Universe Today

Hubble astronomers have uncovered, for the first time, a population of infant stars in the Milky Way satellite galaxy, the Small Magellanic Cloud (SMC, visible to the naked eye in the southern constellation Tucana), located 210,000 light-years away.

Hubble’s exquisite sharpness plucked out an underlying population of infant stars embedded in the nebula NGC 346 that are still forming from gravitationally collapsing gas clouds. They have not yet ignited their hydrogen fuel to sustain nuclear fusion. The smallest of these infant stars is only half the mass of our Sun.

Although star birth is common within the disk of our galaxy, this smaller companion galaxy is more primeval in that it lacks a large percentage of the heavier elements that are forged in successive generations of stars through nuclear fusion.

Fragmentary galaxies like the SMC are considered primitive building blocks of larger galaxies. Most of these types of galaxies existed far away, when the universe was much younger. The SMC offers a unique nearby laboratory for understanding how stars arose in the early universe. Nestled among other starburst regions with the small galaxy, the nebula NGC 346 alone contains more than 2,500 infant stars.

The Hubble images, taken with the Advanced Camera for Surveys, identify three stellar populations in the SMC and in the region of the NGC 346 nebula ? a total of 70,000 stars. The oldest population is 4.5 billion years, roughly the age of our Sun. The younger population arose only 5 million years ago (about the time Earth’s first hominids began to walk on two feet). Lower-mass stars take longer to ignite and become full-fledged stars, so the protostellar population is 5 million years old. Curiously, the infant stars are strung along two intersecting lanes in the nebula, resembling a “T” pattern in the Hubble plot.

The observations, by Antonella Nota of the European Space Agency (ESA) and the Space Telescope Science Institute (STScI), Baltimore, Md., are being presented today at the meeting of the American Astronomical Society in San Diego, Calif.

The other science team members are: M. Sirianni (STScI/ESA), E. Sabbi (Univ. of Bologna), M. Tosi (INAF – Bologna Observ.), J.S. Gallagher (Univ. of Wisconsin), M. Meixner (STScI), M. Clampin (GSFC), S. Oey (Univ. of Michigan), A. Pasquali (ETH Zurich), L. Smith (Univ. College London), and R. Walterbos (New Mexico State Univ.).

Original Source: Hubble News Release

For the first time, astronomers have been able to combine the deepest optical images of the universe, obtained by the Hubble Space Telescope, with equally sharp images in the near-infrared part of the spectrum using a sophisticated new laser guide star system for adaptive optics at the W. M. Keck Observatory in Hawaii. The new observations, presented at the American Astronomical Society (AAS) meeting in San Diego this week, reveal unprecedented details of colliding galaxies with massive black holes at their cores, seen at a distance of around 5 billion light-years, when the universe was just over half its present age.

Observing distant galaxies in the infrared range reveals older populations of stars than can be seen at optical wavelengths, and infrared light also penetrates clouds of interstellar dust more readily than optical light. The new infrared images of distant galaxies were obtained by a team of researchers from the University of California, Santa Cruz, UCLA, and the W. M. Keck Observatory. Jason Melbourne, a graduate student at UC Santa Cruz and lead author of the study, said the initial findings include some surprises and that researchers will continue to analyze the data in the weeks to come.

“We have never been able to achieve this level of spatial resolution in the infrared before,” Melbourne said.

In addition to Melbourne, the research team, led by David Koo of UCSC and James Larkin of UCLA, includes Jennifer Lotz, Claire Max, and Jerry Nelson at UCSC; Shelley Wright and Matthew Barczys at UCLA; and Antonin H. Bouchez, Jason Chin, Scott Hartman, Erik Johansson, Robert Lafon, David Le Mignant, Paul J. Stomski, Douglas Summers, Marcos A. van Dam, and Peter L. Wizinowich at Keck Observatory.

“For the first time in these deep images of the universe we can cover all wavelengths of light from the optical to the infrared with the same level of spatial resolution. This allows us to observe detailed substructures in distant galaxies and study their constituent stars with a precision we couldn’t possibly obtain otherwise,” said Koo, a professor of astronomy and astrophysics at UCSC.

The images were obtained by Wright and the Keck AO team during testing of the laser guide star adaptive optics system on the 10-meter Keck II Telescope. They are the first science-quality images of distant galaxies obtained with the new system. This marks a major step for the Center for Adaptive Optics Treasury Survey (CATS), which will use adaptive optics to observe a large sample of faint, distant galaxies in the early universe, said UCLA’s Larkin.

” We’ve worked very hard for several years taking data around bright stars. But we have been very restricted in terms of the number and types of objects that we can observe. Only with the laser can we now reach the richest and most exciting targets.” Larkin said.

Adaptive optics (AO) corrects for the blurring effect of the atmosphere, which seriously degrades images seen by ground-based telescopes. An AO system precisely measures this blurring and corrects the image using a deformable mirror, applying corrections hundreds of times per second. To measure the blurring, AO requires a bright point-source of light in the telescope’s field of view, which can be created artificially by using a laser to excite sodium atoms in the upper atmosphere, causing them to glow. Without such a laser guide star, astronomers have had to rely on bright stars (“natural guide stars”), which drastically limits where AO can be used in the sky. Furthermore, natural guide stars are too bright to allow observations of very faint, distant galaxies in the same part of the sky, Koo said.

“The advent of the laser guide star at Keck has opened up the sky for adaptive optics observations, and we can now use Keck to focus on those fields where we already have wonderful, deep optical images from the Hubble Space Telescope,” Koo said.

Because the diameter of the Keck Telescope’s mirror is four times larger than Hubble’s, it can obtain images four times sharper than Hubble in the near infrared now that the laser guide star adaptive optics system is available to overcome the blurring effects of the atmosphere.

The images being presented at the AAS meeting were obtained in an area of the sky known as the GOODS-South field, where deep observations have already been made by Hubble, the Chandra X-ray Observatory, and other telescopes. There are six faint galaxies in the images, including two X-ray sources identified by Chandra. The X-ray emissions, combined with the disordered morphology of these objects, suggested recent merger activity, Melbourne said. Mergers can funnel large amounts of matter into the center of a galaxy, and X-ray emissions from the galactic center indicate the presence of a massive black hole that is actively consuming matter.

” We are now fairly certain that we are seeing galaxies that have undergone recent mergers,” Melbourne said. “One of these systems has a double nucleus, so you can actually see the two nuclei of the merging galaxies. The other system is highly disordered–it looks like a train wreck–and is a much stronger X-ray source.”

In addition to lighting up the galactic nucleus with x-ray emissions, mergers also tend to trigger the formation of new stars by shocking and compressing clouds of gas. So the researchers were surprised to find that the system with a double nucleus is dominated by relatively old stars and does not appear to be producing many young stars.

” If we are right about the merger scenario, then this merger is occuring between two galaxies that had already formed most of their stars billions of years before and did not have a lot of gas left over to make new stars,” Melbourne said.

If additional study shows that such objects are common further back in time, these observations could help explain one of the puzzles of galaxy formation. According to the prevailing theory of hierarchical galaxy formation, large galaxies are built up over billions of years through mergers between smaller galaxies. Since mergers trigger star formation, it has been difficult to explain the existence of very large galaxies that lack significant populations of young stars.

“One idea is that you can have a so-called dry merger, where two galaxies full of old stars but little gas merge without forming many new stars. What we are seeing in this object is consistent with a dry merger,” Melbourne said. “Even in a dry merger, there may still be enough gas to feed the black hole, producing X-ray emissions, but not enough to yield a strong burst of star formation.”

Further observations at mid- to far-infrared wavelengths, expected later this year from the Spitzer Space Telescope, may help confirm this. The Spitzer data will provide a better indication of the dust content of the galaxy, a crucial variable in interpreting these observations, Melbourne said.

The laser guide star adaptive optics system was funded by the W. M. Keck Foundation. The artificial laser guide star system was developed and integrated in a partnership between the Lawrence Livermore National Laboratory and the W. M. Keck Observatory. The laser was integrated at Keck with the help of Dee Pennington, Curtis Brown, and Pam Danforth. The NIRC2 near-infrared camera was developed by the California Institute of Technology, UCLA, and the Keck Observatory. The Keck Observatory is operated as a scientific partnership among CalTech, the University of California, and the National Aeronautics and Space Administration.

This work has been supported by the Center for Adaptive Optics, a National Science Foundation Science and Technology Center managed by UC Santa Cruz.

Original Source: Keck News Release

Astronomers using the Gemini South 8-meter telescope in Chile have observed new details in the dusty disk surrounding the nearby star Beta Pictoris which show that a large collision between planetary-sized bodies may have occurred there as recently as the past few decades.

The mid-infrared observations provide the best evidence yet for the occurrence of energetic encounters between planetesimals during the process of planetary formation.

“It is as if we were looking back about 5 billion years and watching our own solar system as it was forming into what we see today,” said Dr. Charles Telesco of the University of Florida who led the team. “Our research is a bit like a detective dusting for fingerprints to figure out a crime scene, only in this case we use the dust as a tracer to show what has happened within the cloud. The properties of the dust show not only that this was a huge collision, but that it probably happened recently in both astronomical and even human timescales.”

The team?s data revealed a significantly higher concentration of small dust grains in one region of the debris disk that gave Beta Pictoris a lopsided appearance in previous observations. According to team member Dr. Scott Fisher of the Gemini Observatory, it is the unique properties of this fine dust that allows speculation on the timing of this collision. “Many of us remember pounding chalk dust out of erasers in school,? he said. ?After you sneeze a few times, you open a window and the fine dust blows away. In Beta Pictoris, the radiation from the star will blow away the fine particles created by the collision quite rapidly. The fact that we still see them in our observations means that the collision probably happened in the past 100 years or so. Almost assuredly my grandparents were alive when this collision occurred.?

Computer models done at the University of Florida by team members Dr. Stanley Dermott, Dr. Tom Kehoe and Dr. Mark Wyatt (of the Royal Observatory, Edinburgh, UK) show that the timescales necessary to remove this fine dust in Beta Pictoris are on the order of decades. “This process moves out the smaller dust particles very quickly and leaves behind the larger debris,” said Dermott. “The larger particles will eventually disperse throughout the cloud as it orbits around the central star and the bright clump we see now will essentially dissolve into the disk.”

Disks of material surrounding stars such as Beta Pictoris are thought to contain objects of all sizes, from small dust grains similar to household dust to large planetesimals, or developing planets. As all of these objects orbit around the star, just like the Earth circles the Sun, they occasionally collide. The largest of these catastrophic encounters leave behind tell-tail debris clouds of fine dust observable at infrared wavelengths. By collecting high-resolution images from across a broad swath of the thermal infrared part of the spectrum, the research team from the US, UK and Chile was able to study such a cloud within the larger Beta Pictoris disk and analyze the images to determine the spatial distribution and estimate the size of the debris particles in the post-collision aftermath.

A collision similar to this one may well have created our own Moon several billion years ago when a Mars-sized body collided with what would eventually become the Earth. While the Moon itself formed out of large rocks and debris created by the collision, the small dust particles were blown away by radiation pressure from the young Sun. In the Beta Pictoris system radiation from the central star blows at about 15 times the intensity of the Sun, clearing out small grains even more quickly.

Because the Beta Pictoris disk is oriented to us edge-on, the observed asymmetry is visible as a bright ?clump? in the cigar-shaped cloud of material orbiting the central star. The Gemini images also reveal new structures in the disk that might show where planets are forming in the system. The team is still studying these features, and follow-up observations are planned using Gemini South?s newly silver-coated 8-meter mirror. This silver coating (now on both Gemini telescopes) makes the twin telescopes the most powerful facilities on Earth for this type of infrared research.

Beta Pictoris was one of the first “circumstellar” disks discovered by astronomers. It was initially detected in IRAS (Infrared Astronomy Satellite) data in 1983 by a team led by Dr. Fred Gillett (formerly Gemini?s Lead Scientist) and then imaged by Dr. Bradley Smith and Dr. Richard Terrile. Its lopsided nature was apparent even then, but until recently, observations yielded insufficient data at high-enough resolutions to show the clumpy nature of this asymmetry and estimate the relative particle distribution in the cloud.

The Gemini data were obtained using the Gemini Thermal-Region Camera Spectrograph (T-ReCS) on the Gemini South Telescope on Cerro Pach?n in Chile.

The international team published their findings and conclusions in the January 13 issue of the journal Nature and in San Diego, California at the 205th meeting of the American Astronomical Society.

Original Source: Gemini News Release

A trio of massive, young star clusters found embedded in a star cloud may shed light on the formation of super-star clusters and globular clusters.

The discovery, made with images taken with the Hubble Space Telescope, is being presented today by You-Hua Chu and Rosie Chen of the University of Illinois at
Urbana-Champaign and Kelsey Johnson of the University of Virginia to the American Astronomical Society meeting in San Diego. This finding indicates that super-star clusters may be formed by coalescence of smaller clusters.

The tightly packed group of clusters was found in the core of the active star formation region NGC 5461, within an arm of the giant spiral galaxy M101. This galaxy is located about 23 million light-years away in the constellation Ursa Major (the Big Dipper).

?NGC 5461 has such a high concentration of light in its core that some astronomers have thought it might host a super-star cluster,? said Chu, who is a professor of astronomy at Illinois and principal investigator of the project. Super-star clusters, with a total mass of up to 1 million times that of the sun, are five to 50 times more massive than the spectacular R136 cluster at the center of the Tarantula Nebula in the Large Magellanic Cloud. They are believed to be the young counterparts of the massive globular clusters in our galaxy.

Hubble Space Telescope images of the core of NGC 5461 revealed a tight group of three massive clusters surrounded by a cloud of stars within a region about 100 light-years in diameter. Although each cluster is comparable to the R136 cluster, the total mass within this small volume is similar to that of a super-star cluster.

?If NGC 5461 were several times farther away, even the Hubble Space Telescope would be unable to resolve this tight group of clusters,? said Chen, a graduate student at Illinois. ?It is possible that some of the super-star clusters previously reported in distant galaxies actually consist of groups of clusters similar to NGC 5461.?

The large amount of mass at the core of NGC 5461 produces a strong gravitational field, causing the clusters and stars to move and interact dynamically. The rapidly fluctuating gravitational field produced by this interaction dissipates the relative motion of the clusters into random motions of individual stars. Eventually, the clusters and surrounding star cloud will merge into one single star cluster.

?The Hubble Space Telescope images of NGC 5461 provide a unique glimpse of a super-star cluster in the making,? said Johnson, a professor of astronomy at Virginia. ?There is no super-star cluster yet, but it is just a matter of time.?

The dynamical evolution of the clusters at the core of NGC 5461 is being simulated by astronomy professor Paul Ricker at Illinois. Preliminary results show that under optimal conditions these clusters may merge within a few million years.

?Fortunately, NGC 5461 is near enough, and young enough for us to resolve it with the Hubble Space Telescope,? Chu said. ?We were indeed lucky to catch it at such an opportune time.?

The work was supported by the National Aeronautics and Space Administration. The researchers will report their findings in the Feb. 1 issue of the Astrophysical Journal.

Original Source: UIUC News Release

Image credit: McDonald Observatory
Observations of the white dwarf star, Sirius B, made with NASA’s Far Ultraviolet Spectroscopic Explorer (FUSE) satellite give astronomers firm new evidence that mathematical models widely used to predict white dwarf star mass and radius are correct.

Jay B. Holberg of the University of Arizona Lunar and Planetary Laboratory is presenting the result today at the American Astronomical Society in San Diego.

The FUSE result is important because Sirius B is one of the few stars that astronomers have to test their ideas on the relationship between mass and radius for white dwarf stars. White dwarf stars are small but astonishingly dense stars. Sirius B is the size of the Earth and as massive as the sun.

Theory that describes how white dwarf stars can exist emerged in the early 1930s, when Subramanyan Chandrasekhar ? or Chandra, as he was known ? calculated the limit to a white dwarf’s mass by applying Einstein’s theory of special relativity. It was one of the first applications of quantum mechanics to large physical systems in the sky.

No white dwarf star could be more than 1.4 times as massive as the sun or it will collapse, Chandra predicted.

“Chandra was the first person to lay out the essential details of how white dwarfs sustain themselves, and it is very, very different from the sun or any other stars,” Holberg said.

Unlike most white dwarfs, Sirius B is part of a binary system, and astronomers can determine the mass of stars in a binary system.

“Having a binary system ? when two stars orbit one another – is virtually the only way you can fundamentally measure the mass of a star,” Holberg said. “You observe their orbits, get the period, know how far away they are, and you can find the sum of the two star masses. If you can time the orbits and know how far apart the stars are, you can determine the individual star masses. That’s the most accurate way, the acceptable way to determine star masses.

“But this star has always been devilishly difficult to observe,” Holberg said. The primary star in the system, Sirius A, is 8 light years from Earth and has twice the mass of the sun. It is the brightest star in the night sky, visible below Orion. Sirius B is 10,000 times dimmer than Sirius A. Astronomers can?t even see the white dwarf companion when it comes closest to the primary star during its 50-year, very elongated orbit around Sirius A.

For the post several years, Holberg and colleagues have observed Sirius B with the Voyager and Extreme Ultraviolet Explorer spacecrafts. They have refined the star’s temperature and gravity – gravity being the gravitational field at the surface of the star – to refine estimates of its mass and radius.

“The methods we’re using are spectroscopic. They infer the mass from synthetic models that we produce from measurements of temperature and gravity, the only two parameters of matter for a white dwarf.”

Holberg and his colleagues published the best determination of Sirius B’s mass-radius relationship in 1998, but that was “still far from definitive,” Holberg said. “That is, the uncertainties are so large, that while these studies define the basic relationship, they don?t tell you lots of details we need to know about these stars.”

The FUSE observations gave Holberg and his colleagues better spectral data on Sirius B’s gravitational field and temperature needed to calculate mass. “And this is a very clean spectrum. We rolled the FUSE spacecraft to keep Sirius A from contaminating the spectrum, and we succeeded very well.

“The mathematical model very well predicts our results on the gravitational field, temperature and brightness of this white dwarf star,” Holberg said. “That helps us determine the radius of the star. What we really want to do is determine mass and radius to within one percent. By verifying the Chandrasekhar limit, you put a great deal of astrophysics on much firmer footing,” he added.

“Astronomy has reached the level where you can make very definitive comparisons between the models and the observations. And it looks like we are going to come out to what we expected,” Holberg said.

Original Source: University of Arizona News Release

Astronomers have numerous technical terms and numbering systems for describing the universe, but one type of mysterious object has yet to be classified. For now, these oddities are named for their strange appearance. They are called blobs.

Today, at the 205th annual meeting of the American Astronomical Society in San Diego, Calif., astronomers presented new evidence in the case of the giant galactic blobs. These blobs are huge clouds of intensely glowing material that envelop faraway galaxies. Using NASA’s Spitzer Space Telescope and its powerful infrared vision, the astronomers caught a glimpse of the galaxies tucked inside the blobs. Their observations reveal monstrously bright galaxies and suggest that blobs might surround not one, but multiple galaxies in the process of merging together.

“It is possible that extremely bright galactic mergers lie at the center of all the mysterious blobs, but we still don’t know how they fuel the blobs themselves,” said Dr. Harry Teplitz, Spitzer Science Center, California Institute of Technology, Pasadena, Calif., co-author of the new research. “It’s like seeing smoke in the distance and now discovering that it’s a forest fire, not a house or car fire, but still not knowing whether it was caused by lightning or arson.”

The findings will ultimately provide a better understanding of how galaxies, including ones like our own Milky Way, form.

Blobs were first discovered about five years ago with visible-light telescopes. They are located billions of light-years away in ancient galactic structures or filaments, where thousands of young galaxies are clustered together. These large, fuzzy galactic halos are made up of hot hydrogen gas and are about 10 times as large as the galaxies they encompass. Astronomers can see glowing blobs, but they don’t know what provides the energy to light them up.

“To figure out what’s going on, we need to better characterize the galaxies at the center of the blobs,” said Dr. James Colbert, Spitzer Science Center, first author of the study.

That’s where Spitzer comes in. Spitzer can sense the infrared glow from the dusty galaxies inside the blobs. When Colbert and colleagues used Spitzer to look at four well-known blobs located in a galactic filament 11 billion light-years away, they discovered that one of them appears to be made up of three galaxies falling into each other — an unusual cosmic event. The finding is intriguing because previous observations from NASA’s Hubble Space Telescope found that another one of the four blobs surrounds a merger between two galaxies. The astronomers speculate that all blobs might share this trait; however, more evidence is needed to close the case.

One clue that the scientists might be on the right track has to do with the infrared brightness of the blob galaxies. To visible-light telescopes, these galaxies appear unremarkable. Spitzer measurements revealed that all four of the galaxies studied are among the brightest in the universe, giving off the equivalent light of trillions of Suns. Such luminous galaxies are often triggered when smaller, gas-rich ones crash together, supporting the notion that galactic mergers might make up the cores of blobs.

Even if galactic mergers are fingered as the culprit, the mystery of the giant galactic blobs will persist. Astronomers will have to figure out why mergers are producing such tremendous clouds of material.

“Far from solving the mystery of the blobs, these observations only deepen it. Not only are the gas clouds bizarre, we now know that they contain some of the brightest and most violent galaxies in the universe,” said Teplitz.

Other authors of this work include Dr. Paul Francis, The Australian National University Canberra, Australia; Dr. Povilas Palunas, University of Texas at Austin; Dr. Gerard Williger, Johns Hopkins University, Baltimore, Md.; and Dr. Bruce E. Woodgate, NASA’s Goddard Space Flight Center, Greenbelt, Md.

NASA’s Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope mission for NASA’s Science Mission Directorate, Washington, D.C. Science operations are conducted at the Spitzer Science Center. The telescope’s multiband imaging photometer, which made the new Spitzer observations, was built by Ball Aerospace Corporation, Boulder, Colo., the University of Arizona, Tucson, and Boeing North American, Canoga Park, Calif. The instrument’s development was led by Dr. George Rieke, University of Arizona.

Images and additional information about the Spitzer Space Telescope are available at http://www.spitzer.caltech.edu/Media.

Original Source: NASA/JPL News Release

Image credit: SWRI
Recently, astronomers reported the surprising discovery of a very large diameter Kuiper Belt planetoid — (90377) Sedna — on a distant, 12,500-year-long, eccentric orbit centered approximately 500 astronomical units from the Sun. Sedna’s estimated diameter is about 1,600 km, two-thirds that of Pluto. Initial studies of Sedna’s origin have speculated that it might have been ejected from the giant planets region of our solar system far inside the orbit of Pluto, or perhaps was captured from a passing star’s Kuiper Belt.

In a report published in the January 2005 issue of The Astronomical Journal, planetary scientist Dr. Alan Stern of the Space Science and Engineering Division at Southwest Research Institute? (SwRI?) shows Sedna could have formed far beyond the distance of Pluto.

“If this is actually what happened,” Stern points out, “it would indicate that our solar system’s planet factory operated across a much larger region than previously thought.” It would also indicate that the mysterious Kuiper Belt “edge” near 50 AU (one AU is the distance from the Earth to the Sun) is not an outer edge, but simply the inner edge of an annular trough, or gap, that is carved out of a much broader structure that has been called the “Kuiper disk.”

The new Sedna formation study used a planetary accretion code developed by Stern with funding from NASA’s Origins of Solar System’s Program in the late 1990s for studies of the formation of Kuiper Belt Objects. This software was used to explore the feasibility of building Sedna from boulder-sized and other small bodies at distances between 75 AU (Sedna’s closest solar approach distance) and 500 AU (Sedna’s average distance from the Sun). Stern’s Sedna formation simulations assumed that Sedna’s original orbit, while distant from the Sun, was circular. Astronomers agree that Sedna could not have formed in its present, eccentric orbit because such an orbit allows only violent collisions that prevent the growth of small bodies. Stern’s simulations further assumed that the solar nebula — the disk of material out of which the planets formed — was much more extended than most previous simulations had assumed.

“The Sedna formation simulations assumed that the primordial solar nebula was a disk about the size of those observed around many nearby middle-aged stars — like the well-known example of the 1,500-AU-wide disk around the star Beta Pictoris,” Stern says.

“The model calculations found that objects as large, or even larger, than Sedna could easily form in circular orbits at distances of 75 to 500 AU, and that their formation time could have been fairly short — just a few percent the age of the solar system,” Stern continues. “If Sedna did form this far out, it is likely to be accompanied by a cohort of other large planetoids in this very distant region of the solar system. One telltale sign that these objects were formed where they are, rather than in another location, would be if a good fraction of them are on near circular orbits.”

Original Source: SWRI News Release