Faint Glow Within Galaxy Clusters Illuminates Dark Matter

Galaxy Clusters Abell S1063 and MACS J0416.1-2403
Credits: NASA, ESA, and M. Montes (University of New South Wales)
Acknowledgment: J. Lotz (STScI) and the HFF team

Two massive galaxy clusters — Abell S1063 (left) and MACS J0416.1-2403 (right) — display a soft blue haze, called intracluster light, embedded among innumerable galaxies. The intracluster light is produced by orphan stars that no longer belong to any single galaxy, having been thrown loose during a violent galaxy interaction, and now drift freely throughout the cluster of galaxies. Astronomers have found that intracluster light closely matches with a map of mass distribution in the cluster's overall gravitational field. This makes the blue "ghost light" a good indicator of how invisible dark matter is distributed in the cluster. Dark matter is a key missing link in our understanding of the structure and evolution of the universe. Abell S1063 and MACS J0416.1-2403 were the strongest examples of intracluster light providing a much better match to the cluster's mass map than X-ray light, which has been used in the past to trace dark matter. Release Images

A new look at Hubble images of galaxies could be a step toward illuminating the elusive nature of dark matter, the unobservable material that makes up the majority of the universe, according to a study published online today in the Monthly Notices of the Royal Astronomical Society.

Utilizing Hubble's past observations of six massive galaxy clusters in the Frontier Fields program, astronomers demonstrated that intracluster light — the diffuse glow between galaxies in a cluster — traces the path of dark matter, illuminating its distribution more accurately than existing methods that observe X-ray light.

Intracluster light is the byproduct of interactions between galaxies that disrupt their structures; in the chaos, individual stars are thrown free of their gravitational moorings in their home galaxy to realign themselves with the gravity map of the overall cluster. This is also where the vast majority of dark matter resides. X-ray light indicates where groups of galaxies are colliding, but not the underlying structure of the cluster. This makes it a less precise tracer of dark matter.

"The reason that intracluster light is such an excellent tracer of dark matter in a galaxy cluster is that both the dark matter and these stars forming the intracluster light are free-floating on the gravitational potential of the cluster itself—so they are following exactly the same gravity," said Mireia Montes of the University of New South Wales in Sydney, Australia, who is co-author of the study. "We have found a new way to see the location where the dark matter should be, because you are tracing exactly the same gravitational potential. We can illuminate, with a very faint glow, the position of dark matter."

Montes also highlights that not only is the method accurate, but it is more efficient in that it utilizes only deep imaging, rather than the more complex, time-intensive techniques of spectroscopy. This means more clusters and objects in space can be studied in less time — meaning more potential evidence of what dark matter consists of and how it behaves.

"This method puts us in the position to characterize, in a statistical way, the ultimate nature of dark matter," Montes said.

"The idea for the study was sparked while looking at the pristine Hubble Frontier Field images," said study co-author Ignacio Trujillo of the Canary Islands Institute of Astronomy in Tenerife, Spain, who along with Montes had studied intracluster light for years. "The Hubble Frontier Fields showed intracluster light in unprecedented clarity. The images were inspiring," Trujillo said. "Still, I did not expect the results to be so precise. The implications for future space-based research are very exciting."

"The astronomers used the Modified Hausdorff Distance (MHD), a metric used in shape matching, to measure the similarities between the contours of the intracluster light and the contours of the different mass maps of the clusters, which are provided as part of the data from the Hubble Frontier Fields project, housed in the Mikulski Archive for Space Telescopes (MAST). The MHD is a measure of how far two subsets are from each other. The smaller the value of MHD, the more similar the two point sets are. This analysis showed that the intracluster light distribution seen in the Hubble Frontier Fields images matched the mass distribution of the six galaxy clusters better than did X-ray emission, as derived from archived observations from Chandra X-ray Observatory's Advanced CCD Imaging Spectrometer (ACIS).

Beyond this initial study, Montes and Trujillo see multiple opportunities to expand their research. To start, they would like to increase the radius of observation in the original six clusters, to see if the degree of tracing accuracy holds up. Another important test of their method will be observation and analysis of additional galaxy clusters by more research teams, to add to the data set and confirm their findings.

The astronomers also look forward to the application of the same techniques with future powerful space-based telescopes like the James Webb Space Telescope and WFIRST, which will have even more sensitive instruments for resolving faint intracluster light in the distant universe.

Trujillo would like to test scaling down the method from massive galaxy clusters to single galaxies. "It would be fantastic to do this at galactic scales, for example exploring the stellar halos. In principal the same idea should work; the stars that surround the galaxy as a result of the merging activity should also be following the gravitational potential of the galaxy, illuminating the location and distribution of dark matter."

The Hubble Frontier Fields program was a deep imaging initiative designed to utilize the natural magnifying glass of galaxy clusters' gravity to see the extremely distant galaxies beyond them, and thereby gain insight into the early (distant) universe and the evolution of galaxies since that time. In that study the diffuse intracluster light was an annoyance, partially obscuring the distant galaxies beyond. However, that faint glow could end up shedding significant light on one of astronomy's great mysteries: the nature of dark matter.

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

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Related Links

This site is not responsible for content found on external links

• The science paper by M. Montes and I. Trujillo
• NASA's Hubble Portal
• Frontier Fields (STScI Archived Blog)
• Hubble Space Telescope Frontier Fields Portal
• Hubble-Europe's Release

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Contacts

Leah Ramsay / Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
667-218-6439 / 410-338-4514
lramsay@stsci.edu / villard@stsci.edu

Mireia Montes
University of New South Wales, Sydney, Australia
mireia.montes.quiles@gmail.com

Source: HubbleSite/News

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Hubble Finds Dead Stars 'Polluted' with Planet Debris

Artist's Impression of a White Dwarf 'Polluted' with Planet Debris 
Credit: NASA, ESA, and G. Bacon (STScI)

NASA's Hubble Space Telescope has found the building blocks for Earth-sized planets in an unlikely place, the atmospheres of a pair of burned-out stars called white dwarfs. The dwarfs are being polluted by asteroid-like debris falling onto them. This discovery suggests that rocky planet assembly is common in stars, say researchers.

The white dwarfs reside 150 light-years away in the Hyades star cluster, residing in the constellation Taurus the Bull. The cluster is relatively young, only 625 million years old.

Hubble's spectroscopic observations identified silicon in the white dwarfs' atmospheres, a major ingredient of the rocky material constituting Earth and other terrestrial planets in our solar system. The silicon may have come from asteroids that were shredded by the white dwarfs' gravity when they veered too close to the stars. The rocky debris likely formed a ring around the dead stars, which then funneled the material onto the stellar relics.

The material detected whirling around the white dwarfs suggests that terrestrial planets formed when these stars were born. After the stars collapsed to white dwarfs, surviving gas-giant planets may have gravitationally perturbed members of any leftover asteroid belts into star-grazing orbits.

"We have identified chemical evidence for the Lego building blocks of rocky planets," says Jay Farihi of the University of Cambridge in England, lead author of a new study that appeared in the May 2 issue of the Monthly Notices of the Royal Astronomical Society. "When these stars were born, they built planets, and there's a good chance they currently retain some of them. The material we are seeing is evidence of this. The debris is at least as rocky as the most primitive terrestrial bodies in our solar system."

Astronomers commonly believe that all stars formed in clusters. But searches for planets outside our solar system have only detected a handful of them orbiting cluster stars. Farihi suggested that it may be harder to make the precision measurements needed to find extrasolar planets in clusters because the stars are young and more active, producing stellar flares and other outbursts.

The team, therefore, searched planets around retired cluster stars. "Using Hubble to analyze the atmospheres of white dwarfs is the best method for finding the signatures of solid planet chemistry and determining their composition," Farihi explains. "Normally, white dwarfs are like blank pieces of paper, containing only the light elements hydrogen and helium. Heavy elements like silicon and carbon sink to the core."

Besides finding silicon in the Hyades stars' atmospheres, Hubble also detected low levels of carbon, another sign of the debris' rocky nature. Astronomers would expect carbon to be depleted or absent in rocky, Earth-like material. Carbon is a key element that helps astronomers determine the properties and origin of the planetary debris raining down onto white dwarfs. It leaves fingerprints only in ultraviolet light, which cannot be observed from ground-based telescopes. Finding its chemical signature required Hubble's Cosmic Origins Spectrograph (COS).

"The one thing the white dwarf pollution technique gives us that we just won't get with any other planet-detection technique is the chemistry of solid planets," Farihi says. "Based on the silicon-to-carbon ratio in our study, for example, we can actually say that this material is basically Earth-like. If you put this stuff into the hand of a child, or an adult, and you ask them, `What is this?' Any human being would be able to respond, ‘It's a rock!' They wouldn't need to be a scientist. They would know exactly what it is, as it's something familiar to all of us."

Farihi suggests that asteroids less than 100 miles (160 kilometers) across were probably gravitationally torn apart by the white dwarfs' strong tidal forces. The pulverized material may have been pulled into a ring that eventually fell onto the dead stars. "It's difficult to imagine another mechanism than gravity that causes material to get close enough to rain down onto the star," he says.

The team estimated each asteroid's size by measuring the amount of dust being gobbled up by the dead stars, about 10 million grams per second, equal to the flow rate of a small river. They then compared that data with measurements of material falling onto other white dwarfs.

The Hyades study offers insight into what will happen in our solar system when our Sun burns out 5 billion years from now. When the Sun exhausts its hydrogen fuel, it will puff up to a red giant and swallow Mercury and Venus, and perhaps the Earth. As the Sun begins to eject its outer layers, it loses mass. The balance of gravitational forces between the Sun and Jupiter changes, disrupting the main asteroid belt. Some of these asteroids could veer too close to the Sun, which breaks them up. The debris could be pulled into a ring around the dead Sun, similar to the inferred rings around the Hyades white dwarfs.

The two "polluted" Hyades white dwarfs are part of the team's search of planetary debris around more than 100 white dwarfs, led by Boris Gänsicke of the University of Warwick in England. Team member Detlev Koester of the University of Kiel in Germany is using sophisticated computer models of white dwarf atmospheres to determine the abundances of various elements that can be traced to planets in the COS data.

The team plans to analyze more white dwarfs using the same technique to identify not only the rocks' composition but also their parent bodies. "The beauty of this technique is that whatever the universe is doing, we'll be able to measure it," Farihi said. "We have been using our solar system as a kind of map, but I don't know what the universe does. Is there another recipe for Earth-like or habitable planets? The chemistry can tell us. Hopefully, with Hubble and its powerful ultraviolet-light camera COS, and with the upcoming ground-based 30- and 40-meter telescopes, we'll be able to tell a story. We hope to create a picture of hundreds of rocky planet building blocks and tell how often they look like Earth and how often they look different, or even exotic. Who knows, maybe we'll find some stuff we haven't thought of yet."

CONTACT

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

Hubble Breaks Record in Search for Farthest Supernova

Supernova Wilson - SN UDS10Wil

Credit: NASA, ESA, A. Riess (STScI and JHU), and D. Jones and S. Rodney (JHU)

More Images

NASA's Hubble Space Telescope has broken the record in the quest to find the farthest supernova of the type used to measure cosmic distances. Supernova UDS10Wil, nicknamed SN Wilson, after the 28th U.S. President, Woodrow Wilson, exploded more than 10 billion years ago (redshift 1.914). At that time, the universe was in its early formative years where stars were being born at a rapid rate.

SN Wilson belongs to a special class called Type Ia supernovae. These bright beacons are prized by astronomers because they provide a consistent level of brightness that can be used as a cosmic yardstick for measuring the expansion of space. They also yield clues to the nature of dark energy, the mysterious force accelerating the rate of expansion.

"The new distance record holder opens a window into the early universe, offering important new insights into how these stars explode," said astronomer David O. Jones of The Johns Hopkins University in Baltimore, Md., lead author on the science paper detailing the discovery. "At that epoch, we can test theories about how reliable these detonations are for understanding the evolution of the universe and its expansion."

One of the debates surrounding Type Ia supernovae is the fuse that ignites them. This latest detection adds credence to one of two competing theories of how they explode. Although preliminary, the evidence favors the explosive merger of two burned-out stars, called white dwarfs.

The discovery was part of a three-year Hubble program, begun in 2010, to survey faraway Type Ia supernovae to determine if they have changed over the 13.8 billion years since the big bang, the explosive birth of the universe. Called the CANDELS+CLASH Supernova Project, the census uses the sharpness and versatility of Hubble's Wide Field Camera 3 (WFC3) to assist astronomers in the search for supernovae in near-infrared light and verify their distance with spectroscopy. The survey searches for supernovae in two large Hubble programs, the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey and the Cluster Lensing and Supernova Survey with Hubble, which study thousands of galaxies. The census is led by Adam Riess of the Space Telescope Science Institute in Baltimore, Md., and The Johns Hopkins University.

Finding remote supernovae provides a powerful method to measure the universe's accelerating expansion due to dark energy. So far, Riess's team has uncovered more than 100 supernovae of all types and distances, ranging from 2.4 billion years ago to more than 10 billion years ago. Of those new discoveries, the team has identified eight Type Ia supernovae that exploded more than 9 billion years ago, including SN Wilson.

The supernova team's search technique involved taking multiple near-infrared images spaced roughly 50 days apart over the span of three years, looking for a supernova's faint glow. The team spotted SN Wilson in December 2010 in the CANDELS survey. They then used WFC3's spectrometer and the European Southern Observatory's Very Large Telescope to verify the supernova's distance and to decode its light, finding the unique signature of a Type Ia supernova.

Though SN Wilson is only four percent farther than the previous distance record holder, it pushes roughly 350 million years further back in time. The last record breaker was announced just three months ago by a separate team led by David Rubin of the U.S. Department of Energy's Lawrence Berkeley National Laboratory in California.

"These supernovae are important tools for studying the dark energy that is speeding up the expansion of space," Riess explained. "This study gives us a chance to 'stress test' the supernovae themselves to test how well we understand them."

Astronomers, however, still have much to learn about the nature of dark energy and how Type Ia supernovae explode.

"The Type Ia supernovae give us the most precise yardstick ever built, but we're not quite sure if it always measures exactly a yard," said team member Steve Rodney of The Johns Hopkins University. "The more we understand these supernovae, the more precise our cosmic yardstick will become."

By finding Type Ia supernovae so early in the universe, astronomers can distinguish between two competing explosion models. In one model the explosion is caused by a merger between two white dwarfs. In another, a white dwarf gradually feeds off its partner, a normal star, and explodes when it accretes too much mass.

The team's preliminary evidence shows a sharp decline in the rate of Type Ia supernova blasts between roughly 7.5 billion years ago and more than 10 billion years ago. The steep drop-off favors the merger of two white dwarfs because it predicts that most stars in the early universe are too young to become Type Ia supernovae.

"If supernovae were popcorn, the question is how long before they start popping?" Riess said. "You may have different theories about what is going on in the kernel. If you see when the first kernels popped and how often they popped, it tells you something important about the process of popping corn."

In the two white-dwarf scenario, the first supernovae pop off about 400 million years after they are born as stars, and then the rate gradually declines over time. "There is a cosmic 'high noon' for star formation at about 10 billion years ago," Rodney explained. "If most of the supernovae were exploding very shortly after their birth, then we would see a cosmic 'high noon' for supernova explosions at about the same time. We are actually finding relatively few supernovae like SN Wilson at the time of peak star formation, and this favors the double white-dwarf model, with a modest time delay between formation and explosion."

Knowing the type of trigger for Type Ia supernovae will also show how quickly the universe enriched itself with heavier elements, such as iron. These exploding stars produce about half of the iron in the universe, the raw material for building planets and life.

The team's results have been accepted for publication in an upcoming issue of The Astrophysical Journal.

CONTACT

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

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

Stellar Motions in Outer Halo Shed New Light on Milky Way Evolution

This illustration shows the disk of our Milky Way galaxy, surrounded by a faint, extended halo of old stars. Astronomers using the Hubble Space Telescope to observe the nearby Andromeda galaxy serendipitously identified a dozen foreground stars in the Milky Way halo. They measured the first sideways motions (represented by the arrows) for such distant halo stars. The motions indicate the possible presence of a shell in the halo, which may have formed from the accretion of a dwarf galaxy. This observation supports the view that the Milky Way has undergone continuing growth and evolution over its lifetime by consuming smaller galaxies.

Illustration Credit: NASA, ESA, and A. Feild (STScI)

Science Credit: NASA, ESA, A. Deason and P. Guhathakurta (University of California, Santa Cruz), and R. van der Marel, T. Sohn, and T. Brown (STScI)

Peering deep into the vast stellar halo that envelops our Milky Way galaxy, astronomers using NASA's Hubble Space Telescope have uncovered tantalizing evidence for the possible existence of a shell of stars that are a relic of cannibalism by our Milky Way.

Hubble was used to precisely measure, for the first time ever, the sideways motions of a small sample of stars located far from the galaxy's center. Their unusual lateral motion is circumstantial evidence that the stars may be the remnants of a shredded galaxy that was gravitationally ripped apart by the Milky Way billions of years ago. These stars support the idea that the Milky Way grew, in part, through the accretion of smaller galaxies.

"Hubble's unique capabilities are allowing astronomers to uncover clues to the galaxy's remote past. The more distant regions of the galaxy have evolved more slowly than the inner sections. Objects in the outer regions still bear the signatures of events that happened long ago," said Roeland van der Marel of the Space Telescope Science Institute (STScI) in Baltimore, Md.

They also offer a new opportunity for measuring the "hidden" mass of our galaxy, which is in the form of dark matter (an invisible form of matter that does not emit or reflect radiation). In a universe full of 100 billion galaxies, our Milky Way "home" offers the closest and therefore best site for detailed study of the history and architecture of a galaxy.

A team of astronomers led by Alis Deason of the University of California, Santa Cruz, and van der Marel identified 13 stars located roughly 80,000 light-years from the galaxy's center. They lie in the Milky Way's outer halo of ancient stars that date back to the formation of our galaxy.

The team was surprised to find that the stars showed more of a sideways, or tangential, amount of motion than they expected. This movement is different from what astronomers know about the halo stars near the Sun, which move predominantly in radial orbits. Stars in these orbits plunge toward the galactic center and travel back out again. The stars' tangential motion can be explained if there is an over-density of stars at 80,000 light-years, like cars backing up on an expressway. This traffic jam would form a shell-like feature, as seen around other galaxies.

Deason and her team plucked the outer halo stars out of seven years' worth of archival Hubble telescope observations of our neighboring Andromeda galaxy. In those observations, Hubble peered through the Milky Way's halo to study the Andromeda stars, which are more than 20 times farther away. The Milky Way's halo stars were in the foreground and considered as clutter for the study of Andromeda. But to Deason's study they were pure gold. The observations offered a unique opportunity to look at the motion of Milky Way halo stars.

Finding the stars was meticulous work. Each Hubble image contained more than 100,000 stars. "We had to somehow find those few stars that actually belonged to the Milky Way halo," van der Marel said. "It was like finding needles in a haystack."

The astronomers identified the stars based on their colors, brightnesses, and sideways motions. The halo stars appear to move faster than the Andromeda stars because they are so much closer. Team member Sangmo Tony Sohn of STScI identified the halo stars and measured both the amount and direction of their slight sideways motion. The stars move on the sky only about one milliarcsecond a year, which would be like watching a golf ball on the Moon moving one foot per month. Nonetheless, this was measured with 5 percent precision, made possible in visible-light observations because of Hubble's razor-sharp view and instrument consistency.

"Measurements of this accuracy are enabled by a combination of Hubble's sharp view, the many years' worth of observations, and the telescope's stability. Hubble is located in the space environment, and it's free of gravity, wind, atmosphere, and seismic perturbations," van der Marel said.

Stars in the inner halo have highly radial orbits. When the team compared the tangential motion of the outer halo stars with their radial motion, they were very surprised to find that the two were equal. Computer simulations of galaxy formation normally show an increasing tendency towards radial motion if one moves further out in the halo. These observations imply the opposite trend. The existence of a shell structure in the Milky Way halo is one plausible explanation of the researchers' findings. Such a shell can form by accretion of a satellite galaxy. This is consistent with a picture in which the Milky Way has undergone continuing evolution over its lifetime due to the accretion of satellite galaxies.

The team compared their results with data of halo stars recorded in the Sloan Digital Sky Survey. Those observations uncovered a higher density of stars at about the same distance as the 13 outer halo stars in their Hubble study. A similar excess of halo stars exists across the Triangulum and Andromeda constellations. Beyond that radius, the number of stars plummets.

Deason immediately thought the two results were more than just coincidence. "What may be happening is that the stars are moving quite slowly because they are at the apocenter, the farthest point in their orbit about the hub of our Milky Way," Deason explained. "The slowdown creates a pileup of stars as they loop around in their path and travel back towards the galaxy. So their in and out or radial motion decreases compared with their sideways or tangential motion."

Shells of stars have been seen in the halos of some galaxies, and astronomers predicted that the Milky Way may contain them, too. But until now there was limited evidence for their existence. The halo stars in our galaxy are hard to see because they are dim and spread across the sky.

Encouraged by this study, the team hopes to search for more distant halo stars in the Hubble archive. "These unexpected results fuel our interest in looking for more stars to confirm that this is really happening," Deason said. "At the moment we have quite a small sample. So we really can make it a lot more robust with getting more fields with Hubble." The Andromeda observations only cover a very small "keyhole view" of the sky.

The team's goal is to put together a clearer picture of the Milky Way's formation history. By knowing the orbits and motions of many halo stars it will also be possible to calculate an accurate mass for the galaxy. "Until now, what we have been missing is the stars' tangential motion, which is a key component. The tangential motion will allow us to better measure the total mass distribution of the galaxy, which is dominated by dark matter. By studying the mass distribution, we can see whether it follows the same distribution as predicted in theories of structure formation," Deason said.

The Hubble study will appear in an upcoming issue of the Astrophysical Journal.

The science team consists of A. Deason and P. Guhathakurta of UCO/Lick Observatory, University of California, Santa Cruz, Calif., and R.P. van der Marel, S.T. Sohn, and T.M. Brown of the Space Telescope Science Institute, Baltimore, Md.

CONTACT

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

Alis Deason
University of California, Santa Cruz, Calif.
831-459-3841
alis@ucolick.org

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

Hubble Reveals Rogue Planetary Orbit for Fomalhaut b

 

 Fomalhaut System

Credit: NASA, ESA, 

and P. Kalas (University of California, Berkeley and SETI Institute)

Release Images

Newly released Hubble Space Telescope images of a vast debris disk encircling the nearby star Fomalhaut, and of a mysterious planet circling it, may provide forensic evidence of a titanic planetary disruption in the system.

Astronomers are surprised to find that the debris belt is wider than previously known, spanning a gulf of space from 14 billion miles to nearly 20 billion miles from the star. Even more surprisingly, the latest Hubble images have allowed a team of astronomers to calculate that the planet follows an unusual elliptical orbit that carries it on a potentially destructive path through the vast dust ring.

The planet, called Fomalhaut b, swings as close to its star as 4.6 billion miles, and the outermost point of its orbit is 27 billion miles away from the star. The orbit was re-calculated from the newest Hubble observation made in 2012. "We are shocked — Fomalhaut b probably passed three times closer to the star than we previously thought, and now it is zipping outward," said Paul Kalas of the University of California at Berkeley and the SETI Institute in Mountain View, Calif.

The Fomalhaut team led by Kalas considers this circumstantial evidence that there may be other planet-like bodies in the system that gravitationally disturbed Fomalhaut b to place it in such a highly eccentric orbit.

His team is presenting their finding on January 8 at the 221st meeting of the American Astronomical Society in Long Beach, Calif.

Among several scenarios to explain Fomalhaut b's 2,000-year-long orbit is the hypothesis that an as yet undiscovered planet gravitationally ejected Fomalhaut b from a position closer to the star, and sent it flying into an orbit that extends beyond the dust belt. "Hot Jupiters get tossed through scattering events, where one planet goes in and one gets thrown out. This could be the planet that gets thrown out," according to co-investigator Mark Clampin of NASA's Goddard Space Flight Center in Greenbelt, Md.

Hubble also found that the dust and ice belt encircling Fomalhaut (the star) has an apparent gap slicing across the belt. This might have been carved out by another undetected planet, researchers said. "Hubble's exquisite view of the dust belt shows irregularities that strongly motivate a search for other planets in the system," Kalas said.

"If its orbit lies in the same plane with the dust belt, then Fomalhaut b will intersect the belt around 2032 on the outbound leg of its orbit. During the crossing, icy and rocky debris in the belt could crash into the planet's atmosphere and create the type of cosmic fireworks seen when comet Shoemaker-Levy 9 crashed into Jupiter," Kalas said. "But if Fomalhaut b is not co-planar with the belt, we may not see anything at all except for a gradual dimming of Fomalhaut b as it travels farther and farther from the star," he explained.

Kalas hypothesized that Fomalhaut b's extreme orbit is a major clue in explaining why the planet is unusually bright in visible light but very dim in infrared light. The planet could be between the mass of Pluto and Jupiter, but the optical brightness possibly originates from a ring or shroud of dust around the planet, reflecting starlight. The dust is rapidly produced by satellites orbiting the planet, which suffer extreme erosion by impacts and gravitational stirring when Fomalhaut b enters into the planetary system after a millennium of deep freeze beyond the main belt. "An analogy can be found by looking at Saturn, which has a tenuous but very large dust ring produced when meteoroids hit the outer moon called Phoebe," Kalas said.

The team has also considered a different scenario where a hypothetical second dwarf planet suffered a catastrophic collision with Fomalhaut b. Kalas explained, "The collision scenario would provide a solution as to why Fomalhaut (the star) has a narrow outer belt linked to an extreme planet. But in this case the belt is young, less than 10,000 years old, and it is difficult to produce energetic collisions far from the star in such young systems."

Two previous papers have confirmed Fomalhaut b's existence as derived in the previous Hubble observations.

"Fomalhaut is a rather special system because it looks like we have a snapshot of what our solar system was doing 4 billion years ago," Kalas said. "The planetary architecture is being redrawn, the comet belts are evolving, and planets may be gaining and losing their moons." Astronomers will continue monitoring Fomalhaut b for decades to come because they may have a chance to observe a planet entering an icy debris belt that is like the Kuiper Belt at the fringe of our own solar system.

CONTACT

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

Paul Kalas
University of California, Berkeley, Calif.
510-642-8285
kalas@astron.berkeley.edu

A Cosmic Holiday Ornament, Hubble-Style

 NGC 5189

Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

'Tis the season for holiday decorating and tree-trimming. Not to be left out, astronomers using NASA's Hubble Space Telescope have photographed a festive-looking nearby planetary nebula called NGC 5189. The intricate structure of this bright gaseous nebula resembles a glass-blown holiday ornament with a glowing ribbon entwined.

Planetary nebulae represent the final brief stage in the life of a medium-sized star like our Sun. While consuming the last of the fuel in its core, the dying star expels a large portion of its outer envelope. This material then becomes heated by the radiation from the stellar remnant and radiates, producing glowing clouds of gas that can show complex structures, as the ejection of mass from the star is uneven in both time and direction.

A spectacular example of this beautiful complexity is seen in the bluish lobes of NGC 5189. Most of the nebula is knotty and filamentary in its structure. As a result of the mass-loss process, the planetary nebula has been created with two nested structures, tilted with respect to each other, that expand away from the center in different directions.

This double bipolar or quadrupolar structure could be explained by the presence of a binary companion orbiting the central star and influencing the pattern of mass ejection during its nebula-producing death throes. The remnant of the central star, having lost much of its mass, now lives its final days as a white dwarf. However, there is no visual candidate for the possible companion.

The bright golden ring that twists and tilts through the image is made up of a large collection of radial filaments and cometary knots. These are usually formed by the combined action of photo-ionizing radiation and stellar winds.

This image was taken with Hubble's Wide Field Camera 3 on October 8, 2012, in filters tuned to the specific colors of fluorescing sulfur, hydrogen, and oxygen atoms. Broad filters in the visible and near-infrared were used to capture the star colors.

For additional information, please contact:

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

Hubble Provides First Census of Galaxies Near Cosmic Dawn

Hubble Ultra Deep Field 2012

Credit: NASA, ESA, R. Ellis (Caltech), and the UDF 2012 Team

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Using NASA's Hubble Space Telescope, astronomers have uncovered a previously unseen population of seven primitive galaxies that formed more than 13 billion years ago, when the universe was less than 3 percent of its present age. The deepest images to date from Hubble yield the first statistically robust sample of galaxies that tells how abundant they were close to the era when galaxies first formed.

The results show a smooth decline in the number of galaxies with increasing look-back time to about 450 million years after the big bang. The observations support the idea that galaxies assembled continuously over time and also may have provided enough radiation to reheat, or reionize, the universe a few hundred million years after the big bang.

These pioneering observations blaze a trail for future exploration of this epoch by NASA's next-generation spacecraft, the James Webb Space Telescope. Looking deeper into the universe also means peering farther back in time. The universe is now 13.7 billion years old. The newly discovered galaxies are seen as they looked 350 million to 600 million years after the big bang. Their light is just arriving at Earth now.

The greater depth of the new Hubble images, together with a carefully designed survey strategy, allows this work to go further than previous studies, thereby providing the first reliable census of this epoch, say the researchers. Notably, one of the galaxies may be a distance record breaker, observed 380 million years after the birth of our universe in the big bang, corresponding to a redshift of 11.9.

The results are from an ambitious Hubble survey of an intensively studied patch of sky known as the Ultra Deep Field (UDF). In the new 2012 campaign, called UDF 2012, a team of astronomers led by Richard Ellis of the California Institute of Technology in Pasadena, Calif., used Hubble's Wide Field Camera 3 (WFC3) to peer deeper into space in near- infrared light than any previous Hubble observation. The observations were made over a period of six weeks during August and September, and the first scientific results are now appearing in a series of scientific papers. The UDF 2012 team is publicly releasing these unique data, after preparing them for other research groups to use.

Astronomers study the distant universe in near-infrared light because the expansion of space stretches ultraviolet and visible light from galaxies into infrared wavelengths, a phenomenon called "redshift." The more distant a galaxy, the higher its redshift.

A major goal of the new program was to determine how rapidly the number of galaxies increases over time in the early universe. This measure is the key evidence for how quickly galaxies build up their constituent stars.

"Our study has taken the subject forward in two ways," Ellis explained. "First, we have used Hubble to make longer exposures than previously. The added depth is essential to reliably probe the early period of cosmic history. Second, we have used Hubble's available color filters very effectively to more precisely measure galaxy distances."

The team estimated the galaxy distances by studying their colors through a carefully chosen set of four filters at specific near-infrared wavelengths. "We added an additional filter, and undertook much deeper exposures in some filters than in earlier work in order to convincingly reject the possibility that some of our galaxies might be foreground objects," said team member James Dunlop of the Institute for Astronomy, University of Edinburgh.

For galaxies whose light has been shifted to infrared wavelengths, Dunlop said, the intervening hydrogen will have absorbed all of the light that was originally emitted as visible light and most of the light initially released at near-infrared wavelengths. Therefore, these galaxies will not be detected in most of Hubble's filters. They will only be seen in Hubble's longer-wavelength infrared filters, which hold the key to discovering the earliest galaxies.

The results from the UDF 2012 campaign mean there may be many undiscovered galaxies even deeper in space waiting to be uncovered by the Webb telescope. "Although we may have reached back as far as Hubble will see, Hubble has, in a sense, set the stage for Webb," noted team member Anton Koekemoer of the Space Telescope Science Institute in Baltimore, Md., who oversaw the Hubble observations and combined the images. "Our work indicates that there may be a rich field of even earlier galaxies that Webb will be able to study."

Astronomers have long debated whether hot stars in such early galaxies could have provided enough radiation to warm the cold hydrogen that formed soon after the big bang. This process, called "reionization," is thought to have occurred 200 million to a billion years after our universe's birth. This process made the universe transparent to light, allowing astronomers to look far back into time. The galaxies in the new study are seen in this early epoch.

"Observations of the microwave afterglow from the big bang tell us that reionization happened more than about 13 billion years ago," said Brant Robertson of the University of Arizona in Tucson. "Our data confirms that reionization was a drawn-out process occurring over several hundred million years with galaxies slowly building up their stars and chemical elements. There wasn't a single dramatic moment when galaxies formed; it was a gradual process."

The team's finding on the distant galaxy census has been accepted for publication in The Astrophysical Journal Letters.

For images and more information about these results, visit:

http://hubblesite.org/news/2012/48http://hubblesite.org/news/2012/48
http://www.nasa.gov/hubble
http://udf12.arizona.edu

CONTACT

Donna Weaver / Ray Villard
Space Telescope Science Institute, Baltimore, Md.
410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu

Richard Ellis
California Institute for Technology, Pasadena, Calif.
626-676-5530
rse@astro.caltech.edu

James Dunlop / Ross McLure
University of Edinburgh, Edinburgh, Scotland, UK 
011-131-668-8477 /011-131-668-8349
jsd@roe.ac.uk / rjm@roe.ac.uk

Brant Robertson
University of Arizona, Tucson, Ariz.
520-626-5909
brant@email.arizona.edu

Anton Koekemoer
Space Telescope Science Institute, Baltimore, Md.
410-338-4815
koekemoer@stsci.edu

'Dark Core' May Not Be So Dark After All

  Abell 520

Credit: [Top] NASA, ESA, and D. Clowe, (Ohio University); 
[Bottom] NASA, ESA, and J. Jee (University of California, Davis

Release images

These composite images, taken by two different teams using the Hubble Space Telescope, show different results concerning the amount of dark matter in the core of the merging galaxy cluster Abell 520.

Dark matter is an invisible form of matter that astronomers deduce is the underlying gravitational "glue" that holds galaxies together.

Top Image: Observations of the cluster, taken by D. Clowe with the Advanced Camera for Surveys, map the amount of dark matter in Abell 520. The map reveals an amount of dark matter astronomers expect based on the number of galaxies in the core. The dark-matter densities are marked in blue, and the dotted circle marks the dark-matter core. The map is superimposed onto visible-light images of the cluster.

Bottom Image: A second team, led by James Jee of the University of California, Davis, used the Wide Field Planetary Camera 2 and found an unusual overabundance of dark matter in the cluster's core, denoted by the bright blue color at image center. The observation was surprising because astronomers expect that dark matter and galaxies should be anchored together, even during a collision between galaxy clusters.

This discrepancy between the two results requires further observation and analysis, say researchers.

The two dark-matter maps were made by detecting how light from distant objects is distorted by the galaxy clusters, an effect called gravitational lensing.

Abell 520 is located 2.4 billion light-years away.

Contacts

Andrea Gibson
Ohio University, Athens, Ohio
740-597-2166
gibsona@ohio.edu

Douglas Clowe
Ohio University, Athens, Ohio
740-593-0063
clowe@ohio.edu

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

NASA Great Observatories Find Candidate for Most Distant Galaxy Yet Known

 Distant Galaxy Lensed by Cluster MACS J0647

 Credit: NASA, ESA, M. Postman and D. Coe (STScI), and the CLASH Team

  Cluster MACS J0647 Provides Three Magnified Views of Distant Galaxy

Credit: NASA, ESA, M. Postman and D. Coe (STScI), and the CLASH Team

Galaxy Cluster MACS J0647

Credit: NASA, ESA, M. Postman and D. Coe (STScI), and the CLASH Team

By combining the power of NASA's Hubble Space Telescope, Spitzer Space Telescope, and one of nature's own natural "zoom lenses" in space, astronomers have set a new distance record for finding the farthest galaxy yet seen in the universe.

The diminutive blob, which is only a tiny fraction of the size of our Milky Way galaxy, offers a peek back into a time when the universe was 3 percent of its present age of 13.7 billion years. The newly discovered galaxy, named MACS0647-JD, is observed 420 million years after the big bang. Its light has traveled 13.3 billion years to reach Earth.

This is the latest discovery from a large program that uses natural zoom lenses to reveal distant galaxies in the early universe. The Cluster Lensing And Supernova survey with Hubble (CLASH) is using massive galaxy clusters as cosmic telescopes to magnify distant galaxies behind them, an effect called gravitational lensing.

Along the way, 8 billion years into its journey, this light took a detour along multiple paths around the massive galaxy cluster MACS J0647+7015. Due to the gravitational lensing, the CLASH research team, an international group led by Marc Postman of the Space Telescope Science Institute in Baltimore, Md., observed three magnified images of MACS0647-JD with the Hubble telescope. The cluster's gravity boosted the light from the faraway galaxy, making the images appear approximately eight, seven, and two times brighter than they otherwise would, enabling astronomers to detect them more efficiently and with greater confidence. Without the cluster's magnification powers, astronomers would not have seen this remote galaxy.

"This cluster does what no manmade telescope can do," said Postman. "Without the magnification, it would require a Herculean effort to observe this galaxy."

The object is so small it may be in the first embryonic steps of forming an entire galaxy. An analysis shows that the galaxy is less than 600 light-years wide. Based on observations of somewhat closer galaxies, astronomers estimate that a typical galaxy of that epoch should be about 2,000 light-years wide. For comparison, the Large Magellanic Cloud, a companion dwarf galaxy to the Milky Way, is 14,000 light-years wide. Our Milky Way is 150,000 light-years across.

"This object may be one of many building blocks of a galaxy," explained Dan Coe of the Space Telescope Science Institute, lead author of the study. "Over the next 13 billion years, it may have dozens, hundreds, or even thousands of merging events with other galaxies and galaxy fragments."

The estimated total mass of the stars in this baby galaxy is roughly equal to 100 million or a billion suns, or about 0.1 percent to 1 percent the mass of our Milky Way's stars.

The galaxy was observed with 17 filters — spanning near-ultraviolet to near-infrared wavelengths — using Hubble's Wide Field Camera 3 (WFC3) and Advanced Camera for Surveys (ACS). Coe, a CLASH team member, discovered the galaxy in February 2012 while pouring through a catalogue of thousands of "lensed" objects found in Hubble observations of 17 clusters in the CLASH survey. MACS0647-JD, unlike all the others, only appeared in the two reddest filters.

"This immediately told us that MACS0647-JD is either a very red object, only shining at red wavelengths, or it is extremely distant and its light has been 'redshifted' to these wavelengths, or some combination of the two," said Coe. "We considered this full range of possibilities."

Coe and his collaborators spent months systematically ruling out these other alternative explanations for the object's identity, including red stars, brown dwarfs, and red (old and/or dusty) galaxies at intermediate distances from Earth, and concluded that a very distant galaxy was the right explanation.

"All three of the lensed galaxy images match fairly well and are in positions you would expect for a galaxy at that remote distance when you look at the predictions from our best lens models for this cluster," Coe said.

The paper will appear in the December 20 issue of The Astrophysical Journal.

Redshift is a consequence of the expansion of space over cosmic time. Coe estimates that MACS0647-JD has a redshift of 11, the highest yet observed. The wavelengths of near-ultraviolet light from the galaxy have been stretched into the near-infrared part of the spectrum as the light traveled through an expanding universe. "At early times, galaxies are ablaze with hot, young blue stars, but they appear extremely red when we see their light with Hubble."

Images of the galaxy at longer wavelengths obtained with the Spitzer Space Telescope played a key role in the analysis. If the object were intrinsically red, it would appear bright in the Spitzer images. Instead, the galaxy was barely detected, if at all, indicating its great distance. The research team plans to use Spitzer to obtain deeper observations of the galaxy, which should yield confident detections as well as estimates of the object's age and dust content.

The first galaxies probably formed somewhere between 100 million and 500 million years after the big bang, the astronomers said. Galaxies formed at earlier times are found to be more pristine, relatively free from heavy-element enrichment by later generations of supernovae.

The small-fry galaxy, however, may be too far away for any current telescope to confirm the distance based on spectroscopy, which spreads out an object's light into thousands of colors. Nevertheless, Coe is confident the fledgling galaxy is the new distance champion based on its unique colors and the research team's extensive analysis. By measuring how bright the object is at various wavelengths, the team determined a reasonably accurate estimate of the object's distance. Near-infrared wavelengths are the most critical to making distance estimates for such far-off objects.

The galaxy will almost certainly be a prime target for the James Webb Space Telescope. "We are reaching the limit of Hubble's vision because the galaxy is barely resolved," Postman said. "To really see finer structure you need a bigger telescope." At near-infrared wavelengths, Webb's resolution will be three times sharper than Hubble's. The Webb telescope's larger mirror will also collect enough light to obtain a spectrum of MACS0647-JD. This will yield a more definitive and precise measurement of the galaxy's distance, as well as the galaxy's mass, age, and amount of heavy elements it contains, which were forged by the first generation of stars.

The new distance champion is the second remote galaxy uncovered in the CLASH survey, a multi-wavelength census of 25 hefty galaxy clusters with Hubble's ACS and WFC3. Earlier this year, the CLASH team announced the discovery of a galaxy that existed when the universe was 490 million years old (redshift 9.6), 70 million years later than the new record-breaking galaxy. So far, the survey has completed observations for 20 of the 25 clusters.

The team hopes to use Hubble to search for more dwarf galaxies at these early epochs. If these infant galaxies are numerous, then they could have provided the energy to burn off the fog of hydrogen that blanketed the universe, a process called reionization. Reionization ultimately made the universe transparent to light.

CONTACT

Donna Weaver / Ray Villard
 Space Telescope Science Institute, Baltimore, Md.
 410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu

Dan Coe / Marc Postman
 Space Telescope Science Institute, Baltimore, Md.
 410-338-4312 / 410-338-4340
dcoe@stsci.edu / postman@stsci.edu

Asteroid Belts of Just the Right Size are Friendly to Life

Scenarios for the Evolution of Asteroid Belts

Illustration Credit: NASA, ESA, and A. Feild (STScI)
Science Credit: NASA, ESA, R. Martin and M. Livio (STScI)

Solar systems with life-bearing planets may be rare if they are dependent on the presence of asteroid belts of just the right mass, according to a study by Rebecca Martin, a NASA Sagan Fellow from the University of Colorado in Boulder, and astronomer Mario Livio of the Space Telescope Science Institute in Baltimore, Md.

They suggest that the size and location of an asteroid belt, shaped by the evolution of the Sun's protoplanetary disk and by the gravitational influence of a nearby giant Jupiter-like planet, may determine whether complex life will evolve on an Earth-like planet.

This might sound surprising because asteroids are considered a nuisance due to their potential to impact the Earth and trigger mass extinctions. But an emerging view proposes that asteroid collisions with planets may provide a boost to the birth and evolution of complex life.

Asteroids may have delivered water and organic compounds to the early Earth. According to the theory of punctuated equilibrium, occasional asteroid impacts might accelerate the rate of biological evolution by disrupting a planet's environment to the point where species must try new adaptation strategies.

The astronomers based their conclusion on an analysis of theoretical models and archival observations of extrasolar Jupiter-sized planets and debris disks around young stars. "Our study shows that only a tiny fraction of planetary systems observed to date seem to have giant planets in the right location to produce an asteroid belt of the appropriate size, offering the potential for life on a nearby rocky planet," said Martin, the study's lead author. "Our study suggests that our solar system may be rather special."

The findings will appear today in the Monthly Notices of the Royal Astronomical Society: Letters (published by Oxford University Press).

Martin and Livio suggest that the location of an asteroid belt relative to a Jupiter-like planet is not an accident. The asteroid belt in our solar system, located between Mars and Jupiter, is a region of millions of space rocks that sits near the "snow line," which marks the border of a cold region where volatile material such as water ice are far enough from the Sun to remain intact. At the time when the giant planets in our solar system were forming, the region just beyond the snow line contained a dense mix of ices, rock, and metals that provided enough material to build giant planets like Jupiter.

When Jupiter formed just beyond the snow line, its powerful gravity prevented nearby material inside its orbit from coalescing and building planets. Instead, Jupiter's influence caused the material to collide and break apart. These fragmented rocks settled into an asteroid belt around the Sun.

"To have such ideal conditions you need a giant planet like Jupiter that is just outside the asteroid belt [and] that migrated a little bit, but not through the belt," Livio explained. "If a large planet like Jupiter migrates through the belt, it would scatter the material. If, on the other hand, a large planet did not migrate at all, that, too, is not good because the asteroid belt would be too massive. There would be so much bombardment from asteroids that life may never evolve."

In fact, during the solar system's infancy, the asteroid belt probably had enough material to make another Earth, but Jupiter's presence and its small migration towards the Sun caused some of the material to scatter. Today, the asteroid belt contains less than one percent of its original mass. Using our solar system as a model, Martin and Livio proposed that asteroid belts in other solar systems would always be located approximately at the snow line. To test their proposal, Martin and Livio created models of protoplanetary disks around young stars and calculated the location of the snow line in those disks based on the mass of the central star.

They then looked at all the existing space-based infrared observations from NASA's Spitzer Space Telescope of 90 stars having warm dust, which could indicate the presence of an asteroid belt-like structure. The temperature of the warm dust was consistent with that of the snow line. "The warm dust falls right onto our calculated snow lines, so the observations are consistent with our predictions," Martin said.

The duo then studied observations of the 520 giant planets found outside our solar system. Only 19 of them reside outside the snow line, suggesting that most of the giant planets that may have formed outside the snowline have migrated too far inward to preserve the kind of slightly-dispersed asteroid belt needed to foster enhanced evolution of life on an Earth-like planet near the belt. Apparently, less than four percent of the observed systems may actually harbor such a compact asteroid belt.

"Based on our scenario, we should concentrate our efforts to look for complex life in systems that have a giant planet outside of the snow line," Livio said.

CONTACT

Donna Weaver / Ray Villard
Space Telescope Science Institute, Baltimore, Md.
410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu

Rebecca Martin
University of Colorado, Boulder, Colo.
rebecca.martin@jila.colorado.edu

Mario Livio
Space Telescope Science Institute, Baltimore, Md.
410-338-4439
mlivio@stsci.edu

NASA Telescopes Spy Ultra-Distant Galaxy Amidst Cosmic 'Dark Ages'

Galaxy Cluster MACS J1149+2223

Credit: NASA, ESA, W. Zheng (JHU),
M. Postman (STScI), and the CLASH Team

News Release Images

With the combined power of NASA's Spitzer and Hubble space telescopes, as well as a cosmic magnification effect, astronomers have spotted what could be the most distant galaxy ever seen. Light from the young galaxy captured by the orbiting observatories first shone when our 13.7-billion-year-old universe was just 500 million years old.

The far-off galaxy existed within an important era when the universe began to transit from the so-called cosmic dark ages. During this period, the universe went from a dark, starless expanse to a recognizable cosmos full of galaxies. The discovery of the faint, small galaxy opens a window onto the deepest, most remote epochs of cosmic history.

"This galaxy is the most distant object we have ever observed with high confidence," said Wei Zheng, a principal research scientist in the department of physics and astronomy at Johns Hopkins University in Baltimore and lead author of a new paper appearing in Nature. "Future work involving this galaxy, as well as others like it that we hope to find, will allow us to study the universe's earliest objects and how the dark ages ended."

Light from the primordial galaxy traveled approximately 13.2 billion light-years before reaching NASA's telescopes. In other words, the starlight snagged by Hubble and Spitzer left the galaxy when the universe was just 3.6 percent of its present age. Technically speaking, the galaxy has a redshift, or "z," of 9.6. The term redshift refers to how much an object's light has shifted into longer wavelengths as a result of the expansion of the universe. Astronomers use redshift to describe cosmic distances.

Unlike previous detections of galaxy candidates in this age range, which were only glimpsed in a single color, or waveband, this newfound galaxy has been seen in five different wavebands. As part of the Cluster Lensing And Supernova Survey with Hubble Program, the Hubble Space Telescope registered the newly described, far-flung galaxy in four visible and infrared wavelength bands. Spitzer measured it in a fifth, longer-wavelength infrared band, placing the discovery on firmer ground.

Objects at these extreme distances are mostly beyond the detection sensitivity of today's largest telescopes. To catch sight of these early, distant galaxies, astronomers rely on gravitational lensing. In this phenomenon, predicted by Albert Einstein a century ago, the gravity of foreground objects warps and magnifies the light from background objects. A massive galaxy cluster situated between our galaxy and the newfound galaxy magnified the newfound galaxy's light, brightening the remote object some 15 times and bringing it into view.

Based on the Hubble and Spitzer observations, astronomers think the distant galaxy was less than 200 million years old when it was viewed. It also is small and compact, containing only about one percent of the Milky Way's mass. According to leading cosmological theories, the first galaxies indeed should have started out tiny. They then progressively merged, eventually accumulating into the sizable galaxies of the more modern universe.

These first galaxies likely played the dominant role in the epoch of reionization, the event that signaled the demise of the universe's dark ages. This epoch began about 400,000 years after the big bang when neutral hydrogen gas formed from cooling particles. The first luminous stars and their host galaxies emerged a few hundred million years later. The energy released by these earliest galaxies is thought to have caused the neutral hydrogen strewn throughout the universe to ionize, or lose an electron, a state that the gas has remained in since that time.

"In essence, during the epoch of reionization, the lights came on in the universe," said paper co-author Leonidas Moustakas, a research scientist at NASA's Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, Calif.

Astronomers plan to study the rise of the first stars and galaxies and the epoch of reionization with the successor to both Hubble and Spitzer, NASA's James Webb Telescope, which is scheduled for launch in 2018. The newly described distant galaxy likely will be a prime target.

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

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

CONTACT

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

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

Hubble Watches Star Clusters on a Collision Course

30 Doradus, 30 Dor, Tarantula Nebula

Acknowledgment: R. O'Connell (University of Virginia)
and the Wide Field Camera 3 Science Oversight Committee
Image Credit: NASA, ESA, and E. Sabbi (ESA/STScI)

Astronomers using data from NASA's Hubble Space Telescope have caught two clusters full of massive stars that may be in the early stages of merging. The clusters are 170,000 light-years away in the Large Magellanic Cloud, a small satellite galaxy to our Milky Way.

What at first was thought to be only one cluster in the core of the massive star-forming region 30 Doradus (also known as the Tarantula Nebula) has been found to be a composite of two clusters that differ in age by about one million years.

The entire 30 Doradus complex has been an active star-forming region for 25 million years, and it is currently unknown how much longer this region can continue creating new stars. Smaller systems that merge into larger ones could help to explain the origin of some of the largest known star clusters.

Lead scientist Elena Sabbi of the Space Telescope Science Institute in Baltimore, Md., and her team began looking at the area while searching for runaway stars, fast-moving stars that have been kicked out of their stellar nurseries where they first formed. "Stars are supposed to form in clusters, but there are many young stars outside 30 Doradus that could not have formed where they are; they may have been ejected at very high velocity from 30 Doradus itself," Sabbi said.

She then noticed something unusual about the cluster when looking at the distribution of the low-mass stars detected by Hubble. It is not spherical, as was expected, but has features somewhat similar to the shape of two merging galaxies where their shapes are elongated by the tidal pull of gravity. Hubble's circumstantial evidence for the impending merger comes from seeing an elongated structure in one of the clusters, and from measuring a different age between the two clusters.

According to some models, the giant gas clouds out of which star clusters form may fragment into smaller pieces. Once these small pieces precipitate stars, they might then interact and merge to become a bigger system. This interaction is what Sabbi and her team think they are observing in 30 Doradus.

Also, there is an unusually large number of high-velocity stars around 30 Doradus. Astronomers believe that these stars, often called "runaway stars" were expelled from the core of 30 Doradus as the result of dynamical interactions. These interactions are very common during a process called core collapse, in which more-massive stars sink to the center of a cluster by dynamical interactions with lower-mass stars. When many massive stars have reached the core, the core becomes unstable and these massive stars start ejecting each other from the cluster.

The big cluster R136 in the center of the 30 Doradus region is too young to have already experienced a core collapse. However, since in smaller systems the core collapse is much faster, the large number of runaway stars that has been found in the 30 Doradus region can be better explained if a small cluster has merged into R136.

Follow-up studies will look at the area in more detail and on a larger scale to see if any more clusters might be interacting with the ones observed. In particular the infrared sensitivity of NASA's planned James Webb Space Telescope (JWST) will allow astronomers to look deep into the regions of the Tarantula Nebula that are obscured in visible-light photographs. In these areas cooler and dimmer stars are hidden from view inside cocoons of dust. Webb will better reveal the underlying population of stars in the nebula.

The 30 Doradus Nebula is particularly interesting to astronomers because it is a good example of how star-forming regions in the young universe may have looked. This discovery could help scientists understand the details of cluster formation and how stars formed in the early universe.

The members of Sabbi's team are D.J. Lennon (ESA/STScI), M. Gieles (University of Cambridge, UK), S.E. de Mink (STScI/JHU), N.R. Walborn, J. Anderson, A. Bellini, N. Panagia, and R. van der Marel (STScI), and J. Maíz Appelániz (Instituto de Astrofísica de Andalucía, CISC, Spain)

CONTACT

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

Elena Sabbi
Space Telescope Science Institute, Baltimore, Md.
410-338-4732
sabbi@stsci.edu

Why Is Earth So Dry?

A Tale of Two Disk Models
Credit: NASA, ESA, and A. Feild (STScI)

With large swaths of oceans, rivers that snake for hundreds of miles, and behemoth glaciers near the north and south poles, Earth doesn't seem to have a water shortage. And yet, less than one percent of our planet's mass is locked up in water, and even that may have been delivered by comets and asteroids after Earth's initial formation.

Astronomers have been puzzled by Earth's water deficiency. The standard model explaining how the solar system formed from a protoplanetary disk, a swirling disk of gas and dust surrounding our Sun, billions of years ago suggests that our planet should be a water world. Earth should have formed from icy material in a zone around the Sun where temperatures were cold enough for ices to condense out of the disk. Therefore, Earth should have formed from material rich in water. So why is our planet comparatively dry?

A new analysis of the common accretion-disk model explaining how planets form in a debris disk around our Sun uncovered a possible reason for Earth's comparative dryness. Led by Rebecca Martin and Mario Livio of the Space Telescope Science Institute in Baltimore, Md., the study found that our planet formed from rocky debris in a dry, hotter region, inside of the so-called "snow line." The snow line in our solar system currently lies in the middle of the asteroid belt, a reservoir of rubble between Mars and Jupiter; beyond this point, the Sun's light is too weak to melt the icy debris left over from the protoplanetary disk. Previous accretion-disk models suggested that the snow line was much closer to the Sun 4.5 billion years ago, when Earth formed.

"Unlike the standard accretion-disk model, the snow line in our analysis never migrates inside Earth's orbit," Livio said. "Instead, it remains farther from the Sun than the orbit of Earth, which explains why our Earth is a dry planet. In fact, our model predicts that the other innermost planets, Mercury, Venus, and Mars, are also relatively dry. "

The results have been accepted for publication in the journal Monthly Notices of the Royal Astronomical Society.

In the conventional model, the protoplanetary disk around our Sun is fully ionized (a process where electrons are stripped off of atoms) and is funneling material onto our star, which heats up the disk. The snow line is initially far away from the star, perhaps at least one billion miles. Over time, the disk runs out of material, cools, and draws the snow line inward, past Earth's orbit, before there is sufficient time for Earth to form.

"If the snow line was inside Earth's orbit when our planet formed, then it should have been an icy body," Martin explained. "Planets such as Uranus and Neptune that formed beyond the snow line are composed of tens of percents of water. But Earth doesn't have much water, and that has always been a puzzle."

Martin and Livio's study found a problem with the standard accretion-disk model for the evolution of the snow line. "We said, wait a second, disks around young stars are not fully ionized," Livio said. "They're not standard disks because there just isn't enough heat and radiation to ionize the disk."

"Very hot objects such as white dwarfs and X-ray sources release enough energy to ionize their accretion disks," Martin added. "But young stars don't have enough radiation or enough infalling material to provide the necessary energetic punch to ionize the disks."

So, if the disks aren't ionized, mechanisms that would allow material to flow through the region and fall onto the star are absent. Instead, gas and dust orbit around the star without moving inward, creating a so-called "dead zone" in the disk. The dead zone typically extends from about 0.1 astronomical unit to a few astronomical units beyond the star. (An astronomical unit is the distance between Earth and the Sun, which is roughly 93 million miles.) This zone acts like a plug, preventing matter from migrating towards the star. Material, however, piles up in the dead zone and increases its density, much like people crowding around the entrance to a concert, waiting for the gates to open.

The dense matter begins to heat up by gravitational compression. This process, in turn, heats the area outside the plug, vaporizing the icy material and turning it into dry matter. Earth forms in this hotter region, which extends to around a few astronomical units beyond the Sun, from the dry material. Martin and Livio's altered version of the standard model explains why Earth didn't wind up with an abundance of water.

Martin cautioned that the revised model is not a blueprint for how all disks around young stars behave. "Conditions within the disk will vary from star to star," Livio said, "and chance, as much as anything else, determined the precise end results for our Earth."

CONTACT

Donna Weaver / Ray Villard
Space Telescope Science Institute, Baltimore, Md.
410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu

Mario Livio
Space Telescope Science Institute, Baltimore, Md.
410-338-4439
mlivio@stsci.edu