Kavli Institute for Astronomy and Astrophysics

Heyuan, China

Kavli Institute for Astronomy and Astrophysics

Heyuan, China
SEARCH FILTERS
Time filter
Source Type

Blakeslee J.P.,National Research Council Canada | Cho H.,Yonsei University | Peng E.W.,Peking University | Peng E.W.,Kavli Institute for Astronomy and Astrophysics | And 4 more authors.
Astrophysical Journal | Year: 2012

We combine new Wide Field Camera 3 IR Channel (WFC3/IR) F160W (H 160) imaging data for NGC1399, the central galaxy in the Fornax cluster, with archival F475W (g 475), F606W (V 606), F814W (I 814), and F850LP (z 850) optical data from the Advanced Camera for Surveys (ACS). The purely optical g 475 - I 814, V 606 - I 814, and g 475 - z 850 colors of NGC1399's rich globular cluster (GC) system exhibit clear bimodality, at least for magnitudes I 814 > 21.5. The optical-IR I 814 - H 160 color distribution appears unimodal, and this impression is confirmed by mixture modeling analysis. The V 606 - H 160 colors show marginal evidence for bimodality, consistent with bimodality in V 606 - I 814 and unimodality in I 814 - H 160. If bimodality is imposed for I 814 - H 160 with a double Gaussian model, the preferred blue/red split differs from that for optical colors; these "differing bimodalities" mean that the optical and optical-IR colors cannot both be linearly proportional to metallicity. Consistent with the differing color distributions, the dependence of I 814 - H 160 on g 475 - I 814 for the matched GC sample is significantly nonlinear, with an inflection point near the trough in the g 475 - I 814 color distribution; the result is similar for the I 814 - H 160 dependence on g 475 - z 850 colors taken from the ACS Fornax Cluster Survey. These g 475 - z 850 colors have been calibrated empirically against metallicity; applying this calibration yields a continuous, skewed, but single-peaked metallicity distribution. Taken together, these results indicate that nonlinear color-metallicity relations play an important role in shaping the observed bimodal distributions of optical colors in extragalactic GC systems. © 2012. The American Astronomical Society. All rights reserved.


Blakeslee J.P.,National Research Council Canada | Cantiello M.,National institute for astrophysics | Peng E.W.,Peking University | Peng E.W.,Kavli Institute for Astronomy and Astrophysics
Astrophysical Journal | Year: 2010

Two recent empirical developments in the study of extragalactic globular cluster (GC) populations are the color-magnitude relation of the blue GCs (the "blue tilt") and the nonlinearity of the dependence of optical GC colors on metallicity. The color-magnitude relation, interpreted as a mass-metallicity relation, is thought to be a consequence of self-enrichment. Nonlinear color-metallicity relations have been shown to produce bimodal color distributions from unimodal metallicity distributions. We simulate GC populations including both a mass-metallicity scaling relation and nonlinear color-metallicity relations motivated by theory and observations. Depending on the assumed range of metallicities and the width of the GC luminosity function (GCLF), we find that the simulated populations can have bimodal color distributions with a "blue tilt" similar to observations, even though the metallicity distribution appears unimodal. The models that produce these features have the relatively high mean GC metallicities and nearly equal blue and red peaks characteristic of giant elliptical galaxies. The blue tilt is less apparent in the models with metallicities typical of dwarf ellipticals; the narrower GCLF in these galaxies has an even bigger effect in reducing the significance of their color-magnitude slopes. We critically examine the evidence for nonlinearity versus bimodal metallicities as explanations for the characteristic double-peaked color histograms of giant ellipticals and conclude that the question remains open. We discuss the prospects for further theoretical and observational progress in constraining the models presented here and for uncovering the true metallicity distributions of extragalactic GC systems. © 2010. The American Astronomical Society. All rights reserved..


News Article | February 23, 2017
Site: www.eurekalert.org

Glowing nebula found at the heart of a huge "rotocluster' of early galaxies appears to be part of the cosmic web of filaments connecting galaxies, but what's lighting it up? Astronomers have found an enormous, glowing blob of gas in the distant universe, with no obvious source of power for the light it is emitting. Called an "enormous Lyman-alpha nebula" (ELAN), it is the brightest and among the largest of these rare objects, only a handful of which have been observed. ELANs are huge blobs of gas surrounding and extending between galaxies in the intergalactic medium. They are thought to be parts of the network of filaments connecting galaxies in a vast cosmic web. Previously discovered ELANs are likely illuminated by the intense radiation from quasars, but it's not clear what is causing the hydrogen gas in the newly discovered nebula to emit Lyman-alpha radiation (a characteristic wavelength of light absorbed and emitted by hydrogen atoms). The newly discovered nebula was found at a distance of 10 billion light years in the middle of a region with an extraordinary concentration of galaxies. Researchers found this massive overdensity of early galaxies, called a "protocluster," through a novel survey project led by Zheng Cai, a Hubble Postdoctoral Fellow at UC Santa Cruz. "Our survey was not trying to find nebulae. We're looking for the most overdense environments in the early universe, the big cities where there are lots of galaxies," said Cai. "We found this enormous nebula in the middle of the protocluster, near the peak density." Cai is first author of a paper on the discovery accepted for publication in the Astrophysical Journal and available online at arxiv.org/abs/1609.04021. His survey project is called Mapping the Most Massive Overdensities Through Hydrogen (MAMMOTH), and the newly discovered ELAN is known as MAMMOTH-1. Coauthor J. Xavier Prochaska, professor of astronomy and astrophysics at UC Santa Cruz, said previously discovered ELANs have been detected in quasar surveys. In those cases, the intense radiation from a quasar illuminated hydrogen gas in the nebula, causing it to emit Lyman-alpha radiation. Prochaska's team discovered the first ELAN, dubbed the "Slug Nebula," in 2014. MAMMOTH-1 is the first one not associated with a visible quasar, he said. "It's extremely bright, and it's probably larger than the Slug Nebula, but there's nothing else visible except the faint smudge of a galaxy. So it's a terrifically energetic phenomenon without an obvious power source," Prochaska said. Equally impressive is the enormous protocluster in which it resides, he said. Protoclusters are the precursors to galaxy clusters, which consist of hundreds to thousands of galaxies bound together by gravity. Because protoclusters are spread out over a much larger area of the sky, they are much harder to find than galaxy clusters. The protocluster hosting the MAMMOTH-1 nebula is massive, with an unusually high concentration of galaxies in an area about 50 million light years across. Because it is so far away (10 billion light years), astronomers are in effect looking back in time to see the protocluster as it was 10 billion years ago, or about 3 billion years after the big bang, during the peak epoch of galaxy formation. After evolving for 10 billion more years, this protocluster would today be a mature galaxy cluster perhaps only one million light years across, having collapsed down to a much smaller area, Prochaska said. The standard cosmological model of structure formation in the universe predicts that galaxies are embedded in a cosmic web of matter, most of which is invisible dark matter. The gas that collapses to form galaxies and stars traces the distribution of dark matter and extends beyond the galaxies along the filaments of the cosmic web. The MAMMOTH-1 nebula appears to have a filamentary structure that aligns with the galaxy distribution in the large-scale structure of the protocluster, supporting the idea that ELANs are illuminated segments of the cosmic web, Cai said. "From the distribution of galaxies we can infer where the filaments of the cosmic web are, and the nebula is perfectly aligned with that structure," he said. Cai and his coauthors considered several possible mechanisms that could be powering the Lyman-alpha emission from the nebula. The most likely explanations involve radiation or outflows from an active galactic nucleus (AGN) that is strongly obscured by dust so that only a faint source can be seen associated with the nebula. An AGN is powered by a supermassive black hole actively feeding on gas in the center of a galaxy, and it is usually an extremely bright source of light (quasars being the most luminous AGNs in visible light). The intense radiation from an AGN can ionize the gas around it (called photoionization), and this may be one mechanism at work in MAMMOTH-1. When ionized hydrogen in the nebula recombines it would emit Lyman-alpha radiation. Another possible mechanism powering the Lyman-alpha emissions is shock heating by a powerful outflow of gas from the AGN. The researchers described several lines of evidence supporting the existence of a hidden AGN energizing the nebula, including the dynamics of the gas and emissions from other elements besides hydrogen, notably helium and carbon. "It has all the hallmarks of an AGN, but we don't see anything in our optical images. I expect there's a quasar that is so obscured by dust that most of its light is hidden," Prochaska said. In addition to Cai and Prochaska at UC Santa Cruz, the team includes coauthors at Steward Observatory, University of Arizona; Korea Astronomy and Space Institute; Mount Stromlo Observatory, Australia; Pontifical Catholic University of Chile; Institute for Astronomy, ETH Zurich; California Institute of Technology; Kavli Institute for Astronomy and Astrophysics, Peking University; and National Astronomical Observatory of Japan. This research was supported by the National Science Foundation and NASA.


News Article | February 24, 2017
Site: spaceref.com

Astronomers have found an enormous, glowing blob of gas in the distant universe, with no obvious source of power for the light it is emitting. Called an "enormous Lyman-alpha nebula" (ELAN), it is the brightest and among the largest of these rare objects, only a handful of which have been observed. ELANs are huge blobs of gas surrounding and extending between galaxies in the intergalactic medium. They are thought to be parts of the network of filaments connecting galaxies in a vast cosmic web. Previously discovered ELANs are likely illuminated by the intense radiation from quasars, but it's not clear what is causing the hydrogen gas in the newly discovered nebula to emit Lyman-alpha radiation (a characteristic wavelength of light absorbed and emitted by hydrogen atoms). The newly discovered nebula was found at a distance of 10 billion light-years in the middle of a region with an extraordinary concentration of galaxies. Researchers found this massive overdensity of early galaxies, called a "protocluster," through a novel survey project led by Zheng Cai, a Hubble Postdoctoral Fellow at UC Santa Cruz. "Our survey was not trying to find nebulae. We're looking for the most overdense environments in the early universe, the big cities where there are lots of galaxies," said Cai. "We found this enormous nebula in the middle of the protocluster, near the peak density." Cai is first author of a paper on the discovery accepted for publication in the Astrophysical Journal and available online. His survey project is called Mapping the Most Massive Overdensities Through Hydrogen (MAMMOTH), and the newly discovered ELAN is known as MAMMOTH-1. Coauthor J. Xavier Prochaska, professor of astronomy and astrophysics at UC Santa Cruz, said previously discovered ELANs have been detected in quasar surveys. In those cases, the intense radiation from a quasar illuminated hydrogen gas in the nebula, causing it to emit Lyman-alpha radiation. Prochaska's team discovered the first ELAN, dubbed the "Slug Nebula," in 2014. MAMMOTH-1 is the first one not associated with a visible quasar, he said. "It's extremely bright, and it's probably larger than the Slug Nebula, but there's nothing else visible except the faint smudge of a galaxy. So it's a terrifically energetic phenomenon without an obvious power source," Prochaska said. Equally impressive is the enormous protocluster in which it resides, he said. Protoclusters are the precursors to galaxy clusters, which consist of hundreds to thousands of galaxies bound together by gravity. Because protoclusters are spread out over a much larger area of the sky, they are much harder to find than galaxy clusters. The protocluster hosting the MAMMOTH-1 nebula is massive, with an unusually high concentration of galaxies in an area about 50 million light-years across. Because it is so far away (10 billion light-years), astronomers are in effect looking back in time to see the protocluster as it was 10 billion years ago, or about 3 billion years after the big bang, during the peak epoch of galaxy formation. After evolving for 10 billion more years, this protocluster would today be a mature galaxy cluster perhaps only one million light-years across, having collapsed down to a much smaller area, Prochaska said. The standard cosmological model of structure formation in the universe predicts that galaxies are embedded in a cosmic web of matter, most of which is invisible dark matter. The gas that collapses to form galaxies and stars traces the distribution of dark matter and extends beyond the galaxies along the filaments of the cosmic web. The MAMMOTH-1 nebula appears to have a filamentary structure that aligns with the galaxy distribution in the large-scale structure of the protocluster, supporting the idea that ELANs are illuminated segments of the cosmic web, Cai said. "From the distribution of galaxies we can infer where the filaments of the cosmic web are, and the nebula is perfectly aligned with that structure," he said. Cai and his coauthors considered several possible mechanisms that could be powering the Lyman-alpha emission from the nebula. The most likely explanations involve radiation or outflows from an active galactic nucleus (AGN) that is strongly obscured by dust so that only a faint source can be seen associated with the nebula. An AGN is powered by a supermassive black hole actively feeding on gas in the center of a galaxy, and it is usually an extremely bright source of light (quasars being the most luminous AGNs in visible light). The intense radiation from an AGN can ionize the gas around it (called photoionization), and this may be one mechanism at work in MAMMOTH-1. When ionized hydrogen in the nebula recombines it would emit Lyman-alpha radiation. Another possible mechanism powering the Lyman-alpha emissions is shock heating by a powerful outflow of gas from the AGN. The researchers described several lines of evidence supporting the existence of a hidden AGN energizing the nebula, including the dynamics of the gas and emissions from other elements besides hydrogen, notably helium and carbon. "It has all the hallmarks of an AGN, but we don't see anything in our optical images. I expect there's a quasar that is so obscured by dust that most of its light is hidden," Prochaska said. * "Discovery of an Enormous Ly-alpha Nebula in a Massive Galaxy Overdensity at z = 2.3," Zheng Cai et al., 2017, to appear in the Astrophysical Journal [http://apj.aas.org, preprint: https://arxiv.org/abs/1609.04021]. * "MApping the Most Massive Overdensities Through Hydrogen (MAMMOTH) I: Methodology," Zheng Cai et al., 2016 Dec. 20, Astrophysical Journal [http://iopscience.iop.org/article/10.3847/1538-4357/833/2/135 , preprint: https://arxiv.org/abs/1512.06859]. In addition to Cai and Prochaska at UC Santa Cruz, the team includes coauthors at Steward Observatory, University of Arizona; Korea Astronomy and Space Institute; Mount Stromlo Observatory, Australia; Pontifical Catholic University of Chile; Institute for Astronomy, ETH Zurich; California Institute of Technology; Kavli Institute for Astronomy and Astrophysics, Peking University; and National Astronomical Observatory of Japan. This research was supported by the National Science Foundation and NASA. Please follow SpaceRef on Twitter and Like us on Facebook.


News Article | January 14, 2016
Site: phys.org

Supernovae are violent stellar explosions and some of the brightest objects in the universe. Human records noting their existence date back nearly 2,000 years. Within the past two decades a rare new category of super-luminous supernovae have been discovered, which shine one hundred to a thousand times brighter than the more-common supernovae. It has been theorized that these super-luminous supernovae are powered by so-called magnetars, neutron stars with extremely powerful magnetic fields, with the magnetism providing the engine for the immense luminosity. According to this theory, the magnetic field's spin magnifies the energy of the explosion, increasing the luminosity. As counterintuitive as it may sound, super-luminous supernovae are difficult for astronomers to spot. This is because they are rare and tend to form in low-luminosity galaxies with vigorous star formation, whereas the sky surveys that have been traditionally used to locate supernovae target bright galaxies with low rates of star formation. The newly found super-luminous supernova was discovered by the All Sky Automated Survey for SuperNovae team (ASAS-SN), an international collaboration headquartered at the Ohio State University, which uses a network of 14-centimeter telescopes around the world to scan the visible sky every two or three nights looking for very bright supernovae. The only all-sky variability survey in existence, it is capable of finding normal supernovae out to about 350 million light years from Earth. "On June 14 of this year, we spotted a newly occurring explosion in a galaxy of an unknown distance," Shappee said. "Subsequent observations—including those made at our Las Campanas Observatory by Nidia Morrell and Ian Thompson—allowed the team to confirm the existence of the supernova ASAS-SN-15lh." The supernova's spectra matched that of other hydrogen-poor super-luminous supernovae. But it wasn't until further follow-up was conducted that the study's lead author Subo Dong of the Kavli Institute for Astronomy and Astrophysics (KIAA) at Peking University and the rest of the team realized how unusual the supernova is. It is two times more luminous than any supernova previously discovered. In fact, ASAS-SN-15lh at peak was almost 50 times more luminous than the entire Milky Way galaxy. "When the first du Pont spectrum was available, as usual, I quickly checked what kind of supernova it was. To my surprise, I was not able to even tell for sure it was a supernova. My first reaction was: 'this is interesting, we should get more data,'" Morrell said. "It was only when we obtained higher resolution spectra from the Southern African Large Telescope and the Magellan Clay Telescope that I realized how distant the host galaxy is and consequently, how luminous the supernova." What's more, they determined that the galaxy where ASAS-SN-15lh formed is very atypical for a super-luminous supernova, which raises questions about how these types of supernovae form. Its host galaxy isn't the typical low-luminosity, star-forming galaxy where previous super-luminous supernovae have been spotted. ASAS-SN-15lh's galaxy is, in fact, more luminous than our own Milky Way. "The astounding amount of energy released by this supernova strains the magnetar-formation theory," Shappee explained. "More work will be necessary to understand this extraordinary object's power source and whether there are other similar supernovae out there in the universe."


News Article | January 14, 2016
Site: www.rdmag.com

A team of astronomers, including Carnegie's Benjamin Shappee, Nidia Morrell, and Ian Thompson, has discovered the most-luminous supernova ever observed, called ASAS-SN-15lh. Their findings are published in Science. Supernovae are violent stellar explosions and some of the brightest objects in the universe. Human records noting their existence date back nearly 2,000 years. Within the past two decades a rare new category of super-luminous supernovae have been discovered, which shine one hundred to a thousand times brighter than the more-common supernovae. It has been theorized that these super-luminous supernovae are powered by so-called magnetars, neutron stars with extremely powerful magnetic fields, with the magnetism providing the engine for the immense luminosity. According to this theory, the magnetic field's spin magnifies the energy of the explosion, increasing the luminosity. As counterintuitive as it may sound, super-luminous supernovae are difficult for astronomers to spot. This is because they are rare and tend to form in low-luminosity galaxies with vigorous star formation, whereas the sky surveys that have been traditionally used to locate supernovae target bright galaxies with low rates of star formation. The newly found super-luminous supernova was discovered by the All Sky Automated Survey for SuperNovae team (ASAS-SN), an international collaboration headquartered at the Ohio State University, which uses a network of 14-centimeter telescopes around the world to scan the visible sky every two or three nights looking for very bright supernovae. The only all-sky variability survey in existence, it is capable of finding normal supernovae out to about 350 million light years from Earth. "On June 14 of this year, we spotted a newly occurring explosion in a galaxy of an unknown distance," Shappee said. "Subsequent observations--including those made at our Las Campanas Observatory by Nidia Morrell and Ian Thompson--allowed the team to confirm the existence of the supernova ASAS-SN-15lh." The supernova's spectra matched that of other hydrogen-poor super-luminous supernovae. But it wasn't until further follow-up was conducted that the study's lead author Subo Dong of the Kavli Institute for Astronomy and Astrophysics (KIAA) at Peking University and the rest of the team realized how unusual the supernova is. It is two times more luminous than any supernova previously discovered. In fact, ASAS-SN-15lh at peak was almost 50 times more luminous than the entire Milky Way galaxy. "When the first du Pont spectrum was available, as usual, I quickly checked what kind of supernova it was. To my surprise, I was not able to even tell for sure it was a supernova. My first reaction was: 'this is interesting, we should get more data,'" Morrell said. "It was only when we obtained higher resolution spectra from the Southern African Large Telescope and the Magellan Clay Telescope that I realized how distant the host galaxy is and consequently, how luminous the supernova." What's more, they determined that the galaxy where ASAS-SN-15lh formed is very atypical for a super-luminous supernova, which raises questions about how these types of supernovae form. Its host galaxy isn't the typical low-luminosity, star-forming galaxy where previous super-luminous supernovae have been spotted. ASAS-SN-15lh's galaxy is, in fact, more luminous than our own Milky Way. "The astounding amount of energy released by this supernova strains the magnetar-formation theory," Shappee explained. "More work will be necessary to understand this extraordinary object's power source and whether there are other similar supernovae out there in the universe."


News Article | January 14, 2016
Site: news.yahoo.com

CAPE CANAVERAL, Fla. (Reuters) - Astronomers have found a distant supernova, or exploded star, 20 times brighter than the Milky Way galaxy, according to research published on Thursday. The massive supernova is about 3.8 billion light-years away in a galaxy roughly three times the size of the Milky Way, scientists wrote in a report in this week's issue of the journal Science. A light-year is the distance that light travels in one year, moving at 186,000 miles (300,000 km) per second. The cosmic blast was first spotted on June 14, 2015, in an automated search for supernovas conducted by a global network of small telescopes. "It didn't look like any of the other 200 or so supernovae we had discovered at that point," astronomer Subo Dong, with the Kavli Institute for Astronomy and Astrophysics at Peking University in China, wrote in an email. Dong and colleagues do not know what triggered the blast, which is more than twice as bright as any previously discovered supernova. They plan to use the Hubble Space Telescope later this year to get a better look at the supernova's host galaxy for clues. If the supernova, known as ASASSN-151h, is at the center of its galaxy it could have been triggered by a massive black hole. Black holes are objects so dense with matter that not even photons of light can escape their gravitational pull. Massive black holes are believed to exist at the centers of most, if not all, large galaxies, including the Milky Way. Another theory is that the supernova was spawned by a magnetar, a rare, rapidly rotating neutron star with an extremely powerful magnetic field, according to Ohio State University astronomer Todd Thompson. "Like many mysteries in astronomy, it may take years, if not decades, of observational and theoretical efforts to unravel it," Dong said.


News Article | January 27, 2016
Site: phys.org

Instead of having all their stellar progeny at once, globular clusters can somehow bear second or even third sets of thousands of sibling stars. Now a new study led by researchers at the Kavli Institute for Astronomy and Astrophysics (KIAA) at Peking University, and including astronomers at Northwestern University, the Adler Planetarium and the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC), might explain these puzzling, successive stellar generations. Using observations by the Hubble Space Telescope, the research team has for the first time found young populations of stars within globular clusters that have apparently developed courtesy of star-forming gas flowing in from outside of the clusters themselves. This method stands in contrast to the conventional idea of the clusters' initial stars shedding gas as they age in order to spark future rounds of star birth. The study will be published in the Jan. 28 issue of the journal Nature. "This study offers new insight on the problem of multiple stellar populations in star clusters," said study lead author Chengyuan Li, an astronomer at KIAA and NAOC who also is affiliated with the Chinese Academy of Sciences' Purple Mountain Observatory. "Our study suggests the gaseous fuel for these new stellar populations has an origin that is external to the cluster, rather than internal." In a manner of speaking, globular clusters appear capable of "adopting" baby stars—or at least the material with which to form new stars—rather than creating more "biological" children as parents in a human family might choose to do. "Our explanation that secondary stellar populations originate from gas accreted from the clusters' environments is the strongest alternative idea put forward to date," said Richard de Grijs, also an astronomer at KIAA and Chengyuan's Ph.D. advisor. "Globular clusters have turned out to be much more complex than we once thought." Globular clusters are spherical, densely packed groups of stars orbiting the outskirts of galaxies. Our home galaxy, the Milky Way, hosts several hundred. Most of these local, massive clusters are quite old, however, so the KIAA-led research team turned their attention to young and intermediate-aged clusters found in two nearby dwarf galaxies, collectively called the Magellanic Clouds. Specifically, the researchers used Hubble observations of the globular clusters NGC 1783 and NGC 1696 in the Large Magellanic Cloud, along with NGC 411 in the Small Magellanic Cloud. Scientists routinely infer the ages of stars by looking at their colors and brightnesses. Within NGC 1783, for example, Li, de Grijs and colleagues identified an initial population of stars aged 1.4 billion years, along with two newer populations that formed 890 million and 450 million years ago. What is the most straightforward explanation for these unexpectedly differing stellar ages? Some globular clusters might retain enough gas and dust to crank out multiple generations of stars, but this seems unlikely, said study co-author Aaron M. Geller of Northwestern University and the Adler Planetarium in Chicago. "Once the most massive stars form, they are like ticking time bombs, with only about 10 million years until they explode in powerful supernovae and clear out any remaining gas and dust," Geller said. "Afterwards, the lower-mass stars, which live longer and die in less violent ways, may allow the cluster to build up gas and dust once again." The KIAA-led research team proposes that globular clusters can sweep up stray gas and dust they encounter while moving about their respective host galaxies. The theory of newborn stars arising in clusters as they "adopt" interstellar gases actually dates back to a 1952 paper. More than a half-century later, this once speculative idea suddenly has key evidence to support it. In the study, the KIAA researchers analyzed Hubble observations of these star clusters, and then Geller and his Northwestern colleague Claude-André Faucher-Giguère carried out calculations that show this theoretical explanation is possible in the globular clusters this team studied. "We have now finally shown that this idea of clusters forming new stars with accreted gas might actually work," de Grijs said, "and not just for the three clusters we observed for this study, but possibly for a whole slew of them." Future studies will aim to extend the findings to other Magellanic Cloud as well as Milky Way globular clusters. More information: Formation of new stellar populations from gas accreted by massive young star clusters, Nature, nature.com/articles/doi:10.1038/nature16493


News Article | January 28, 2016
Site: www.techtimes.com

A team of scientists from China and the United States have discovered how relatively young stars are able to make their way into dense collections of older stars known as globular clusters. While it was initially thought that these clusters form their stars all at the same time, it has been revealed that they are also capable of producing thousands of second and even third generations of sibling stars. In a study featured in the journal Nature, researchers from Peking University and the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC) in China and the Adler Planetarium and Northwestern University (NU) in the U.S. have found that globular clusters are capable of taking in gas from outside sources, which can then lead to the formation of new stars. This discovery contradicts an earlier notion that it is the aging stars themselves that shed gas in order to trigger the creation of newer stars. Chengyuan Li, a researcher from Peking's Kavli Institute for Astronomy and Astrophysics (KIAA) and lead author of the study, explained that their research offers new perspective on how several stellar populations are able to form as part of star clusters. Their findings suggest that the gas from which new stars are formed likely originates from outside globular clusters rather than from the inside. This event can be compared to how some people choose to adopt kids instead of having biological children of their own with their partners. Globular clusters are capable of producing their own progeny of stars, but it appears that they would rather "adopt" young stars, or at the very least the materials with which new stars can be formed. "Our explanation that secondary stellar populations originate from gas accreted from the clusters' environments is the strongest alternative idea put forward to date," KIAA astronomer Richard de Grijs said. "Globular clusters have turned out to be much more complex than we once thought." The Milky Way is known to contain hundreds of spherical and densely packed globular clusters at its outskirts. A large number of these clusters are already quite old, which is why Li and his colleagues chose to focus their study on younger clusters. The team found their target clusters in two dwarf galaxies known as the Magellanic Clouds. Through the use of data gathered from observations of the Hubble Space Telescope, the researchers were able to identify three particular globular clusters: NGC 1696 and NGC 1783 found in the Large Magellanic Cloud and NGC 411 found in the Small Magellanic Cloud. In the NGC 1783 star cluster, Li and his team identified an initial stellar population that is already 1.4 billion years old, as well as two other stellar populations that are 450 million years old and 890 million years old. The difference in the ages of the star populations was first thought to be because of their ability to retain dust and gas enough to produce several generations of stars. However, this appears to be unlikely according to the researchers. NU astronomer Aaron M. Geller said that after massive stars are formed, they only have an estimated 10 million years before they meet their end in powerful supernovae that can eliminate any remaining dust or gas in the surrounding area. After the explosion, lower-mass stars would then trigger an accumulation of dust and gas in the area once again. The researchers believe globular clusters take up material from stray dust and gas as they move about their host galaxies. Li and his colleagues are now planning to extend their study to globular clusters in the Milky Way other than Magellanic Cloud.


News Article | January 28, 2016
Site: www.rdmag.com

At a first glance, the globular cluster NGC 1783 looks like a high concentration of pockmarks of light bursting their way through a black expanse. Located roughly 160,000 light-years from the Earth, the massive stellar cluster boasts a mass equivalent to 170,000 suns. The cluster is one of the biggest and brightest located in the Large Magellanic Cloud satellite galaxy. Previously, scientists thought stellar clusters formed in a single outburst from a progenitor cloud. But by studying clusters like NGC 1783, scientists from the Kavli Institute for Astronomy and Astrophysics, the National Astronomical Observatories of the Chinese Academy of Sciences, Northwestern Univ., and the Adler Planetarium have found that globular clusters can give birth to second and third sets of thousands of sibling stars after the initial progeny. Their research was published in Nature. According to the researchers, NGC 1783 bore populations of stars around 4.1 billion years ago, 890 million years ago, and 450 million years ago. “Such clusters could have accreted sufficient gas from new stars if they had orbited in their host galaxies’ gaseous disks throughout the period between their initial formation and the more recent bursts of star formation,” the researchers wrote. “This process may eventually give rise to ubiquitous multiple stellar populations in globular clusters.” NGC 1783 was first observed by English astronomer John Herschel in 1835. The researchers also studied NGC 1696, which is located in the Large Magellanic Cloud, and NGC 411, located in the Small Magellanic Cloud. Each of the star clusters is between 1 and 2 billion years old. According to Space.com, NGC 1696 is about 160,000 light-years from Earth and is 50,000 solar masses; NGC 411 is about 190,000 light-years away and about 32,000 solar masses. At first, the researchers considered the theory that globular clusters retain enough gas and dust to create new stars throughout their lifetimes. However, this explanation was refuted by the idea that massive stars in globular clusters live for about 10 million years before exploding as supernovae. This would cause any gas and dust nearby to be blown away. The sweeping theory adopted by the researchers was actually proposed in 1952. “We have now finally shown that this idea of clusters forming new stars with accreted gas might actually work,” said Richard de Grijs, of the Kavli Institute for Astronomy and Astrophysics, “and not just for the three clusters we observed for this study, but possibly for a whole slew of them.”

Loading Kavli Institute for Astronomy and Astrophysics collaborators
Loading Kavli Institute for Astronomy and Astrophysics collaborators