and whizz around the galaxy’s centre much faster than otherwise.

From investigating the motion of these central stars, astronomers can estimate the mass of the central black hole — for NGC 4845 this is estimated to be hundreds of thousands times heavier than the Sun. This same technique was also used to discover the supermassive black hole at the centre of our own Milky Way — Sagittarius A* — which hits some four million times the mass of the Sun (potw1340a).

The galactic core of NGC 4845 is not just supermassive, but also super-hungry. In 2013 researchers were observing another galaxy when they noticed a violent flare at the centre of NGC 4845. The flare came from the central black hole tearing up and feeding off an object many times more massive than Jupiter. A brown dwarf or a large planet simply strayed too close and was devoured by the hungry core of NGC 4845.

Source: ESA/Hubble - Space Telescope

IDCS J1426.5+3508: NASA's Great Observatories Weigh Massive Young Galaxy Cluster

IDCS J1426.5+3508

Credit X-ray: NASA/CXC/Univ of Missouri/M.Brodwin et al; 
Optical: NASA/STScI; Infrared: JPL/CalTech

JPEG (505.9 kb) - Large JPEG (4 MB) - Tiff (9.3 MB) - More Images

 

Astronomers have made the most detailed study yet of an extremely massive young galaxy cluster using three of NASA's Great Observatories, as described in our latest press release . This multi-wavelength image shows this galaxy cluster, called IDCS J1426.5+3508 (IDCS J1426 for short), in X-rays from the Chandra X-ray Observatory in blue, visible light from the Hubble Space Telescope in green, and infrared light from the Spitzer Space Telescope in red. 

This rare galaxy cluster, which is located 10 billion light years from Earth, weighs almost 500 trillion Suns. This object has important implications for understanding how these mega-structures formed and evolved early in the Universe. Astronomers have observed IDCS J1426 when the Universe was less than a third of its current age. It is the most massive galaxy cluster detected at such an early age.

First discovered by the Spitzer Space Telescope in 2012, IDCS J1426 was then observed using the Hubble Space Telescope and the Keck Observatory to determine its distance. Observations from the Combined Array for Millimeter Wave Astronomy indicated it was extremely massive. New data from the Chandra X-ray Observatory confirm the galaxy cluster mass and show that about 90% of the mass of the cluster is in the form of dark matter, the mysterious substance that has so far been detected only through its gravitational pull on normal matter composed of atoms. 

There is a region of bright X-ray emission (seen as blue-white) near the middle of the cluster, but not exactly at the center. The location of this "core" of gas suggests that the cluster has had a collision or interaction with another massive system of galaxies relatively recently, perhaps within about the last 500 million years. This would cause the core to "slosh" around like wine in a moving glass and become offset as it appears to be in the Chandra data. Such a merger would not be surprising, given that astronomers are observing IDCS J1426 when the Universe was only 3.8 billion years old. So that an enormous structure can form so rapidly, scientists think mergers with smaller clusters would likely play a role in a large cluster's growth.

This core, while still extremely hot, contains cooler gas than its surroundings. This is the most distant galaxy cluster where such a "cool core" of gas has been observed. Astronomers think these cool cores are important in understanding how quickly hot gas cools off in clusters, influencing the rate of stars at which stars are born. This cooling rate can be slowed down by outbursts from a supermassive black hole in the center of the cluster. Apart from the cool core, the hot gas in the cluster is remarkably symmetrical and smooth. This is another piece of evidence that IDCS J1426 formed very rapidly and quickly in the early Universe. Despite the high mass and rapid evolution of this cluster, its existence does not pose a threat to the standard model of cosmology.

These results were presented at the 227th meeting of the American Astronomical Society meeting being held in Kissimmee, Florida. A paper describing these results has been accepted for publication in The Astrophysical Journal and is available online. NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program for NASA's Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, controls Chandra's science and flight operations.

Fast Facts for IDCS J1426.5+3508:

Scale: Image is 1.9 arcmin across (about 3 million light years)
Category: Groups & Clusters of Galaxies
Coordinates (J2000): RA 14h 26m 28.80s | Dec +35° 07' 48.00"
Constellation: Boötes
Observation Date: 05 and 23 March 2014
Observation Time: 27 hours 47 min (1 day 3 hours 47 min)
Obs. ID: 15168, 16321
Instrument: ACIS
References: Brodwin, M. et al. 2015, ApJ (accepted); arXiv:1504.01397
Color Code: X-ray: Blue, Optical: Green, Infrared: Red
Distance Estimate: About 10 billion light years (z=1.75)

Source: NASA’s Chandra X-ray Observatory

Runaway Stars Leave Infrared Waves in Space

Bow shocks thought to mark the paths of massive, speeding stars are highlighted in these images from NASA's Spitzer Space Telescope and Wide-field Infrared Survey Explorer, or WISE. Image Credit: NASA/JPL-Caltech/University of Wyoming.  › Full image and caption

Astronomers are finding dozens of the fastest stars in our galaxy with the help of images from NASA's Spitzer Space Telescope and Wide-field Infrared Survey Explorer, or WISE.

When some speedy, massive stars plow through space, they can cause material to stack up in front of them in the same way that water piles up ahead of a ship. Called bow shocks, these dramatic, arc-shaped features in space are leading researchers to uncover massive, so-called runaway stars.

"Some stars get the boot when their companion star explodes in a supernova, and others can get kicked out of crowded star clusters," said astronomer William Chick from the University of Wyoming in Laramie, who presented his team's new results at the American Astronomical Society meeting in Kissimmee, Florida. "The gravitational boost increases a star's speed relative to other stars."

Our own sun is strolling through our Milky Way galaxy at a moderate pace. It is not clear whether our sun creates a bow shock. By comparison, a massive star with a stunning bow shock, called Zeta Ophiuchi (or Zeta Oph), is traveling around the galaxy faster than our sun, at 54,000 mph (24 kilometers per second) relative to its surroundings. Zeta Oph's giant bow shock can be seen in this image from the WISE mission: http://www.nasa.gov/mission_pages/WISE/multimedia/gallery/pia13455.html

 

Both the speed of stars moving through space and their mass contribute to the size and shapes of bow shocks. The more massive a star, the more material it sheds in high-speed winds. Zeta Oph, which is about 20 times as massive as our sun, has supersonic winds that slam into the material in front of it.

The result is a pile-up of material that glows. The arc-shaped material heats up and shines with infrared light. That infrared light is assigned the color red in the many pictures of bow shocks captured by Spitzer and WISE.

Chick and his team turned to archival infrared data from Spitzer and WISE to identify new bow shocks, including more distant ones that are harder to find. Their initial search turned up more than 200 images of fuzzy red arcs. They then used the Wyoming Infrared Observatory, near Laramie, to follow up on 80 of these candidates and identify the sources behind the suspected bow shocks. Most turned out to be massive stars. 

The findings suggest that many of the bow shocks are the result of speedy runaways that were given a gravitational kick by other stars. However, in a few cases, the arc-shaped features could turn out to be something else, such as dust from stars and birth clouds of newborn stars. The team plans more observations to confirm the presence of bow shocks.

"We are using the bow shocks to find massive and/or runaway stars," said astronomer Henry "Chip" Kobulnicky, also from the University of Wyoming. "The bow shocks are new laboratories for studying massive stars and answering questions about the fate and evolution of these stars."

Another group of researchers, led by Cintia Peri of the Argentine Institute of Radio Astronomy, is also using Spitzer and WISE data to find new bow shocks in space. Only instead of searching for the arcs at the onset, they start by hunting down known speedy stars, and then they scan them for bow shocks.

"WISE and Spitzer have given us the best images of bow shocks so far," said Peri. "In many cases, bow shocks that looked very diffuse before, can now be resolved, and, moreover, we can see some new details of the structures."

Some of the first bow shocks from runaway stars were identified in the 1980s by David Van Buren of NASA's Jet Propulsion Laboratory in Pasadena, California. He and his colleagues found them using infrared data from the Infrared Astronomical Satellite (IRAS), a predecessor to WISE that scanned the whole infrared sky in 1983.

Kobulnicky and Chick belong to a larger team of researchers and students studying bow shocks and massive stars, including Matt Povich from the California State Polytechnic University, Pomona. The National Science Foundation funds their research.

Images from Spitzer, WISE and IRAS are archived at the NASA Infrared Science Archive housed at the Infrared Processing and Analysis Center at the California Institute of Technology in Pasadena. Caltech manages JPL for NASA.

More information about Spitzer is online at: http://www.nasa.gov/spitzer -  http://spitzer.caltech.edu

More information about WISE is at: http://www.nasa.gov/wise


Media Contact

Whitney Clavin
Jet Propulsion Laboratory, Pasadena, California
818-354-4673
whitney.clavin@jpl.nasa.gov

Source: JPL-Caltech

NASA's Spitzer, Hubble Find "Twins" of Superstar Eta Carinae in Other Galaxies

Eta Carinae's great eruption in the 1840s created the billowing Homunculus Nebula, imaged here by Hubble, and transformed the binary into a unique object in our galaxy. Astronomers cannot yet explain what caused this eruption. The discovery of likely Eta Carinae twins in other galaxies will help scientists better understand this brief phase in the life of a massive star. Credits: NASA, ESA, and the Hubble SM4 ERO Team.

The nearby spiral galaxy M83 is currently the only one known to host two potential Eta Carinae twins. This composite of images from the Hubble Space Telescope's Wide Field Camera 3 instrument shows a galaxy ablaze with newly formed stars. A high rate of star formation increases the chances of finding massive stars that have recently undergone an Eta Carinae-like outburst. Bottom: Insets zoom into Hubble data to show the locations of M83's Eta twins. Credits: NASA, ESA, the Hubble Heritage Team (STScI/AURA) and R. Khan (GSFC and ORAU). Hi-res image

In a follow-on survey in 2015, the team found two candidate Eta twins in the galaxy M83, located 15 million light-years away, and one each in NGC 6946, M101 and M51, located between 18 and 26 million light-years away. These five objects mimic the optical and infrared properties of Eta Carinae, indicating that each very likely contains a high mass star buried in five to 10 solar masses of gas and dust. Further study will let astronomers more precisely determine their physical properties. The findings were published in the Dec. 20 edition of The Astrophysical Journal Letters.

Researchers found likely Eta twins in four galaxies by comparing the infrared and optical brightness of each candidate source. Infrared images from NASA's Spitzer Space Telescope revealed the presence of warm dust surrounding the stars. Comparing this information with the brightness of each source at optical and near-infrared wavelengths as measured by instruments on Hubble, the team was able to identify candidate Eta Carinae-like objects. Top: 3.6-micron images of candidate Eta twins from Spitzer's IRAC instrument. Bottom: 800-nanometer images of the same sources from various Hubble instruments. Credits: NASA, ESA, and R. Khan (GSFC and ORAU)

NASA's James Webb Space Telescope, set to launch in late 2018, carries an instrument ideally suited for further study of these stars. The Mid-Infrared Instrument (MIRI) has 10 times the angular resolution of instruments aboard Spitzer and is most sensitive at the wavelengths where Eta twins shine brightest. 

"Combined with Webb's larger primary mirror, MIRI will enable astronomers to better study these rare stellar laboratories and to find additional sources in this fascinating phase of stellar evolution," said Sonneborn, NASA's project scientist for Webb telescope operations. It will take Webb observations to confirm the Eta twins as true relatives of Eta Carinae.

The Spitzer Space Telescope is managed by NASA's Jet Propulsion Laboratory in Pasadena, California. The Spitzer Science Center at the California Institute of Technology in Pasadena conducts science operations. 

The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. NASA's Goddard Space Flight Center 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.

For more information about Spitzer, visit:  http://www.nasa.gov/spitzer
For more information about Hubble, visit: http://www.nasa.gov/hubble

Francis Reddy
NASA's Goddard Space Flight Center, Greenbelt, Maryland

 Source: NASA/Solar System and Beyond

Andromeda Galaxy Scanned with High-Energy X-ray Vision

NASA's Nuclear Spectroscope Telescope Array, or NuSTAR, has imaged a swath of the Andromeda galaxy -- the nearest large galaxy to our own Milky Way galaxy. Image credit: NASA/JPL-Caltech/GSFC.   › Full image and caption | Annotated Version | Inset

NASA's Nuclear Spectroscopic Telescope Array, or NuSTAR, has captured the best high-energy X-ray view yet of a portion of our nearest large, neighboring galaxy, Andromeda. The space mission has observed 40 "X-ray binaries" -- intense sources of X-rays comprised of a black hole or neutron star that feeds off a stellar companion.

The results will ultimately help researchers better understand the role of X-ray binaries in the evolution of our universe. According to astronomers, these energetic objects may play a critical role in heating the intergalactic bath of gas in which the very first galaxies formed.

"Andromeda is the only large spiral galaxy where we can see individual X-ray binaries and study them in detail in an environment like our own," said Daniel Wik of NASA Goddard Space Flight Center in Greenbelt, Maryland, who presented the results at the 227th meeting of American Astronomical Society in Kissimmee, Florida.¬¬¬¬ "We can then use this information to deduce what's going on in more distant galaxies, which are harder to see."

Andromeda, also known as M31, can be thought of as the big sister to our own Milky Way galaxy. Both galaxies are spiral in shape, but Andromeda is slightly larger than the Milky Way in size. Lying 2.5 million light-years away, Andromeda is relatively nearby in cosmic terms. It can even be seen by the naked eye in dark, clear skies.

Other space missions, such as NASA's Chandra X-ray Observatory, have obtained crisper images of Andromeda at lower X-ray energies than the high-energy X-rays detected by NuSTAR. The combination of Chandra and NuSTAR provides astronomers with a powerful tool for narrowing in on the nature of the X-ray binaries in spiral galaxies.

In X-ray binaries, one member is always a dead star or remnant formed from the explosion of what was once a star much more massive than the sun. Depending on the mass and other properties of the original giant star, the explosion may produce either a black hole or neutron star. Under the right circumstances, material from the companion star can "spill over" its outermost edges and then be caught by the gravity of the black hole or neutron star. As the material falls in, it is heated to blazingly high temperatures, releasing a huge amount of X-rays.

With NuSTAR's new view of a swath of Andromeda, Wik and colleagues are working on identifying the fraction of X-ray binaries harboring black holes versus neutron stars. That research will help them understand the population as a whole. 

"We have come to realize in the past few years that it is likely the lower-mass remnants of normal stellar evolution, the black holes and neutron stars, may play a crucial role in heating of the intergalactic gas at very early times in the universe, around the cosmic dawn," said Ann Hornschemeier of NASA Goddard, the principal investigator of the NuSTAR Andromeda studies.

"Observations of local populations of stellar-mass-sized black holes and neutron stars with NuSTAR allow us to figure out just how much power is coming out from these systems." 

The new research also reveals how Andromeda may differ from our Milky Way. Fiona Harrison, the principal investigator of the NuSTAR mission, added, "Studying the extreme stellar populations in Andromeda tells us about how its history of forming stars may be different than in our neighborhood."

Harrison will be presenting the 2015 Rossi Prize lecture at the AAS meeting. The prize, awarded by the AAS's High-Energy Astrophysics Division, honors physicist Bruno Rossi, an authority on cosmic-ray physics and a pioneer in the field of X-ray astronomy.

For more information about NuSTAR, visit:  http://www.nustar.caltech.edu/

Media Contact

Whitney Clavin
Jet Propulsion Laboratory, Pasadena, California
818-354-4673
whitney.clavin@jpl.nasa.gov

 Source: JPL-Caltech

NGC 5195: NASA's Chandra Finds Supermassive Black Hole Burping Nearby

 NGC 5194
Credit  X-ray: NASA/CXC/Univ of Texas/E.Schlegel et al; Optical: NASA/STScI
 JPEG (660.4 kb) - Large JPEG (9.1 MB)  - Tiff (17.6 MB) - More Images
 

 X-ray close-up

  Credit  X-ray: NASA/CXC/Univ of Texas/E.Schlegel et al

Astronomers have used NASA's Chandra X-ray Observatory to discover one of the nearest supermassive black holes to Earth that is currently undergoing powerful outbursts, as described in our latest press release. This galactic burping was found in the Messier 51 galaxy, which is located about 26 million light years from Earth and, contains a large spiral galaxy NGC 5194 (also known by its nickname of the "Whirlpool"), merging with a smaller companion galaxy NGC 5195.

This main panel of this graphic shows M51 in visible light data from the Hubble Space Telescope (red, green, and blue). The box at the top of the image outlines the field of view by Chandra in the latest study, which focuses on the smaller component of M51, NGC 5195. 

The inset to the right shows the details of the Chandra data (blue) of this region. Researchers found a pair of arcs in X-ray emission close to the center of the galaxy, which they interpret as two outbursts from the galaxy's supermassive black hole (mouse over annotated image for additional information). The authors estimate that it took about one to three million years for the inner arc to reach its current position, and three to six million years for the outer arc.

Just outside the outer X-ray arc is a slender region of hydrogen emission detected in an optical image. This suggests that the X-ray emitting gas has "snow-plowed" or swept-up the hydrogen gas from the center of the galaxy. This is a clear case where a supermassive black hole is affecting its host galaxy, in a phenomenon that astronomers called "feedback." 

This arc of hydrogen gas contains what appears to be two or three small "HII regions." An HII (pronounced "H-two") region is created when the radiation from hot, young stars strips away the electrons from neutral hydrogen atoms (HI) to form clouds of ionized hydrogen (HII). This suggests that the outer arc has plowed up enough material to trigger the formation of new stars.

The outbursts of the supermassive black hole in NGC 5195 may have been triggered by the interaction of this galaxy with the large spiral galaxy in M51, causing gas to be disrupted and then funneled down towards the black hole.

These results were presented at the 227th meeting of the American Astronomical Society meeting in Kissimmee, Florida. They are also in a paper submitted to The Astrophysical Journal and the authors are Eric Schlegel (University of Texas at San Antonio), Christine Jones (Harvard-Smithsonian Center for Astrophysics), Marie Marachek (CfA), and Laura Vega (Fisk University and Vanderbilt University Bridge Program).

NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program for NASA's Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, controls Chandra's science and flight operations. Swift is managed by NASA's Goddard Space Flight Center in Greenbelt, Maryland.

Fast Facts for NGC 5195:

Scale: Main image is about 6 x 10 arcmin (About 45,000 x 76,000 light years) Pullout image is 3 arcmin across (about 23,000 light years)
Category: Normal Galaxies & Starburst Galaxies
Coordinates (J2000): RA 13h 29m 59.9s | Dec +47° 15' 58.00"
Constellation: Canes Venatici
Observation Date: 11 pointings between Mar 2000 and Oct 2012
Observation Time: 232 hours 10 min. (9 days 16 hours 10 min)
Obs. ID: 353,354,1622,3932,13812-13816,15496,15553
Instrument: ACIS
References: Schlegel, E. et al, AAS 227, 5-8 Jan 2016
Color Code: X-ray (Blue); Optical (Red, Green, Blue)
Distance Estimate: About 26 million light years

Source: NASA’s Chandra X-ray Observatory

No Fireworks in the Galactic Center

A false-color infrared image of the region around the nucleus of our Milky Way, the supermassive black hole SagA*. The marker shows the location of the black hole, which glows faintly due to accretion of material; the other objects are stars or dense clouds either orbiting the black hole or in its general vicinity. The size scale of the image end is about one light-year. Credit: Stefan Gillessen and the Max-Planck Institute fur Extraterrestrische Physik

The center of our Milky Way galaxy, about twenty-five thousand light years from Earth, is invisible to us in optical light because of the extensive amounts of absorbing, intervening dust. Radiation at many other wavelengths, however, including the infrared, radio, and energetic X-rays, can penetrate the veiling material. At the heart of the galactic center is a supermassive black hole, SagA*, with about four million solar-masses of material. It is a relatively dim object, and shows some slight flickering that is thought to be the result of small blobs of material randomly accreting onto a disk around it. Its general passivity distinguishes SagA* from many other supermassive black holes in other galactic centers that actively accrete and heat large amounts of material, and then eject powerful bipolar jets of fast-moving charged particles.

A few years ago astronomers spotted a large cloud of gas (estimated to be three Earth-masses in size) moving relatively quickly towards SagA*. Some models projected that the cloud (known as G2) would be disrupted by the black hole during 2015, an event that might be accompanied by detectable radiation that could in turn shed light on a black hole's feeding mechanisms. That did not happen; the year passed without any fireworks, possibly because G2 was too dense to break up.

CfA astronomer Michael McCourt and his colleague have been able to make the most out of this recent non-event. They recognized that the ongoing X-ray emission from SgrA* implied an inflow rate of about a few Earth-masses per year from ambient material, but that this rate was inconsistent with almost every other measurement, including among other things the total luminosity of SgrA*. Sorting out the possible solutions required knowing the distribution of the gas very near the black hole - distances less than the distance of the Earth from the Sun. The scientists realized that they could use the changes in the orbit of G2 as it passed through this medium to probe the innermost gas. Even though the object was not devoured as expected, its path would be altered. They also took advantage of a second small cloud in the system, G1, to pin down some parameters as the two traveled during the year along highly eccentric, nearly co-planar orbits around SgrA*.

The scientists modeled slight changes in the orbital parameters of G1 and G2 as they moved, assuming the changes were due to encounters with the local material. Their analysis provides the first determination of the rotation axis of the accretion flow, and points to the source of the accretion as being from the large torus of molecular gas that is about 4 light-years from the black hole, rather than from the winds of stars that are present in the intervening volume. The result is an important clue to the0 nature of the black hole’s environment, and moreover it has some observational consequences that might be tested in the next decade, including the future paths of G2 and G2 and the geometry of the emission close to the black hole's boundary.

Reference(s):

"Going with the flow: using gas clouds to probe the accretion flow feeding Sgr A∗," Michael McCourt and Ann-Marie Madigan, MNRAS, 455, 2187, 2015.

Source:  Harvard-Smithsonian Center for Astrophysics (CfA)

The diversity of stellar halos in massive disk galaxies

Figure 1: The observed stellar halo of NGC 474 showing the complex substructure present in the outer regions of this large elliptical galaxy. Shells and tidal stellar streams are formed as a consequence of the interactions with and accretion of smaller galaxies. Image credit: P.-A. Duc (CEA), J.-C. Cuillandre (CFHT) et al. 2013, IAUS, 295, 358

The stellar halos of galaxies are diffuse and faint components which provide scientists with a window into the assembling history of galaxies. A research team at MPA has investigated the properties of stellar halos in large disk galaxies by using both observations and state-of-the-art simulations of galaxy formation. They find a great diversity in the halo properties for galaxies that are – otherwise – alike in terms of morphology, mass, and luminosity. Observed properties, such as a mean metallicity as a function of galactocentric distance, can be reproduced by the simulations if they are analyzed in the same way as the data. 

Stellar halos are diffuse and faint components surrounding most large galaxies. They form largely from the accretion and disruption of smaller satellite galaxies, although their inner regions also contain stars which were formed in the disk of their host galaxy. Stellar halos are highly important to understand galaxy formation and evolution since they preserve a record of the accretion history of galaxies and serve as probes of the total matter distribution of galaxies. However, stellar halos are far less luminous than the main galactic disk. Observing them is therefore an extremely challenging task.

Over the past decades, different approaches have been developed to reveal this diffuse component of galaxies. Observations of galaxies with very long exposure times have produced stunning images, an example of which is shown in Figure 1. These observations helped characterize the role of accretion and mergers in shaping the outskirts of galaxies as well as unveil their vast extent, reaching distances larger than 100 kpc from their centre. 

Figure 2: Optical image of one of the galaxies in the GHOSTS sample, NGC 4945. Yellow squares indicate Hubble Space Telescope (HST) observations obtained in the stellar halo of this galaxy. © GHOSTS project

To study the properties of stellar halos in detail, however, the astronomers need to observe their constituent stars. By doing so, it is possible to estimate the mean age and chemical composition (called metallicity, when all elements but hydrogen and helium are considered) of the halo stellar population as a function of distance from the galactic centre. Until recently, studies of halo stars were confined largely to our own Milky Way and the neighbouring Andromeda galaxy, due to the proximity of their stars. However, during the past years the GHOSTS survey has extended this study to nearby disk galaxies, thanks to the exquisite resolution of the Hubble Space Telescope (Figure 2).

In her recent work, MPA scientist Antonela Monachesi has studied individual stars in the halos of nearby galaxies similar to the Milky Way using GHOSTS observations. This study showed that there is a great diversity in the metallicity of the observed stellar halos. In addition, the way this metallicity depends on the distance from the galactic centre also varies from galaxy to galaxy. In half the currently analyzed galactic halos the mean metallicity decreases with galactocentric distance, whereas the other half shows no significant variation with distance.

Moreover, the results from the GHOSTS observations showed that stellar halo properties differ greatly among galaxies that are otherwise alike in morphology, mass, and luminosity. The process of merger and accretion of satellites is rather stochastic and the properties of the stellar halos reflect the properties of the different accreted satellite galaxies.

The variety in the observed metallicity behaviour of stellar halos, with half galaxies presenting flat metallicity trends with distance, appeared to be at odds with previous results from hydrodynamical simulations, which had predicted large metallicity variations with distance as ubiquitous features. In order to investigate this seeming discrepancy, the research team at MPA used state-of-the art hydrodynamical cosmological simulations called “Auriga”. These simulations include the main physical processes responsible for the formation and evolution of galaxies in the universe and self-consistently follow the evolution of gas, stars and dark matter over time.

Figure 3: Metallicity trends of stellar halos as a function of galactocentric distance for two simulated galaxies using the Auriga simulations. This figure shows how the metallicity behaviour of stellar halos differs as a function of galactocentric distance if the metallicities are measured by averaging the properties of stars in concentric spherical shells (black), by projecting along a direction that is perpendicular to the galactic disk (blue), or on the disk plane (red). Dashed lines are linear fits to the black and blue lines to show trends more clearly. The blue lines are much flatter than the other two, showing that the metallicity variations will be much less, when averaging perpendicular to the disk – the method preferred by observations. © MPA

The MPA group analyzed the metallicity trends in stellar halos of simulated galaxies with similar mass and morphology as the Milky Way and the GHOSTS galaxies. As the main result, the scientists find from this analysis that the behaviour of the mean stellar halo metallicity with galactocentric distance depends strongly on the way it is measured. If the mean metallicities are derived by spherically averaging the properties of halo stars around the galactic center, as done in previous simulations, large variations in the mean metallicity with distance are obtained. However, if the mean metallicities are derived along a direction that is perpendicular to the galactic disk, the metallicity tends to be much more uniform (Figure 3). In observational studies, the latter direction is preferred since it allows the scientists to minimize contamination from galactic disk stars.

The MPA group has thus clearly highlighted the crucial importance of preforming careful comparisons between models and observations of halo metallicity distributions. This is the only way to alleviate tension between theory and observations.

 In future work, the MPA group will perform a very careful and detailed analysis of the simulations to compare against observations and to interpret the observations. This will allow them to decipher the accretion history of the GHOSTS galaxies.

Antonela Monachesi, Facundo Gomez and Guinevere Kauffmann

Source:  Max Planck Institute for Astrophysics