NASA's Hubble Finds Strong Evidence for Intermediate-Mass Black Hole in Omega Centauri

Omega Centauri
Credits: Image: ESA/Hubble, NASA, Maximilian Häberle (MPIA)

IMBH Candidate in Omega Centauri
Credits: Image: ESA/Hubble, NASA, Maximilian Häberle (MPIA)

Omega Centauri (cropped)
Credits: Image: ESA/Hubble, NASA, Maximilian Häberle (MPIA)

Images

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Most known black holes are either extremely massive, like the supermassive black holes that lie at the cores of large galaxies, or relatively lightweight, with a mass of under 100 times that of the Sun. Intermediate-mass black holes (IMBHs) are scarce, however, and are considered rare "missing links" in black hole evolution.

Now, an international team of astronomers has used more than 500 images from NASA's Hubble Space Telescope — spanning two decades of observations — to search for evidence of an intermediate-mass black hole by following the motion of seven fast-moving stars in the innermost region of the globular star cluster Omega Centauri.

These stars provide new compelling evidence for the presence of the gravitational pull from an intermediate-mass black hole tugging on them. Only a few other IMBH candidates have been found to date.

Omega Centauri consists of roughly 10 million stars that are gravitationally bound. The cluster is about 10 times as massive as other big globular clusters — almost as massive as a small galaxy.

Among the many questions scientists want to answer: Are there any IMBHs, and if so, how common are they? Does a supermassive black hole grow from an IMBH? How do IMBHs themselves form? Are dense star clusters their favored home?

The astronomers have now created an enormous catalog for the motions of these stars, measuring the velocities for 1.4 million stars gleaned from the Hubble images of the cluster. Most of these observations were intended to calibrate Hubble's instruments rather than for scientific use, but they turned out to be an ideal database for the team's research efforts.

"We discovered seven stars that should not be there," explained Maximilian Häberle of the Max Planck Institute for Astronomy in Germany, who led this investigation. "They are moving so fast that they would escape the cluster and never come back. The most likely explanation is that a very massive object is gravitationally pulling on these stars and keeping them close to the center. The only object that can be so massive is a black hole, with a mass at least 8,200 times that of our sun."

Several studies have suggested the presence of an IMBH in Omega Centauri. However, other studies proposed the mass could be contributed by a central cluster of stellar-mass black holes, and had suggested the lack of fast-moving stars above the necessary escape velocity made an IMBH less likely in comparison.

"This discovery is the most direct evidence so far of an IMBH in Omega Centauri," added team lead Nadine Neumayer of the Max Planck Institute for Astronomy in Germany, who initiated the study, together with Anil Seth from the University of Utah, Salt Lake City. "This is exciting because there are only very few other black holes known with a similar mass. The black hole in Omega Centauri may be the best example of an IMBH in our cosmic neighborhood."

If confirmed, at a distance of 17,700 light-years the candidate black hole resides closer to Earth than the 4.3-million-solar-mass black hole in the center of the Milky Way, located 26,000 light-years away.

Omega Centauri is visible from Earth with the naked eye and is one of the favorite celestial objects for stargazers living in the southern hemisphere. Located just above the plane of the Milky Way, the cluster appears almost as large as the full Moon when seen from a dark rural area. It was first listed in Ptolemy’s catalog nearly 2,000 years ago as a single star. Edmond Halley reported it as a nebula in 1677. In the 1830s the English astronomer John Herschel was the first to recognize it as a globular cluster.

The discovery paper led by Häberle et al. is published online today in the journal Nature.

The Hubble Space Telescope has been operating for over three decades and continues to make ground-breaking discoveries that shape our fundamental understanding of the universe. Hubble is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope and mission operations. Lockheed Martin Space, based in Denver, Colorado, also supports mission operations at Goddard. The Space Telescope Science Institute in Baltimore, Maryland, which is operated by the Association of Universities for Research in Astronomy, conducts Hubble science operations for NASA.

Source: HubbleSite/News

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NASA's Chandra Catches Spider Pulsars Destroying Nearby Stars

Omega Centauri
Credit: X-ray: NASA/CXC/San Francisco State Univ./A. Cool et al.;
Optical: NASA/ESA/STScI; IR: NASA/JPL/Caltech; Image Processing: NASA/CXC/SAO/N. Wolk

JPEG (472.7 kb) - Large JPEG (8 MB) - Tiff (37.1 MB) - More Images

A Tour of Spider Pulsars in Omega Centauri - More Videos

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A group of dead stars known as “spider pulsars” are obliterating companion stars within their reach. Data from NASA’s Chandra X-ray Observatory of the globular cluster Omega Centauri is helping astronomers understand how these spider pulsars prey on their stellar companions.

A pulsar is the spinning dense core that remains after a massive star collapses into itself to form a neutron star. Rapidly rotating neutron stars can produce beams of radiation. Like a rotating lighthouse beam, the radiation can be observed as a powerful, pulsing source of radiation, or pulsar. Some pulsars spin around dozens to hundreds of times per second, and these are known as millisecond pulsars.

Spider pulsars are a special class of millisecond pulsars, and get their name for the damage they inflict on small companion stars in orbit around them. Through winds of energetic particles streaming out from the spider pulsars, the outer layers of the pulsar’s companion stars are methodically stripped away.

Astronomers recently discovered 18 millisecond pulsars in Omega Centauri — located about 17,700 light-years from Earth — using the Parkes and MeerKAT radio telescopes. A pair of astronomers from the University of Alberta in Canada then looked at Chandra data of Omega Centauri to see if any of the millisecond pulsars give off X-rays.

They found 11 millisecond pulsars emitting X-rays, and five of those were spider pulsars concentrated near the center of Omega Centauri. The researchers next combined the data of Omega Centauri with Chandra observations of 26 spider pulsars in 12 other globular clusters.

Close-up of Omega Centauri, in X-ray & optical light, showing the locations of some of the objects. Credit: X-ray: NASA/CXC/San Francisco State Univ./A. Cool et al.; Optical: NASA/ESA/STScI/AURA; Image Processing: NASA/CXC/SAO/N. Wolk

There are two varieties of spider pulsars based on the size of the star being destroyed. “Redback” spider pulsars are damaging companion stars weighing between a tenth and a half the mass of the Sun. Meanwhile, the “black widow” spider pulsars are damaging companion stars with less than 5 percent of the Sun’s mass.

The team found a clear difference between the two classes of spider pulsars, with the redbacks being brighter in X-rays than the black widows, confirming previous work. The team is the first to show a general correlation between X-ray brightness and companion mass for spider pulsars, with pulsars that produce more X-rays being paired with more massive companions. This gives clear evidence that the mass of the companion to spider pulsars influences the X-ray dose the star receives.

The X-rays detected by Chandra are mainly thought to be generated when the winds of particles flowing away from the pulsars collide with winds of matter blowing away from the companion stars and produce shock waves, similar to those produced by supersonic aircraft.

Spider pulsars are typically separated from their companions by only about one to 14 times the distance between the Earth and Moon. This close proximity — cosmically speaking — causes the energetic particles from the pulsars to be particularly damaging to their companion stars.

This finding agrees with theoretical models that scientists have developed. Because more massive stars produce a denser wind of particles, there is a stronger shock — producing brighter X-rays — when their wind collides with the particles from the pulsar. The proximity of the companion stars to their pulsars means the X-rays can cause significant damage to the stars, along with the pulsar’s wind.

Chandra's sharp X-ray vision is crucial for studying millisecond pulsars in globular clusters because they often contain large numbers of X-ray sources in a small part of the sky, making it difficult to distinguish sources from each other. Several of the millisecond pulsars in Omega Centauri have other, unrelated X-ray sources only a few arc seconds away. (One arc second is the apparent size of a penny seen at a distance of 2.5 miles.)

The paper describing these results will be published in the December issue of the Monthly Notices of the Royal Astronomical Society, and a preprint of the accepted paper is available online. The authors of the paper are Jiaqi (Jake) Zhao and Craig Heinke, both from the University of Alberta in Canada.

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

Quick Look: Chandra Catches Spider Pulsars Destroying Nearby Stars

Source: NASA’s Chandra X-ray Observatory

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Visual Description:

This release features a composite image of a globular cluster, a sphere-shaped collection of stars bound together by gravity. In this conglomeration are eleven spider pulsars, which obliterate their companion stars with strong winds of particles.

In the main image of this release, scores of tiny white stars dot the blackness of space, many appearing to glow with a white or hot pink aura. Like other globular clusters, this conglomeration, named Omega Centauri, is more densely packed near the center.

A close up image of this densely-packed center reveals even more stars, blanketing the frame from corner to corner. This close up is presented fully labeled, with a white ring encircling nine of the eleven marked spider pulsars.

Each spider pulsar is the spinning core of a collapsed massive star. As they spin, the spider pulsars emit beams of radiation, like light from a lighthouse. The pulsars also produce winds of particles that methodically strip away their companion stars, layer by layer.

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Fast Facts for Omega Centauri:

Scale: Image is about 16 arcmin (80 light-years) across.
Category: Normal Stars & Star Clusters
Coordinates (J2000): RA 13h 26m 47s | Dec -47° 28´ 46"
Constellation: Centaurus
Observation Dates: 4 observations from Jan 24, 2000 to Apr 16, 2012
Observation Time: 80 hours 48 minutes (3 days, 8 hours 48 minutes)
Obs. ID: 653, 1519, 13726, 13727
Instrument: ACIS
References: Zhao, J. and Heinke, C, 2023, MNRAS, 526, 2736; arXiv:2309.13189
Distance Estimate: About 17,700 light-years

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Omega Centauri’s lost stars

The Milky Way, as seen by the Gaia satellite. Streams of co-moving stars are shown colored according to their motions as measured by Gaia. The “Fimbulthul” stream which is due to stars lost from the omega Centauri globular cluster (white box) has been highlighted. Credit R. Ibata. Hi-res image

A team of researchers from the Strasbourg Astronomical Observatory, Bologna Observatory and the University of Stockholm has identified a stream of stars that was torn off the globular cluster Omega Centauri. Searching through the 1.7 billion stars observed by the ESA Gaia mission, they have identified 309 stars that suggest that this globular cluster may actually be the remnant of a dwarf galaxy that is being torn apart by the gravitational forces of our Galaxy.

In 1677, Edmond Halley gave the name “Omega Centauri” (ω Cen) to what he thought was a star in the Centaurus constellation. Later in 1830 John Herschel realized that it was in fact a globular cluster that could be resolved into individual stars. We now know that Omega Centauri is the most massive globular cluster in the Milky Way: it is about 18,000 light years from us and contains several million stars that are about 12 billion years old. The nature of this object has been the subject of much debate: is it really a globular cluster, or could it be the heart of a dwarf galaxy whose periphery has been dispersed by the Milky Way?

This last hypothesis is based on the fact that ω Cen contains several stellar populations, with a large range of metallicities (i.e. heavy element content) that betray a formation over an extended period of time. An additional argument in favor of this hypothesis would be to find debris from the cluster scattered along its orbit in the Milky Way. Indeed, when a dwarf galaxy interacts with a massive galaxy like our own, stars are torn off by gravitational tidal forces, and these stars remain visible for a time as stellar streams, before becoming dispersed in the vast volumes of interstellar space surrounding the massive galaxy.

By analyzing the motions of stars measured by the Gaia satellite with an algorithm called STREAMFINDER developed by the team, the researchers identified several star streams. One of them, named “Fimbulthul” (after one of the rivers in Norse mythology that existed at the beginning of the world), contains 309 stars stretching over 18° in the sky. By modeling the trajectories of the stars, the team showed that the Fimbulthul structure is a stellar tidal stream torn off ω Cen, extending up to 28° from the cluster. Spectroscopic observations of 5 stars of this stream with the Canada-France Hawaii Telescope show that their velocities are very similar, and that they have metallicities comparable to the stars of ω Cen itself, which reinforces the idea that the tidal stream is linked to ω Cen.

“The stars that the team observed were quite faint for the instrument we were using,” says Dr. Nadine Manset, instrument scientist for Espadons and CFHT’s astronomy group manager. “It is great to see such challenging observations reinforce the Fimbulthul structure’s link to ω Cen.”

The researchers were then able to show that the stream is also present in the very crowded area of sky in the immediate vicinity of the cluster. Further modeling of the tidal stream will constrain the dynamical history of the dwarf galaxy that was the progenitor of ω Cen, and allow us to find even more stars lost by this system into the halo of the Milky Way.

The team’s paper appeared in the April 22nd edition of Nature.

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Additional information:

Nature paper (Subscription required.)
arXiv preprint (no login required)

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Contact Information:
 
Media contact:

Mary Beth Laychak, Outreach manager
Canada-France-Hawaii Telescope  
laychak@cfht.hawaii.edu

Science contacts:  

Rodrigo Ibata, +33 3 68 85 23 91
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Khyati Malhan, +46 72 085 22 05  
Nicolas Martin, +33 3 68 85 24 67  
Paolo Bianchini, +33 3 68 85 24 02

Source: Canada-France-Hawaii Telescope (CFHT)

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Oxygen in Stars

An optical image of the brightest globular cluster, Omega Centauri, a group of over ten million stars older than the Sun. Astronomers have developed a new computational method to determine the abundance of oxygen in these and similar stars, and in particular in giant stars. The code finds values that are more self-consistent than previous estimates. Credit: Joaquin Polleri & Ezequiel Etcheverry, Observatorio Panameño en San Pedro de Atacama

Oxygen is the third most abundant element in the universe, after hydrogen and helium. It is an important constituent of the clouds of gas and dust in space, especially when combined in molecules with other atoms like carbon, and it is from this interstellar material that new stars and planets develop. Oxygen is, of course, also essential for life as we know it, and all known life forms require liquid water and its oxygen content. Oxygen in molecular form, especially as water, was supposed to be relatively abundant, but over the past decade considerable attention has been paid to observations suggesting that at least in molecular form oxygen is scarcer than expected, a deficit that has not yet been entirely resolved.

Atomic oxygen by contrast, seen most prominently in the light of stars, was thought to be in good agreement with expectations. The neutral oxygen atom produces strong lines that are frequently used to calculate its abundance. Models fit the line strengths by taking into account the radiation field, the star's hot gas motions, and the internal structure of the star (for example, the way the temperature and pressure change with radius). It turns out, however, that varying assumptions in these calculations can result in oxygen abundance predictions that differ significantly, and in the case of giant stars, which are larger and cooler and often have hot outer chromospheres, those abundance results can disagree with one another by as much as a factor of 15. This discrepancy has often been discounted by scientists arguing that some of the proposed stellar models are themselves unrealistic.

CfA astronomers Andrea Dupree, Eugene Avrett, and Bob Kurucz have tacked this fundamental problem with Avrett's PANDORA code for stellar atmospheres. In particular, they include the effects of a hot outer atmosphere in giant stars, something that was typically ignored. Moreover, they do not tie the excitation of oxygen atoms (and the corresponding line strengths) to the local temperature. That constraint, imposed by most previous methods in order to simplify the calculations, does not take more complex situations (like the hot atmosphere) adequately into account. The astronomers find that their new computations can resolve several outstanding issues. The lines themselves are actually as much as three times stronger than previously thought, reducing the implied oxygen abundances, and thereby also affecting details of the stellar interior models, especially for giants seen in globular clusters of stars. Similar improvements are seen in the results for stars known to be lacking other heavier elements, and even some normal, Sun-like stars. The possible implications extend to estimating more accurately the amount of oxygen present in a solar nebula when exoplanets form.

Reference(s):

"Chromospheric Models and the Oxygen Abundance in Giant Stars," A. K. Dupree, E. H. Avrett, and R. L. Kurucz, ApJ 821, L7, 2016.

Source: Smithsonian Astrophysical Observatory (SAO)

Seeing Where Stars Collide

Gemini Observatory near-infrared image of the globular cluster Liller 1 obtained with the GeMS adaptive optics system on the Gemini South telescope in Chile.  Credit: Gemini Observatory/AURA.  Full-resolution image

Scientists have imaged a cluster of stars, heavily obscured by material in our galaxy, where stars are so densely packed that it is likely a rare environment where stars can collide. “It’s a bit like a stellar billiards table; where the probability of collisions depends on the size of the table and on the number of billiard balls on it,” said Francesco R. Ferraro of the University of Bologna (Italy), one of the team members who used the Gemini Observatory to make the observations. 

The cluster of stars, known as Liller 1, is a difficult target to study due to its distance and also because it is located close to the center of the Milky Way (about 3,200 light-years away from it), where the obscuration by dust is very high. The unprecedented ultra-sharp view of the cluster reveals a vast city of stars estimated by the team to contain a total mass of at least 1.5 million suns, very similar to the most massive globular clusters in our galaxy: Omega Centauri and Terzan 5. 

“Although our galaxy has upwards of 200 billion stars, there is so much vacancy between stars that there are very few places where suns actually collide,” said Douglas Geisler, Principal Investigator of the original observing proposal, from University of Concepcion (Chile). “The congested overcrowded central regions of globular clusters are one of these places. Our observations confirmed that, among globular clusters, Liller 1 is one of the best environments in our galaxy for stellar collisions.” 

Geisler’s team specializes in the study of globular clusters near the center of the Milky Way, while Ferraro’s team is adept at the reduction of infrared data on globular clusters. Both groups worked together to obtain the beautiful and detailed observations of Liller 1 with Gemini. 

Liller 1 is a tight sphere of stars known as a globular cluster. Globular clusters orbit in a large halo around the center, or nucleus, of our galaxy and many of the closer globular clusters are spectacular showpieces, even in small telescopes or binoculars. “This isn’t one of these showpieces, it is so obscured by material in the central bulge of our galaxy that is almost completely invisible in visual light,” observed Sara Saracino, lead author on the paper, from the University of Bologna. Indeed, Liller 1 is located at almost 30,000 light years from Earth, in one of the most inaccessible regions of our galaxy, where thick clouds of dust prevent the optical light from emerging. “Only infrared radiation can travel across these clouds and bring us direct information on its stars,” commented Emanuele Dalessandro of University of Bologna. 

The observations of the tightly packed cluster used Gemini Observatory’s powerful adaptive optics system at the Gemini South telescope in Chile. 

A technical jewel named GeMS (derived from “Gemini Multi-conjugate adaptive optics System”), in combination with the powerful infrared camera Gemini South Adaptive Optics Imager (GSAOI), was able to penetrate the dense fog surrounding Liller 1 and to provide astronomers with this unprecedented view of its stars. This has been made possible thanks to the combination of two specific characteristics of GeMS: first, the capability of operating at near-infrared wavelengths (especially in the K pass-band); second, an innovative and revolutionary way to remove the distortions (blurriness) that the Earth’s turbulent atmosphere inflicts on astronomical images. To compensate for the degradation effects of the Earth’s atmosphere, the GeMS system uses three natural guide stars, a constellation of five laser guide stars, and multiple deformable mirrors. The correction is so fine that astronomers are provided with images of unprecedented sharpness. In the best K-band exposures of Liller 1, stellar images have an angular resolution of only 75 milliarcseconds, just slightly larger than the theoretical limit of Gemini’s 8-meter mirror (known as the diffraction limit). This means that GeMS performed with almost perfect corrections of atmospheric distortions. 

The international research team published the results in The Astrophysical Journal (article 152, volume 806, issue 2, June 15, 2015). The astro-ph version of the article can be found here. 

The observations for this project also included several other globular clusters. The results achieved on their first target, Liller 1, have been so important that they have increased their collaboration and are currently working on the other clusters which promise to deliver even more exciting science.


Background: Stellar Collisions

Stellar collisions are important because they can provide the key to understand the origin of exotic objects that cannot be interpreted in terms of the passive evolution of single stars. Nearly head-on collisions in which the stars actually merge, mixing their nuclear fuel and re-stoking the fire of the nuclear fusion are suggested to be the origin of (at least part) of the so-called Blue Straggler Stars. But collisions can also involve binary systems, with the effect of shrinking the initial size of the system and thus promoting the two components to interact and producing a variety of objects like Low mass X-ray binaries, Millisecond pulsars etc. In particular Millisecond pulsars are old neutron stars reaccelerated to millisecond rotation period by mass accretion from a companion in a binary system. Indeed Liller 1 is suspected to have a large population of such exotic objects. Although no millisecond pulsar has been directly observed up to now, a large hidden population has been suggested because of the detection of an intense γ-ray emission (the most intense detected so far from a globular cluster). The Gemini observations indeed confirm that this is possible.

“Indeed our observations confirm Liller 1 as one of the best “laboratories” where the impact of star cluster dynamics on stellar evolution can be studied: it opens the window to a sort of stellar sociology study, aimed at measuring the impact of the reciprocal influence of stars when they are forced to live in conditions of extreme crowding and stress.” concludes Ferraro.

Additional information can be found at http://www.cosmic-lab.eu/Cosmic-Lab/Liller1.html

Media Contacts:

Peter Michaud
Public Information and Outreach Office
Gemini Observatory, Hilo, HI
Email: pmichaud@gemini.edu
Cell: (808) 936-6643

Manuel Paredes
Public Information and Outreach Office
Gemini Observatory, La Serena, Chile
Email: mparedes@gemini.edu
Phone: (56-51) 220-5671

Science Contacts:

Douglas Geisler
Departamento de Astronomía
Universidad de Concepción
Email: dgeisler@astro-udec.cl
Phone: (56-41) 220-3092

Francesco R. Ferraro
Department of Physics and Astronomy, University of Bologna
Email: francesco.ferraro3@unibo.it
Office: +39 051 2095774

Source: Gemini Observatory

An Image Gallery Gift from NASA's Swift

Of the three telescopes carried by NASA's Swift satellite, only one captures cosmic light at energies similar to those seen by the human eye. Although small by the standards of ground-based observatories, Swift's Ultraviolet/Optical Telescope (UVOT) plays a critical role in rapidly pinpointing the locations of gamma-ray bursts (GRBs), the brightest explosions in the cosmos.

 But as the proxy to the human eye aboard Swift, the UVOT takes some amazing pictures. The Swift team is celebrating eight years of UVOT operations by collecting more than 100 of the instrument's best snapshots in a web-based photo gallery. The images also can be viewed with the free Swift Explorer Mission iPhone app developed by the Swift Mission Operations Center (MOC), which is located in State College, Pa., and operated by Penn State.

The Crab Nebula is the wreckage of an exploded star, or supernova, observed in the year 1054. The expanding cloud of gas is located 6,500 light-years away in the constellation Taurus. This composite of three Swift UVOT ultraviolet images highlights the luminous hot gas in the supernova remnant. The image is constructed from exposures using these filters: uvw1, centered at 2,600 angstroms (shown as red); uvm2, centered at 2,246 angstroms (green); and uvw2, centered at 1,928 angstroms (blue). Credit: NASA/Swift/E. Hoversten, PSU

› Larger image (labeled) - › Larger image (unlabeled)

 Swift has detected an average of about 90 GRBs a year since its launch in 2004. "When we aren't studying GRBs, we use the satellite's unique capabilities to engage in other scientific investigations, some of which produce beautiful images from the UVOT that we're delighted to be able to share with the public," said Michael Siegel, the lead scientist on the UVOT and a research associate in astronomy and astrophysics at the MOC.

 The targets range from comets and star clusters to supernova remnants, nearby galaxies and active galaxies powered by supermassive black holes.

 "One of our more challenging projects in the past was completing an ultraviolet mosaic of M31, the famous Andromeda galaxy," said Stefan Immler, a member of the Swift team at NASA's Goddard Space Flight Center in Greenbelt, Md. "Because the galaxy is so much larger than the UVOT field of view, we had to take dozens of pictures and blend them together to show the whole object."

 An ongoing mosaic project targets the Large and Small Magellanic Clouds, two small satellite galaxies orbiting our own, and makes the Andromeda effort look like child's play. Although the galaxies are much smaller than M31, they are both much closer to us and extend over much larger areas of the sky. The task involves acquiring and aligning hundreds of images and is far from complete.

 With the UVOT's wavelength range of 1,700 to 6,000 angstroms, Swift remains one of few missions that study ultraviolet light, much of which is blocked by Earth's atmosphere.

Omega Centauri (also known as NGC 5139) is the largest, brightest and most massive of our galaxy's retinue of 150 or so globular star clusters. Packing some 10 million stars into a region just 150 light-years across, Omega Centauri is easily visible to the unaided eye despite lying nearly 16,000 light-years away. Unlike other star clusters, whose members all have similar age and chemical makeup, Omega Centauri displays a wide range of age and chemistry, from the ancient (12 billion years) to the relatively recent. The presence of different stellar populations suggests that Omega Centauri is not, in fact, a globular cluster, but the remnant core of a dwarf galaxy torn to shreds by the Milky Way’s gravity. The false-color ultraviolet composite from Swift UVOT's uvw1, uvm2 and uvw2 filters reveals a treasure trove of rare stars in various stages of demise. Credit: NASA/Swift/S. Holland (Goddard), M. Siegel and E. Fonseca (PSU)

› Larger image (labeled) - › Larger image (unlabeled)

 The 6.5-foot-long (2 meter) UVOT is centered on an 11.8-inch (30 cm) primary mirror. Designed and built by the Mullard Space Science Laboratory in Surrey, England, the telescope module includes the primary and secondary mirrors, an external baffle to reduce scattered light, two redundant detectors -- only one has been used to date -- and a power supply.

 Each detector lies behind an identical filter wheel. The wheel holds color filters that transmit a broad range of wavelengths as well as devices called grisms, which spread out incoming light in much the same way as a prism spreads sunlight into a rainbow of component colors. The detectors retain information on the position and arrival time of each photon of light, an operating mode similar to typical X-ray telescopes.

 Because most ultraviolet light never reaches the ground, Swift's UVOT provides a unique perspective on the cosmos. For example, it can measure the amount of water produced in passing comets by detecting the ultraviolet emission of hydroxyl (OH), one of the molecular fragments created when ultraviolet sunlight breaks up water molecules. Other types of UVOT science include exploring emissions from the centers of active galaxies, studying regions undergoing star formation, and identifying some of the rarest and most exotic stars known.

Technicians prepare Swift's UVOT for vibration testing on Aug. 1, 2002, more than two years before launch, in the High Bay Clean Room at NASA's Goddard Space Flight Center in Greenbelt, Md. Credit: NASA's Goddard Space Flight Center. › Larger image

Toward the end of its energy-producing life, a star like the sun will blow away its outer layers as its core transforms into a compact, Earth-sized remnant known as a white dwarf. This chapter of stellar evolution, known to astronomers as the post-asymptotic giant branch phase, lasts only about 100,000 years -- just an eye-blink in comparison to the star's total lifetime. To better understand the process, astronomers need to study large numbers of these unusual stars.

"The UVOT's capabilities give us a great tool for surveying stellar populations and cataloging rare types of ultraviolet-bright stars," Siegel explained.

One of the first targets for the stellar survey was the giant cluster Omega Centauri, which hosts millions of stars and may be the remains of a small galaxy. Thanks to Swift's UVOT, astronomers at Goddard and Penn State have cataloged hundreds of rare stellar types in the cluster and are now comparing their properties and numbers to predictions from theoretical models describing how stars evolve.

Related Links

• Penn State's Cool UVOT Pictures gallery

• The Swift Explorer Mission iPhone app

• Young Supernovae Observed by Swift

• "Swift Makes Best-ever Ultraviolet Portrait of Andromeda Galaxy" (09.16.09)

• UVOT first-light images at Mullard Space Science Laboratory

• Additional Swift imagery

Francis Reddy
NASA's Goddard Space Flight Center, Greenbelt, Md.

Under 'dark halo' old galaxies have many more stars

Omega Centauri: the tiny red stars (blue is hot red is cold) are just the sort of faint stars that can be imaged in a nearby cluster like this one but cannot be seen in distant galaxies. However, by measuring their combined mass contribution it is possible to discover that old galaxies are dominated by little red stars like these. Click here for a high res image

Some of the oldest galaxies in the Universe have three times more stellar mass, and so many more stars, than all current models of galaxy evolution predict. The finding comes from the Atlas3D international team, led by Michele Cappellari (Oxford), and including ASTRON astronomers Paolo Serra, Raffaella Morganti and Tom Oosterloo, who found a way to remove the 'halo' of dark matter that has clouded previous calculations.

The team's analysis means that all current models, which assumed for decades that the light we observe from a galaxy can be used to infer its stellar mass, will have to be revised. It also suggests that researchers have a new riddle to ponder: exactly how galaxies forming so early in the life of the Universe got to be massive so fast. A report of the research is published in this week's Nature.

'The light we see from galaxies is just the tip of the iceberg, but what we really need to measure are galaxy masses that all models directly predict,' said Dr Michele Cappellari of Oxford University's Department of Physics, who led the work. 'Galaxies can contain huge numbers of small stars, planets or black holes that have lots of mass but give out very little or no light at all. Up until now models assumed that stellar light could be used to infer the stellar masses and any remaining discrepancy with the observed total mass could be hidden behind a 'halo' of dark matter. Our analysis shows that they can't hide any longer: galaxies are diverse and some have many more stars and are even stranger than we'd assumed.'

Up to now the key limitation on what it was possible to say about the stellar mass of galaxies was the difficulty in separating this out from the mass contributed by dark matter. Various attempts from independent groups failed to provide a conclusive answer. he new analysis succeeded thanks to the availability of two-dimensional maps of stellar motions for a large sample of galaxies, combined with sophisticated models. By disentangling stellar mass from dark matter the team was able to show that instead of the relationship between observable light and stellar mass being universal, it varies between different types of galaxies - with some older galaxies having three times the mass suggested by the light they give off.

'The question of how you should turn light from a galaxy into a prediction of its mass has been hotly debated but up until now nobody has been able to kill off the idea that there's a simple and universal way to convert observed light into mass,' said Dr Cappellari. 'We now think we've done that by eliminating the 'fuzziness' in models caused by dark matter. It's exciting because it reveals how much more there is to discover about how galaxies, and the early Universe itself, evolved.'

This research is part of the Atlas3D project and is part-funded by the Science and Technology Facilities Council, the UK sponsors of astronomy and of the William Herschel Telescope (WHT) which was used by the team. More information about the project and its team can be found here: http://www-astro.physics.ox.ac.uk/atlas3d/

For more information contact Prof. Dr Tom Oosterloo +31 (0)521 595 100 or Dr Michele Cappellari +44 (0)1865 273647

Hubble Data Used to Look 10,000 Years into the Future

Globular Star Cluster Omega Centauri
Illustration Credit: NASA, ESA, and G. Bacon (STScI)

Science Credit: NASA, ESA, and J. Anderson and R. van der Marel (STScI)

The globular star cluster Omega Centauri has caught the attention of sky watchers ever since the ancient astronomer Ptolemy first catalogued it 2,000 years ago. Ptolemy, however, thought Omega Centauri was a single star. He didn't know that the "star" was actually a beehive swarm of nearly 10 million stars, all orbiting a common center of gravity.

The stars are so tightly crammed together that astronomers had to wait for the powerful vision of NASA's Hubble Space Telescope to peer deep into the core of the "beehive" and resolve individual stars. Hubble's vision is so sharp it can even measure the motion of many of these stars, and over a relatively short span of time.

A precise measurement of star motions in giant clusters can yield insights into how stellar groupings formed in the early universe, and whether an "intermediate mass" black hole, one roughly 10,000 times as massive as our Sun, might be lurking among the stars.

Analyzing archived images taken over a four-year period by Hubble's Advanced Camera for Surveys, astronomers have made the most accurate measurements yet of the motions of more than 100,000 cluster inhabitants, the largest survey to date to study the movement of stars in any cluster.

"It takes high-speed, sophisticated computer programs to measure the tiny shifts in the positions of the stars that occur in only four years' time," says astronomer Jay Anderson of the Space Telescope Science Institute in Baltimore, Md., who conducted the study with fellow Institute astronomer Roeland van der Marel. "Ultimately, though, it is Hubble's razor-sharp vision that is the key to our ability to measure stellar motions in this cluster."

Adds van der Marel: "With Hubble, you can wait three or four years and detect the motions of the stars more accurately than if you had waited 50 years on a ground-based telescope."

The astronomers used the Hubble images, which were taken in 2002 and 2006, to make a movie simulation of the frenzied motion of the cluster's stars. The movie shows the stars' projected migration over the next 10,000 years.

Identified as a globular star cluster in 1867, Omega Centauri is one of roughly 150 such clusters in our Milky Way Galaxy. The behemoth stellar grouping is the biggest and brightest globular cluster in the Milky Way, and one of the few that can be seen by the unaided eye. Located in the constellation Centaurus, Omega Centauri is viewable in the southern skies.

CONTACT

Donna Weaver
Space Telescope Science Institute, Baltimore, Md.
410-338-4493
dweaver@stsci.edu

Jay Anderson/Roeland van der Marel
Space Telescope Science Institute, Baltimore, Md.
jayander@stsci.edu / marel@stsci.edu
410-338-4982 / 410-338-4931

Hubble Opens New Eyes on the Universe

Credit: NASA, ESA, and the Hubble SM4 ERO Team

These four images are among the first observations made by the new Wide Field Camera 3 aboard the upgraded NASA Hubble Space Telescope.

The image at top left shows NGC 6302, a butterfly-shaped nebula surrounding a dying star. At top right is a picture of a clash among members of a galactic grouping called Stephan's Quintet. The image at bottom left gives viewers a panoramic portrait of a colorful assortment of 100,000 stars residing in the crowded core of Omega Centauri, a giant globular cluster. At bottom right, an eerie pillar of star birth in the Carina Nebula rises from a sea of greenish-colored clouds.  Object Names: NGC 6302, Stephan's Quintet, Omega Centauri, Jet in Carina

WASHINGTON — Astronomers declared NASA's Hubble Space Telescope a fully rejuvenated observatory with the release Wednesday of observations from four of its six operating science instruments.

Topping the list of new views are colorful, multi-wavelength pictures of far-flung galaxies, a densely packed star cluster, an eerie "pillar of creation," and a "butterfly" nebula. Hubble's suite of new instruments allows it to study the universe across a wide swath of the light spectrum, from ultraviolet all the way to near-infrared. In addition, scientists released spectroscopic observations that slice across billions of light-years to probe the cosmic-web structure of the universe and map the distribution of elements that are fundamental to life as we know it.

"This marks a new beginning for Hubble," said Ed Weiler, associate administrator for NASA's Science Mission Directorate at NASA Headquarters in Washington. "The telescope was given an extreme makeover and now is significantly more powerful than ever, well-equipped to last into the next decade."

The new instruments are more sensitive to light and, therefore, will improve Hubble's observing efficiency significantly. It is able to complete observations in a fraction of the time that was needed with prior generations of Hubble instruments. The space observatory today is significantly more powerful than it ever has been.

"We couldn't be more thrilled with the quality of the images from the new Wide Field Camera 3 (WFC3) and repaired Advanced Camera for Surveys (ACS), and the spectra from the Cosmic Origins Spectrograph (COS) and the Space Telescope Imaging Spectrograph (STIS)," said Keith Noll, leader of a team at the Space Telescope Science Institute in Baltimore, which planned the early release observations. "The targets we've selected to showcase the telescope reveal the great range of capabilities in our newly upgraded Hubble."

These results are compelling evidence of the success of the STS-125 servicing mission in May, which has brought the space observatory to the apex of its scientific performance. Two new instruments, the WFC3 and COS, were installed, and two others, the ACS and STIS, were repaired at the circuit board level. Mission scientists also announced Wednesday that the Near Infrared Camera and Multi-Object Spectrometer was brought back into operation during the three months of calibration and testing.

"On this mission we wanted to replenish the 'tool kit' of Hubble instruments on which scientists around the world rely to carry out their cutting-edge research," said David Leckrone, senior project scientist for Hubble at NASA's Goddard Space Flight Center in Greenbelt, Md. "Prior to this servicing mission, we had only three unique instrument channels still working, and today we have 13. I'm very proud to be able to say, 'mission accomplished.' "

For the past three months, scientists and engineers at the Space Telescope Science Institute and Goddard have been focusing, testing, and calibrating the instruments. Hubble is one of the most complex space telescopes ever launched, and the Hubble servicing mission astronauts performed major surgery on the 19-year-old observatory's multiple systems. This orbital verification phase was interrupted briefly July 23 to observe Jupiter in the aftermath of a collision with a suspected comet.

Hubble now enters a phase of full science observations. The demand for observing time will be intense. Observations will range from studying the population of Kuiper Belt objects at the fringe of our solar system to surveying the birth of planets around other stars and probing the composition and structure of extrasolar planet atmospheres. There are ambitious plans to take the deepest-ever near-infrared portrait of the universe to reveal never-before-seen infant galaxies that existed when the universe was less than 500 million years old. Other planned observations will attempt to shed light on the behavior of dark energy, a repulsive force that is pushing the universe apart at an ever-faster rate.

The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. Goddard manages the telescope. The Space Telescope Science Institute conducts Hubble science operations. The institute is operated for NASA by the Association of Universities for Research in Astronomy, Inc. in Washington, and is an International Year of Astronomy 2009 program partner.

CONTACT

J.D. Harrington
Headquarters, Washington
202-358-5241
j.d.harrington@nasa.gov

Susan Hendrix
Goddard Space Flight Center, Greenbelt, Md.
301-286-7745
susan.m.hendrix@nasa.gov

Ray Villard
Space Telescope Science Institute, Baltimore, Md.
410-338-4514
villard@stsci.edu


For images and more information about the Hubble Space Telescope, visit:

• http://www.nasa.gov/hubble
• http://hubblesite.org/news/2009/25http://www.hubblesite.org/newscenter/archive/releases/2009/25/image/a/
• http://www.spacetelescope.org/news/html/heic0910.html

Source: HubbleSite