Surfing on the light of planet forming dust clouds

Combination of radiation pressure from the star and the disk creates a net force that enables dust grains to surf along the disk surface from inner to outer regions of the disk.

High resolution image: TIFF, JPG

VIDEO: A short animation showing how the newly proposed mechanism of dust movement works.

wmv format - HighRes (9.8Mb)
wmv format - LowRes (4.8Mb)

Rocky planets such as Earth are all believed to have begun as dust circling newly born stars. Ample evidence of this is found in today's meteorites and comets of the solar system, as well as observations of circumstellar disks around young stars. However, details of the evolution of dust and how it eventually comes to form larger objects such as comets and planets are still largely unexplained.
One of the key ingredients in planet and comet formation studies is to understand and account for all mechanisms that influence dust movements through the "protoplanetary disk" - the thick disk of dust and gas where planets and comets are forming. In a paper published in this week's Nature, Dejan Vinković, associate professor of astrophysics at the University of Split, Croatia, describes a new mechanism that moves particles in a previously unexpected way and has the ability to transport them over large distances.

The force produced by the light shining on an object is a well known phenomenon called radiation pressure. We do not feel it in daily lives because we are too massive for this effect to be noticeable. For very small particles, on the other hand, this force can be larger even than the gravity that keeps particles in the orbit around the star. Investigations have been focused so far only to the radiation pressure due to the starlight. The results showed that individual grains would not travel far and would be pushed deeper into the disk.

However, Vinković points out that it is not only the star, but also the disk that shines. When studying effects on protoplanetary dust grains larger than one micrometer, which is comparable to the particle size of cigarette smoke, Vinković has discovered that the intense infrared light from the hottest regions of the protoplanetary disk is capable of pushing such dust out of the disk. Infrared radiation is what we can feel as "heat" on our skin. Combination of radiation pressure from the star and the disk creates a net force that enables dust grains to surf along the disk surface from inner to outer regions of the disk.

The temperatures in this hot region reach around 1500 degrees Kelvin (2200 degrees Fahrenheit), enough to vaporize solid dust particles or to alter their physical and chemical structure. The mechanism that Vinković describes in his paper would transfer such altered dust particles to colder disk regions away form the star. This can explain why comets contain a puzzling combination of ices and particles altered at high temperatures. Astronomers have been perplexed by this mixture, since comets form in cold disk regions out of frozen substances like water, carbon dioxide or methane. Rocky dust particles that end up mixed with ices are therefore expected to never experience high temperatures.

This mechanism is also important for our understanding of the structure of the inner hot part of protoplantary disk, as well as in the interpretation of images and spectra of inner disks that astronomers detect around young stars. Theories aiming at reconstructing the planet formation process need a realistic description of the initial conditions and the disk structure. Vinković points out that his mechanism has to be taken into account because it helps with local mixing of dust grains in the inner disk.

Source: Dejan Vinkovic'
Contact: vinkovic@pmfst.hr

New Horizons Team Remembers Venetia Phair, the ‘Girl Who Named Pluto’

Venetia Burney at age 11, when she suggested the name "Pluto" for the newly discovered ninth planet in 1930. Credit: Venetia Burney Phair (via the BBC)

New Horizons Principal Investigator Alan Stern presents a plaque(1) to Venetia Burney Phair in December 2006, commemorating the name “Venetia” for the New Horizons Student Dust Counter. Read Stern’s account of their meeting at the end of this “PI Perspective” entry.

The team guiding the first mission to Pluto is fondly remembering Venetia Burney Phair, the “little girl” who named the ninth planet when it was discovered nearly 80 years ago. Mrs. Phair died April 30 at her home in Epsom, England, at age 90.

“Venetia's interest and success in naming Pluto as a schoolgirl caught the attention of the world and earned her a place in the history of planetary astronomy that lives on,” says New Horizons Principal Investigator Alan Stern.

In June 2006, the New Horizons team renamed the spacecraft’s Student Dust Counter instrument in her honor, calling it the “Venetia Burney Student Dust Counter” (VBSDC, or just “Venetia” for short). Six months later, in a small ceremony in Mrs. Phair’s home, Stern and SDC Principal Investigator Mihaly Horanyi presented her with a plaque, certificate and spacecraft model to commemorate the renaming. “She was a thoroughly intelligent, likable and endearing woman,” Stern says. “The entire New Horizons team is saddened by her passing.”

The New Horizons dust counter is the first the first science instrument on a NASA planetary mission to be designed, built and operated by students, and by late next year it will be operating farther out in the solar system than any dust measurement instrument in history. Stern and the SDC team members thought it fitting to name instrument built by students after Mrs. Phair, who was just an 11-year-old student herself when she made her historic suggestion of a name for Pluto in 1930.

“Her death deeply saddens the former and current crew of the VBSDC instrument,” says Horanyi, who, like the dust counter student team, is from the Laboratory for Atmospheric and Space Physics at the University of Colorado, Boulder. “Her contribution will be lasting, not only by naming Pluto, but also by giving an example to young people of the value of intellectual curiosity and the rewards of a lifelong interest in science and discovery.”

(1) Plaque Commemorating the Venetia Burney SDC:
"New Horizons, the first mission to Pluto and the Kuiper Belt, is proud to announce that the student instrument aboard our spacecraft is hereby named “The Venetia Burney Student Dust Counter” in honor of Mrs. Venetia Burney Phair, who at age of eleven nominated the name Pluto for our solar system's ninth planet. May “Venetia” inspire a new generation of students to explore our solar system, to make discoveries which challenge the imagination, and to pursue learning all through their lives."

Links:

Read about the instrument renaming and see photos of the Venetia Burney Student Dust Counter

The Guardian: Venetia Phair, who named Pluto, dies at 90

The Telegraph: Venetia Phair

Forty Thousand Meteor Origins Across the Sky

Forty Thousand Meteor Origins Across the Sky
Credit & Copyright: SonotaCo Network, Japan
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Explanation: Where do meteors come from? Visible meteors are typically sand-sized grains of ice and rock that once fragmented from comets.

Many a meteor shower has been associated with a known comet, although some intriguing orphan showers do remain. Recently, a group of meteor enthusiasts created a network of over 100 video cameras placed at 25 well-separated locations across Japan.

This unprecedented network recorded not only 240,000 optically bright meteors over two years, but almost 40,000 meteors seen by more than one station.

These multiple-station events were particularly interesting because they enabled the observers to extrapolate meteor trajectories back into the Solar System.

The resulting radiant map is shown above, with many well known meteor showers labelled by the first three letters of the home constellation.

Besides known meteor showers, eleven new showers were identified by new radiants on the sky from which meteors appear to flow.

The meteor sky is ever changing, and it may be possible that new shower radiants will appear in the future.

Research like this could also potentially identify previously unknown comets or asteroids that might one day pass close to the Earth.

Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (UMCP)
NASA Official: Phillip Newman Specific rights apply.
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A service of: ASD at NASA / GSFC
& Michigan Tech. U.

Hubble Photographs a Planetary Nebula to Commemorate Decommissioning of Super Camera

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

Compass and Scale File for K 4-55

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

The Hubble community bids farewell to the soon-to-be decommissioned Wide Field Planetary Camera 2 (WFPC2) onboard the Hubble Space Telescope. In tribute to Hubble's longest-running optical camera, a planetary nebula has been imaged as WFPC2's final "pretty picture."

This planetary nebula is known as Kohoutek 4-55 (or K 4-55). It is one of a series of planetary nebulae that were named after their discoverer, Czech astronomer Lubos Kohoutek. A planetary nebula contains the outer layers of a red giant star that were expelled into interstellar space when the star was in the late stages of its life. Ultraviolet radiation emitted from the remaining hot core of the star ionizes the ejected gas shells, causing them to glow.

In the specific case of K 4-55, a bright inner ring is surrounded by a bipolar structure. The entire system is then surrounded by a faint red halo, seen in the emission by nitrogen gas. This multi-shell structure is fairly uncommon in planetary nebulae.

This Hubble image was taken by WFPC2 on May 4, 2009. The colors represent the makeup of the various emission clouds in the nebula: red represents nitrogen, green represents hydrogen, and blue represents oxygen. K 4-55 is nearly 4,600 light-years away in the constellation Cygnus.

The WFPC2 instrument, which was installed in 1993 to replace the original Wide Field/Planetary Camera, will be removed to make room for Wide Field Camera 3 during the upcoming Hubble Servicing Mission.

During the camera's amazing, nearly 16-year run, WFPC2 provided outstanding science and spectacular images of the cosmos. Some of its best-remembered images are of the Eagle Nebula pillars, Comet P/Shoemaker-Levy 9's impacts on Jupiter's atmosphere, and the 1995 Hubble Deep Field — the longest and deepest Hubble optical image of its time.

The scientific and inspirational legacy of WFPC2 will be felt by astronomers and the public alike, for as long as the story of the Hubble Space Telescope is told.

WFPC2 was developed and built by NASA's Jet Propulsion Laboratory, Pasadena, Calif.

Acknowledgment: R. Sahai and J. Trauger (Jet Propulsion Laboratory)

Refined Hubble Constant Narrows Possible Explanations for Dark Energy

Image Type: Astronomical/Illustration
Credit: NASA, ESA, and A. Riess (STScI/JHU)

About this image: This is a Hubble Space Telescope photo of the spiral galaxy NGC 3021. This was one of several hosts of recent Type Ia supernovae observed by astronomers to refine the measure of the universe's expansion rate, called the Hubble constant. Hubble made precise measurements of Cepheid variable stars in the galaxy, highlighted by green circles in the four inset boxes. These stars pulsate at a rate that is matched closely to their intrinsic brightness. This makes them ideal for measuring intergalactic distances. The Cepheids are used to calibrate an even brighter milepost marker that can be used over greater distances, a Type Ia supernova. The supernova was observed in the galaxy in 1995. The images in the boxes were taken with the Near Infrared Camera and Multi-Object Spectrometer (NICMOS).

Image Type: Illustration

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

About this image: Hubble measurements have simplified the cosmic "distance ladder," which is needed to calculate a more precise value for the universe's expansion rate, called the Hubble constant. At select host galaxies, Cepheid variable stars — known as reliable milepost markers — are cross-calibrated to Type Ia supernovae in the same host galaxy. The new technique reduced the distance ladder to three "rungs": (1) The distance to galaxy NGC 4258 is measured using straightforward geometry and Kepler's laws; (2) Cepheids in six more distant galaxies are used to calibrate the luminosity of Type Ia supernovae; (3) The Hubble constant is measured by observing a brighter milepost marker, Type Ia supernovae, in more distant galaxies hundreds of millions of light-years away, embedded in the expanding universe.

INTRODUCTION

Less than 100 years ago scientists didn't know if the universe was coming or going, literally. It even fooled the great mind of Albert Einstein. He assumed the universe must be static. But to keep the universe from collapsing under gravity like a house of cards, Einstein hypothesized there was a repulsive force at work, called the cosmological constant, that counterbalanced gravity's tug. Along came Edwin Hubble in 1923 who found that galaxies were receding from us at a proportional rate, called the Hubble constant, which meant the universe was uniformly expanding, so there was no need to shore it up with any mysterious force from deep space. In measuring how this expansion was expected to slow down over time, 11 years ago, two studies, one led by Adam Riess of the Space Telescope Science Institute and the Johns Hopkins University and Brian Schmidt of Mount Stromlo Observatory, and the other by Saul Perlmutter of Lawrence Berkeley National Laboratory, independently discovered dark energy, which seems to behave like Einstein's cosmological constant.

To better characterize dark energy, Riess used Hubble Space Telescope's crisp view (combined with 2003 data from NASA's Wilkinson Microwave Anisotropy Probe, WMAP) to refine the value of the universe's expansion rate to a precision of three percent. That's a big step from 20 years ago when astronomers' estimates for the Hubble constant disagreed by a factor of two. This new value implies that dark energy really is a steady push on the universe as Einstein imagined, rather than something more effervescent (like the early inflationary universe) that changes markedly over time.

Whatever dark energy is, explanations for it have less wiggle room following a Hubble Space Telescope observation that has refined the measurement of the universe's present expansion rate to a precision where the error is smaller than five percent. The new value for the expansion rate, known as the Hubble constant, or H0 (after Edwin Hubble who first measured the expansion of the universe nearly a century ago), is 74.2 kilometers per second per megaparsec (error margin of ± 3.6). The results agree closely with an earlier measurement gleaned from Hubble of 72 ± 8 km/sec/megaparsec, but are now more than twice as precise.

The Hubble measurement, conducted by the SHOES (Supernova H0 for the Equation of State) Team and led by Adam Riess, of the Space Telescope Science Institute and the Johns Hopkins University, uses a number of refinements to streamline and strengthen the construction of a cosmic "distance ladder," a billion light-years in length, that astronomers use to determine the universe's expansion rate.

Hubble observations of pulsating stars called Cepheid variables in a nearby cosmic mile marker, the galaxy NGC 4258, and in the host galaxies of recent supernovae, directly link these distance indicators. The use of Hubble to bridge these rungs in the ladder eliminated the systematic errors that are almost unavoidably introduced by comparing measurements from different telescopes.

Riess explains the new technique: "It's like measuring a building with a long tape measure instead of moving a yard stick end over end. You avoid compounding the little errors you make every time you move the yardstick. The higher the building, the greater the error."

Lucas Macri, professor of physics and astronomy at Texas A&M, and a significant contributor to the results, said, "Cepheids are the backbone of the distance ladder because their pulsation periods, which are easily observed, correlate directly with their luminosities. Another refinement of our ladder is the fact that we have observed the Cepheids in the near-infrared parts of the electromagnetic spectrum where these variable stars are better distance indicators than at optical wavelengths."

This new, more precise value of the Hubble constant was used to test and constrain the properties of dark energy, the form of energy that produces a repulsive force in space, which is causing the expansion rate of the universe to accelerate.

By bracketing the expansion history of the universe between today and when the universe was only approximately 380,000 years old, the astronomers were able to place limits on the nature of the dark energy that is causing the expansion to speed up. (The measurement for the far, early universe is derived from fluctuations in the cosmic microwave background, as resolved by NASA's Wilkinson Microwave Anisotropy Probe, WMAP, in 2003.)

Their result is consistent with the simplest interpretation of dark energy: that it is mathematically equivalent to Albert Einstein's hypothesized cosmological constant, introduced a century ago to push on the fabric of space and prevent the universe from collapsing under the pull of gravity. (Einstein, however, removed the constant once the expansion of the universe was discovered by Edwin Hubble.)

"If you put in a box all the ways that dark energy might differ from the cosmological constant, that box would now be three times smaller," says Riess. "That's progress, but we still have a long way to go to pin down the nature of dark energy."

Though the cosmological constant was conceived of long ago, observational evidence for dark energy didn't come along until 11 years ago, when two studies, one led by Riess and Brian Schmidt of Mount Stromlo Observatory, and the other by Saul Perlmutter of Lawrence Berkeley National Laboratory, discovered dark energy independently, in part with Hubble observations. Since then astronomers have been pursuing observations to better characterize dark energy.

Riess's approach to narrowing alternative explanations for dark energy—whether it is a static cosmological constant or a dynamical field (like the repulsive force that drove inflation after the big bang)—is to further refine measurements of the universe's expansion history.

Before Hubble was launched in 1990, the estimates of the Hubble constant varied by a factor of two. In the late 1990s the Hubble Space Telescope Key Project on the Extragalactic Distance Scale refined the value of the Hubble constant to an error of only about ten percent. This was accomplished by observing Cepheid variables at optical wavelengths out to greater distances than obtained previously and comparing those to similar measurements from ground-based telescopes.

The SHOES team used Hubble's Near Infrared Camera and Multi-Object Spectrometer (NICMOS) and the Advanced Camera for Surveys (ACS) to observe 240 Cepheid variable stars across seven galaxies. One of these galaxies was NGC 4258, whose distance was very accurately determined through observations with radio telescopes. The other six galaxies recently hosted Type Ia supernovae that are reliable distance indicators for even farther measurements in the universe. Type Ia supernovae all explode with nearly the same amount of energy and therefore have almost the same intrinsic brightness.

By observing Cepheids with very similar properties at near-infrared wavelengths in all seven galaxies, and using the same telescope and instrument, the team was able to more precisely calibrate the luminosity of supernovae. With Hubble's powerful capabilities, the team was able to sidestep some of the shakiest rungs along the previous distance ladder involving uncertainties in the behavior of Cepheids.

Riess would eventually like to see the Hubble constant refined to a value with an error of no more than one percent, to put even tighter constraints on solutions to dark energy.

CONTACT

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

Adam Riess
Space Telescope Science Institute/Johns Hopkins University, Baltimore, Md.
410-516-4474
ariess@stsci.edu

ESA to launch two large observatories to look deep into space and time

Herschel and Planck

Credits: ESA

Two of the most sophisticated astronomical spacecraft ever built – Herschel and Planck – will be launched by ESA this month towards deep space orbits around a special observation point beyond the Moon’s orbit.

From there, both spacecraft will begin a revolutionary observation campaign that will further our understanding of the history of the Universe.
Herschel is a large far-infrared space telescope designed to study some of the coldest objects in space, in a part of the electromagnetic spectrum still mostly unexplored. Planck is a telescope that will map the fossil light of the Universe - light from the Big Bang – with unprecedented sensitivity and accuracy. The two missions are among the most ambitious ever carried out by Europe and mark the crossing of new frontiers in the field of space-based astronomy.

The pair will be lofted in tandem by an Ariane 5 ECA launcher. Lift-off is now scheduled for 15:12 CEST (13:12 GMT) on Thursday 14 May, from Europe’s Spaceport in French Guiana. Herschel and Planck will separate shortly after launch and head independently towards the L2 Lagrangian point of the Sun-Earth system, a gravitational stability point suspended in space some 1.5 million kilometres from Earth in the opposite direction to the Sun. While orbiting around that point, they will be able to conduct continuous observations in a thermally-stable environment, far from radiation disturbance caused by the Sun, Earth and Moon.

The 7.5-m-tall, 4-m-wide Herschel is the largest infrared telescope ever launched. The extremely smooth surface of its 3.5-m-diameter primary mirror – made of lightweight silicon carbide – is almost one and a half times bigger than that of Hubble’s, and six times bigger than that of its predecessor ISO launched by ESA in 1995.

With its huge light-collection capability and set of sophisticated detectors cooled to the vicinity of absolute zero by over 2000 litres of superfluid helium, Herschel will look at the faintest and farthest infrared sources and peer into the as-yet uncharted far infrared and submillimetric parts of the spectrum.

Herschel will be able to see through the opacity of cosmic dust and gas and observe structures and events far away that date back to the early Universe – such as the birth and evolution of early stars and galaxies – ten thousand million years ago, in an effort to determine exactly how it all started. Closer by, within our galaxy, Herschel will also observe extremely cold objects, such as the clouds of dust and interstellar gases from which stars and planets are formed, and even the atmosphere around comets, planets and their moons in our own solar system.

Featuring a 1.5 m telescope and instruments sensitive to microwave radiation, Planck will measure temperature variations in the very early Universe. It will monitor the so- called Cosmic Microwave Background, the relic of the very first light ever emitted in space about 380 thousand years after the Big Bang, when the density and temperature of the young Universe had decreased enough to finally allow light to separate from matter and travel freely in space.

With its ‘heart’ operating at unprecedented low temperatures, Planck will deliver unrivalled sensitivity and resolution. By measuring the tiny fluctuations in the temperature of the microwave background radiation, scientists will extract at least 15 times more information about the Universe’s origin, evolution and future than with its most recent predecessor.

Herschel’s detectors will be cooled down to 0.3 degrees above absolute zero. Planck’s detectors will reach even colder temperatures, just 0.1 degrees above 0 K. Indeed, throughout the mission, the coldest points of the Universe may well be inside its payload. The satellite is planned to take some 500 thousand million of raw samples to produce a set of multi-million-pixel sky maps that will also help scientists to understand the Universe’s structure and account as never before for all of its constituents. Planck will be able to determine the total amount of atoms in the Universe, infer the total density of dark matter – an elusive component still inaccessible to direct observations but ‘visible’ through its effects on the surroundings – and even shed new light on the nature of the mysterious dark energy.

Herschel and Planck, two impressive missions designed to revolutionise our understanding of the cosmos, also represent a tremendous technological challenge that has been overcome by ESA thanks to the mobilising of over 100 industrial partners and institutes in Europe, the United States and elsewhere.

Attending the launch

The main launch event for Herschel/Planck will be held at ESOC, the Agency’s establishment in Darmstadt, Germany. There, ESA senior management and programme specialists will be on hand to give explanations and interviews

The Press Centre at ESOC will be open from 10:00 to 18:00 hours, hosting a media workshop from 11:00 to 12:15 hours and the launch event from 14:00 to 16:15 hours.

A live TV transmission of the launch will supply images from Kourou and from mission control at ESOC/Darmstadt to broadcasters (further details will soon be available at http://television.esa.int).

The general public can also follow the launch video transmission via web-streaming at: http://www.esa.int

Media representatives wishing to follow the event at ESOC or watch the launch live from another ESA establishment are requested to fill in the attached accreditation form (linked from the right menu on this page) and fax it back to the venue of their choice.

Note for editors

Herschel and Planck under ESA’s Science Programme

Herschel and Planck are the last missions to be launched under ESA’s Horizon 2000 long-term plan for space science initiated in 1985, which has already brought the worldwide scientific community a series of trail-blazing successes including: the Integral gamma-ray and XMM-Newton X-ray observatories; the Huygens probe that landed on Saturn’s largest moon, Titan; the Ulysses, Soho and Cluster missions monitoring the Sun, its sphere of influence and Sun-Earth interaction; the Smart-1, Mars Express and Venus Express lunar and planetary explorers; and the Rosetta comet chaser currently mid-way to its final target, the nucleus of comet Churyumov-Gerasimenko. Over the past 25 years, the Horizon 2000 plan and its successors Horizon 2000+ and Cosmic Vision, have set the standard for successful space science in Europe and laid the foundations for the future scientific exploration of space, giving Europe international stature when it comes to cooperation.

Herschel and Planck – the result of a huge international effort
Herschel and Planck have been built under a common engineering programme carried out on ESA’s behalf by Thales Alenia Space (Cannes, France) as prime contractor, also responsible for the Planck payload module, heading a consortium of industrial partners with Astrium (Friedrichshafen, Germany) responsible for the Herschel payload module and Thales Alenia Space (Turin, Italy) responsible for the service modules. Astrium (Toulouse, France) provided the Herschel telescope. The Planck telescope was provided through collaboration between ESA and the Danish National Space Centre.

Academic and industrial consortia from across the world have designed and manufactured the Herschel and Planck onboard instruments.

Herschel features three instruments: the HIFI high-resolution spectrometer, consortium led by the SRON Netherlands Institute for Space Research (The Netherlands); the PACS camera and imaging spectrometer, consortium led by the Max Planck Institute for Extraterrestrial Physics (Germany); and the SPIRE camera and imaging spectrometer, consortium led by Cardiff University (Wales, United Kingdom). Important contributions were also made by the United States (NASA), Canada and Poland.

Planck features two instruments: the HFI high-frequency instrument, consortium led by the Institut d’Astrophysique Spatiale (France); and the LFI low-frequency instrument, consortium led by the Istituto di Astrofisica Spaziale e Fisica Cosmica (Italy), with the United States (NASA) making an important contribution.

NASA's Spitzer Telescope Warms up to New Career

The primary mission of NASA's Spitzer Space Telescope is about to end after more than five and a half years of probing the cosmos with its keen infrared eye. Within about a week of May 12, the telescope is expected to run out of the liquid helium needed to chill some of its instruments to operating temperatures.

The end of the coolant will begin a new era for Spitzer. The telescope will start its "warm" mission with two channels of one instrument still working at full capacity. Some of the science explored by a warm Spitzer will be the same, and some will be entirely new.

"We like to think of Spitzer as being reborn," said Robert Wilson, Spitzer project manager at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "Spitzer led an amazing life, performing above and beyond its call of duty. Its primary mission might be over, but it will tackle new scientific pursuits, and more breakthroughs are sure to come."

Spitzer is the last of NASA's Great Observatories, a suite of telescopes designed to see the visible and invisible colors of the universe. The suite also includes NASA's Hubble and Chandra space telescopes. Spitzer has explored, with unprecedented sensitivity, the infrared side of the cosmos, where dark, dusty and distant objects hide.

For a telescope to detect infrared light — essentially heat — from cool cosmic objects, it must have very little heat of its own. During the past five years, liquid helium has run through Spitzer's "veins," keeping its three instruments chilled to -456 degrees Fahrenheit (-271 Celsius), or less than 3 degrees above absolute zero, the coldest temperature theoretically attainable. The cryogen was projected to last as little as two and a half years, but Spitzer's efficient design and careful operations enabled it to last more than five and a half years.

Spitzer's new "warm" temperature is still quite chilly at -404 degrees Fahrenheit (-242 Celsius) — much colder than a winter day in Antarctica when temperatures sometimes reach -75 degrees Fahrenheit (-59 Celsius). This temperature rise means two of Spitzer's instruments — its longer wavelength multiband imaging photometer and its infrared spectrograph — will no longer be cold enough to detect cool objects in space.

However, the telescope's two shortest-wavelength detectors in its infrared array camera will continue to function perfectly. They will still pick up the glow from a range of objects: asteroids in our solar system, dusty stars, planet-forming disks, gas-giant planets and distant galaxies. In addition, Spitzer still will be able to see through the dust that permeates our galaxy and blocks visible-light views.

"We will do exciting and important science with these two infrared channels," said Spitzer Project Scientist Michael Werner of JPL. Werner has been working on Spitzer for more than 30 years. "Our new science program takes advantage of what these channels do best. We're focusing on aspects of the cosmos that we still have much to learn about."

Since its launch from Cape Canaveral, Fla., on Aug. 25, 2003, Spitzer has made countless breakthroughs in astronomy. Observations of comets both near and far have established that the stuff of comets and planets is similar throughout the galaxy. Breathtaking photos of dusty stellar nests have led to new insights into how stars are born. And Spitzer's eye on the very distant universe, billions of light-years away, has revealed hundreds of massive black holes lurking in the dark.

Perhaps the most revolutionary and surprising Spitzer finds involve planets around other stars, called exoplanets. Exoplanets are, in almost all cases, too close to their parent stars to be seen from our Earthly point of view. Nevertheless, planet hunters continue to uncover them by looking for changes in the parent stars. Before Spitzer, everything we knew about exoplanets came from indirect observations such as these.

In 2005, Spitzer detected the first light, or photons, from an exoplanet. In a clever technique, now referred to as the secondary-eclipse method, Spitzer was able to collect the light of a hot, gaseous exoplanet and learn about its temperature. Further detailed spectroscopic studies later revealed more about the atmospheres, or "weather," on similar planets. More recently, Spitzer witnessed changes in the weather on a wildly eccentric gas exoplanet — a storm of colossal proportions brewing up in a matter of hours before quickly settling down.

"Nobody had any idea Spitzer would be able to directly study exoplanets when we designed it," Werner said. "When astronomers planned the first observations, we had no idea if they would work. To our amazement and delight, they did."

These are a few of Spitzer's achievements during the past five and a half years. Data from the telescope are cited in more than 1,500 scientific papers. And scientists and engineers expect the rewards to keep on coming during Spitzer's golden years.

Some of Spitzer's new pursuits include refining estimates of Hubble's constant, or the rate at which our universe is stretching apart; searching for galaxies at the edge of the universe; assessing how often potentially hazardous asteroids might impact Earth by measuring the sizes of asteroids; and characterizing the atmospheres of gas-giant planets expected to be discovered soon by NASA's Kepler mission. As was true during the cold Spitzer mission, these and the other programs are selected through a competition in which scientists from around the world are invited to participate.

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

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Galactic X-ray emissions originate from stars

Astronomers identify the origin of the diffuse radiation
in the plane of the Milky Way

A 25-year old astronomical mystery has been solved: Most of the diffuse X-ray emissions in the Milky Way do not originate from one single source but from so-called white dwarfs and from stars with active outer gas layers. Mikhail Revnivtsev from the Excellence Cluster Universe at the TU Munich and his colleagues at the Max Planck Institute for Astrophysics in Garching, the Space Research Institute in Moscow and the Harvard-Smithsonian Center for Astrophysics in Cambridge have now succeeded in proving this. (Nature, April 30, 2009).

Fig.: The plane of the Milky Way, recorded with the Chandra satellite in three colours: Photons with energies between 0.5 and 1keV appear red, those between 1 and 3keV green, and those between 3 and 7keV blue. Discrete sources are indicated by circles. Image credit: Mikhail Revnivtsev

It is now 25 years since scientists discovered diffuse X-ray emissions from the vicinity of the Milky Way plane. Since then, a whole generation of astronomers has been racking its brains as to their origin. Energetic X-ray emissions usually originate from very hot gases in a temperature range between 10 and 100 million degrees Celsius. And this "Galactic Ridge X-ray Emission" (GRXE) is also typical for very hot, optically thin plasma.

A gas with these thermal properties would, however, immediately escape from our galaxy - the Milky Way would continuously lose colossal amounts of energy and finally collapse as the existing energy sources, such as stars and supernovae, would not be sufficient to replenish such a loss. Cosmic particles colliding with the interstellar medium could also be ruled out as an explanation for the GRXE.

It is only recently that observations with the RXTE and Integral satellites have shown that the X-ray emissions of the Milky Way exhibit the same distribution pattern as the stars. Since then, it has been assumed that a large portion of the GRXE originates from individual stars. These findings motivated the international team to carry out more precise measurements with the Chandra X-ray telescope. The test area chosen was a small celestial region near the centre of the Milky Way.

The region chosen, about half as big as a full moon, lent itself to the observations for two reasons: On the one hand because of the high GRXE intensity, which minimized the "interfering radiation" from extra-galactic X-ray sources; and on the other hand because the interstellar matter at this position absorbs only small amounts of radiation so that it was even possible to detect weak discrete sources with Chandra.

Chandra actually managed to identify 473 point sources of X-rays in a sector of the search field covering only 2.6 arcminutes. In a further step, the group used measurements from the Spitzer satellite observatory to prove that the results of the sector observed could be applied to the whole galaxy.

Most of the 473 X-ray sources are probably white dwarfs, which accrete matter from their surroundings, as well as stars with high activity in their outermost gas layer, the corona. White dwarfs are the remnants of extinct, low-mass suns. These cooling dead stars frequently orbit a partner, and in such a binary star system the white dwarf extracts matter from its larger partner until it becomes a Type Ia supernova.

The resolution of the diffuse X-ray emissions in our galaxy into discrete sources has far-reaching consequences for our understanding of a number of astrophysical phenomena. Astronomers can use the GRX emission as a calibration for the spatial distribution of star populations within the Milky Way, for example. The results were also relevant for research into other galaxies: It now seems clear that the diffuse X-ray radiation from these objects originates from white dwarfs and active stars.

Original work:

Mikhail Revnivtsev, Sergey Sazonov, Eugene Churazov, William Forman, Alexey Vikhlinin and Rashid Sunyaev
Discrete sources as the origin of the Galactic X-ray ridge emission
Nature, Vol. 458, No. 7242, April 30, 2009

PDF (244 KB)

Contact:

Dr. Mona Clerico, Press Officer Max Planck Institute for Astrophysics and
Max-Planck-Institute for Extraterrestrial Physics, Garching
Tel.: +49 89 30000-3980
E-mail: clerico@mpe.mpg.de

Dr. Eugene Churazov
Max Planck Institute for Astrophysics, Garching
Tel.: +49 89 30000-2219
E-mail: echurazov@mpa-garching.mpg

Prof. Dr. Rashid Sunyaev
Max Planck Institute for Astrophysics, Garching
Tel.: +49 89 30000-2244

E-mail: rsunyaev@mpa-garching.mpg.de

Touching the Edge of the Universe world premiere

Touching the Edge of the Universe - ESA's IYA2009 planetarium show
Videos (1)

ESA will present the world premiere of Touching the Edge of the Universe, a stunning new planetarium show, starting 7 May 2009 at 30 planetaria in Germany, Austria and Switzerland. The premiere comes just days before the launch of Herschel & Planck, two of the show's starring missions, scheduled for 14 May.

Both missions will make fundamental contributions to astronomy and cosmology and serve as Europe’s cornerstone contribution to the 2009 Year of Astronomy.
In 1609, Galileo Galilei pointed his telescope at the sky, discovering worlds unknown and proving that Aristotle's long-held theories on the cosmos were in fact wrong. His findings marked the beginning of an intellectual revolution that continues today, underpinning much of modern science.

Touching the Edge of the Universe tells the story of astronomy from the time of Galileo and his simple optical telescope to today’s sophisticated space astronomy missions. Viewers will experience an entirely new view of the cosmos conveyed through stunning 3D graphics and a professionally acted script, much of which was shot on location at various ESA Establishments.

Planck scans the sky during Touching the Edge of the Universe - ESA's IYA2009 planetarium show. Videos (2)

Content based on latest knowledge

The show includes the most current knowledge based on the research of scientists working on present and future ESA missions.

Providing a 360° 'full dome' projection, the show takes the audience on a breathtaking voyage of discovery, from Galileo's 16th Century Tuscan villa to the tense countdown, launch and orbiting of the next generation of space telescopes - and out into the Universe.

"The ESA planetarium show provides a lively and compelling picture of space exploration today and what it means for people in everyday life," says Jocelyne Landeau-Constantin, ESA's Project Manager for the show.

"It also reminds us that scientific exploration remains a grand project, just as during the time of Galileo and Kepler, 400 hundred years ago," she adds.

ESA partnership with European planetaria

More than 30 German-language planetaria located in Germany, Austria and Switzerland are partners in Touching the Edge of the Universe. These planetaria offer some of the best connections between formal academic learning and the power of infotainment.

Herschel separation - a dramatic moment during Touching the Edge of the Universe - ESA's YA2009 planetarium show. Videos (3)

"While books, TV and many other media present the topic of space to the general public, nowhere else can we experience the fascination of space as impressively as in a planetarium," says Fernando Doblas, Head of ESA’s Communication and Knowledge Department.

Digital projection systems using full dome video technology are fast displacing traditional analogue projection systems and planetaria can now provide audiences with a full-surround, cinematic experience.

But digital production techniques are far more demanding and with Touching the Edge of the Universe, the Agency relied on the expertise of the creative team at the Media Faculty of the Kiel University of Applied Sciences, Kiel, Germany.

Show poster - with German-language title 'Augen im All'

In addition to complex digital imagery recorded using 'green screen' substitution techniques, the show includes detailed and accurate 3D renderings of Herschel, Planck and ESA’s future Mars Rover, as well as of Europe’s Ariane 5 launcher.

Professional actors from the Kiel theatre were employed to ensure a truly authentic educational experience.

ESA will also release the English-language version in June this year.

These premieres will be followed by further releases throughout 2009. ESA scientists and managers will be on hand at several of the premieres for an introductory talk.

Further information at http://www.planetariumshow.eu

Contact

Jocelyne Landeau-Constantin
Head of Corporate Communication Office,
ESA/ESOC, Darmstadt, Germany

Tel: +49-6151-902696
jlc @ esa.int

Show premieres

World premiere in Berlin, Vienna and Lucern, 7 May 2009

• Vienna - Zeiss Planetarium Wien

• Berlin - Zeiss Großplanetarium Berlin

• Lucern - Planetarium Luzern

Grand openings in Germany, Austria and Switzerland, May 2009

8 May 2009
Planetarium Hamburg
Nicolaus Copernicus Planetarium, Nürnberg
Planetarium Klagenfurt
Planetarium Sigmund Jähn Rodewisch, Rodewisch

9 May 2009
Planetarium Herzberg
Planetarium Drebach
LWL Planetarium Münster
Planetarium Laupheim

11 May 2009
Planetarium Cottbus

12 May 2009
Zeiss Planetarium Bochum
Planetarium Osnabrück

13 May 2009
Mediendom der Fachhochschule Kiel

14 May 2009
Planetarium Jena
Wilhelm Foerster Sternwarte Berlin

Top Five Breakthroughs From Hubble's Workhorse Camera

Several hundred never before seen galaxies are visible in this "deepest-ever" view of the universe, called the Hubble Deep Field (HDF), made with the Wide Field and Planetary Camera 2 aboard NASA's Hubble Space Telescope. Image credit: NASA/STScI

Full image and caption

Deepest photograph of the universe. Hubble's famous "Deep Field" picture (above), taken by the Wide Field and Planetary Camera 2, left the world with its mouth agape when it was first revealed in 1996. In just a small patch of sky, more than 1,000 galaxies located billions of light-years away could be seen floating in space like sea creatures at the bottom of an endless ocean. Our world and our galaxy suddenly seemed very small.

Observations of comet collision with Jupiter. The Wide Field and Planetary Camera 2 gave the world a rare, stunning view of Comet Shoemaker-Levy 9 plunging into the gas giant Jupiter in 1994. The images revealed the event in great detail, including ripples expanding outward from the impact.

The birth and death of stars. The Wide Field and Planetary Camera 2 brought the cosmos down to Earth with its exquisite pictures of stars in all stages of development. Its famed picture of the "Pillars of Creation" and other images of colorful dying stars offered the first, glorious views of a star's life. The camera also took the first pictures of the dusty disks around stars where planets are born, demonstrating that planet-forming environments are common in the universe.

The age and rate of expansion of our universe. Our universe formed from a colossal explosion known as the Big Bang, and has been stretching apart ever since. Hubble's Wide Field and Planetary Camera 2, by observing stars that vary periodically in brightness, was able to calculate the pace of this expansion to an unprecedented degree of error of 10 percent. The camera also played a leading role in discovering that the expansion of the universe is accelerating, driven by a mysterious force called "dark energy." Together, these findings led to the calculation that our universe is approximately 13.7 billion years old.

Most galaxies harbor huge black holes. Before Hubble, astronomers suspected, but had no proof, that supermassive black holes lurk deep in the bellies of galaxies. The Wide Field and Planetary Camera 2, together with spectroscopy data from Hubble, showed that most galaxies in the universe do indeed harbor monstrous black holes up to billions of times the mass of our sun.

Media contact: DC Agle/JPL
(818) 393-9011

Friends of the RAS

Interested in Astronomy?
Enjoy popular lectures?

Know someone who does?

Why not become a Friend of the RAS and attend the next Friends meeting on Naming Pluto? This will be at 1800 on TUESDAY 19 MAY,in the RAS at Burlington House, consisting of the screening of a new film about Pluto followed by a talk by Oxford historian Allan Chapman.

The film tells the story of how an 11-year-old Oxford schoolgirl, Venetia Burney Phair, named the planet over breakfast on March 14, 1930, after her grandfather read about its discovery in 'The Times'. In this new documentary she recalls how she suggested 'Pluto' to her grandfather. He liked the name and mentioned it to his friend Herbert Hall Turner, a former President of the Royal Astronomical Society. The suggestion was then sent by telegram to the Lowell Observatory in Arizona, which had the planet’s naming rights, and the title was made official on May 1, 1930. However, until 2007 Mrs Burney Phair had never seen her planet through a telescope. The film records the quest for her to see it. During the film’s production, the International Astronomical Union, controversially, demoted Pluto to a ‘dwarf planet’ following a reclassification of the solar system, and the British weather caused a year-long hold up as it did its best to deny Mrs Burney Phair a clear view!

Dr Allan Chapman FRAS will explore the history of Pluto, possibly our most mysterious neighbour. Following the screening, introduced by the film maker Ginita Jimenez, ( which we hope Venetia Burney Phair may be well enough to attend) and talk there will be a drinks reception.

There is no charge for attending this event - however it is restricted to 'Friends of the RAS'.'Friends' will not meet the requirements to become , nor have the same concerns as, a Fellow of the RAS. Rather, membership of the RAS as a 'Friend' recognises the appeal of astronomy to the general public.

To become a 'Friend' (see below * for a fuller description of activities and benefits) for the remainder of 2009, send your name, address, email address and telephone number with a cheque (made out to ‘The Royal Astronomical Society’) for £15.00 to the Membership Secretary, Royal Astronomical Society, Burlington House, Piccadilly, London W1J 0BQ.

• Become a Friend of the RAS and enjoy:
  

• Free priority seat reservation at our popular lunch-time lectures

• Invitations to social events and meetings in Burlington House

• Use of the Society's historic library in Burlington House

• Escorted visits to observatories and other places of interest

• Discounted admission to places of interest

• Discounted subscription to the Astronomy & Geophysics magazine
  

In addition every Friend will receive the RAS 2009 Diary and a colour guide to Astronomy in the UK.

For further information see http://friends.ras.org.uk

Streams of Stars Provide "Missing Link" in the Evolution of Galaxy Disks

"Missing Link" Found
NASA/JPL-Caltech/University of the Witwatersrand

About this image: Here we see two different views of the spiral galaxy, Messier 81. On the left is an image taken in blue light, while on the right is a specially-processed version of an image taken with the Spitzer Space Telescope's infrared array camera (IRAC) at 4.5 microns. The processed image reveals myriads of tiny arclets, a representative sample of which are arrowed. Each of these arclets represents a young star stream in the disk of the galaxy.

Observing the galaxy in the infrared is the only way to directly see the youngest stars, since the shroud of dust and gas that surrounds them is opaque to visible light, but transparent in the IR. Even so, the unprocessed infrared image was still dominated by the light from the smooth, older disk of the galaxy rather than the faint tracks of young stars. Further processing using a mathematical technique called Fourier filtering allowed the team to pick out structures on the physical scale on which star formation occurs, revealing these streams of young stars flowing away from their stellar nurseries.

M81 is one of several galaxies that were observed in this way. Taken together, this sample is the first time that young star streams have been discovered in the disks of galaxies millions of light years distant, filling in the "missing link" in the evolution of galaxy disks.

Why Are Galaxies So Smooth?

NASA/JPL-Caltech

About this image: This latest image from NASA's Spitzer Space Telescope is of the spiral galaxy, NGC 2841. Located about 46 million light-years from Earth in the constellation Ursa Major, this spectacular galaxy is helping astronomers solve one of the oldest puzzles in astronomy: Why do galaxies look so smooth, with stars sprinkled evenly throughout? An international team of astronomers has discovered that rivers of young stars flow from their hot, dense stellar nurseries, dispersing out to form the large, smooth distribution that we see in spiral galaxies like this one.

This image is a composite of three different wavelengths from Spitzer's infrared array camera . The shortest wavelengths are displayed in blue, and mostly show the older stars in NGC 2841, as well as foreground stars in our own Milky Way galaxy. The cooler areas are highlighted in red, and show the dusty, gaseous regions of the galaxy. Blue shows infrared light of 3.6 microns, green represents 4.5-micron light and red, 8.0-micron light. The contribution from starlight measured at 3.6 microns has been subtracted from the 8.0-micron image to enhance the visibility of the dust features.

Using NASA's Spitzer Space Telescope, an international team of astronomers has discovered streams of young stars flowing from their natal cocoons in distant galaxies. These distant rivers of stars provide an answer to one of astronomy's most fundamental puzzles: how do young stars that form clustered together in dense clouds of dust and gas disperse to form the large, smooth distribution seen in the disks of spiral galaxies like the Milky Way?

"When you look at the disks of galaxies in the infrared they are remarkably smooth. All of the older stars are evenly distributed. But stars aren't born that way; they're born in clusters and associations like the Pleiades cluster, or the association of young stars in the Orion constellation of our own Milky Way galaxy. So the question is - why are the disks of galaxies so smooth?" said team leader David Block of the University of the Witwatersrand in South Africa.

Astronomers know that the clusters where stars form begin to disappear when their ages reach several hundred million years. A few mechanisms are thought to explain this: some clusters evaporate when random internal motions kick out stars one by one, and other clusters disperse as a result of collisions among the clouds where they were born. Zooming out to mechanisms operating on larger scales still, shearing motions caused by the galaxy's rotation around its center disperses the clusters of clusters of young stars.

"Our analysis now answers the grand puzzle. By finding a myriad of streams of young stars all over the disks of galaxies we studied, we see that the mechanism for pulling the clusters of young stars apart is shearing motions of the parent galaxy. These streams are the 'missing link' we needed to understand how the disks of galaxies evolve to look the way they do," said Block.

Crucial to this discovery was finding a way to image previously hidden young stellar streams in galaxies millions of light-years away. To do this the team used high-resolution infrared observations from the Spitzer.

Using infrared rather than visible light to look at the galaxies allowed the group to pick out stars at just the right age when the stars are just starting to spread out from their clusters.

"Spitzer observes in the infrared where 100-million-year-old populations of stars dominate the light," noted co-author Bruce Elmegreen, from IBM's Research Division in New York. "Younger regions shine more in the visible and ultraviolet parts of the spectrum, and older regions get too faint to see. So we can filter out all the stars we don't want by taking pictures with an infrared camera."

Infrared is also important because light in this part of the spectrum can penetrate the dense dust clouds surrounding the clusters where stars form.

"Dust blocks optical starlight very effectively," said Robert Gehrz of the University of Minnesota, "but infrared light with its longer wavelength goes right around the dust particles blocking our view. This allows the infrared light from young stars to be seen more clearly."

But even when the images are taken in the infrared, they are still dominated by the light from the smooth older disks of galaxies, not the faint tracks of young dispersing clusters. Special mathematical manipulations were needed to pick out the clusters, whose faint tracks can still be seen precisely because they are not smooth.

Team member Ivanio Puerari of the Instituto Nacional de Astrofisica in Puebla, Mexico used a technique invented by mathematician Jean Baptiste Fourier in the early 1800's. The technique is effectively a spatial filter that picks out structure on the physical scale where star formation occurs. "The structures cannot be seen on the original Spitzer images with the human eye," noted Puerari.

"The combination of the Fourier filtering and infrared images highlighted regions of just the right size and the right age. To then unveil so many star streams in the disks of galaxies was unimaginable a year ago. This discovery continues to highlight the enormous potential of the Spitzer Space Telescope to make contributions none of us could have dreamed possible," commented Giovanni Fazio from the Harvard-Smithsonian Center for Astrophysics, project leader for the Spitzer Infrared Array Camera team used to take the pictures, and co-author of the discovery.

"Galileo, as both astronomer and mathematician, would have been proud. It is a wonderful interplay between the use of astronomical observations and mathematics and computers, exactly 400 years since Galileo used his telescope to examine our Milky Way galaxy in 1609," Fazio concluded.

Debra Elmegreen, Maria Mitchell Professor at Vassar College and President elect of the American Astronomical Society was also a member of the team. The results appeared in the March 20, 2009 issue of the Astrophysical Journal.

Starbursts in Dwarf Galaxies are a Global Affair

Credit: NASA, ESA, K. McQuinn (University of Minnesota, Minneapolis),
and I. Karachentsev (Special Astrophysical Observatory of
the Russian Academy of Sciences, Russia)

These images, taken by NASA's Hubble Space Telescope, show myriad stars residing in the central regions of the three dwarf galaxies NGC 4163, NGC 4068, and IC 4662.

The bluish dots are younger stars; the reddish dots, older stars. The irregularly shaped red blobs in the images of NGC 4163 and IC 4662 are regions of current starburst activity. Starbursts are areas of intense star formation.

The three galaxies are part of a Hubble study of starbursts in nearby, small, or dwarf, galaxies. Based on this study, astronomers have found that starbursts continue 100 times longer than first thought, lasting 200 million to 400 million years. These galaxies show that starbursts are not isolated events, but sweep across a galaxy.

Each of the three starburst galaxies has a different shape. The collection of stars in NGC 4163 is more spherical, with a higher concentration of stars forming in the center.

By contrast, the grouping of stars in NGC 4068 is more elongated and has fewer new stars than the other two galaxies. Astronomers think the starburst in this galaxy is ending. In the image of IC 4662 the clumpy red blobs peppered throughout the galaxy indicate active regions of star birth. One such region extends off the image's top, right edge.

This galaxy exhibits the strongest star formation of the three galaxies in the study.
The distances of the galaxies range from 8 million to 14 million light-years away.
The images were taken in 2004 by the Advanced Camera for Surveys.

Bursts of star making in a galaxy have been compared to a Fourth of July fireworks display: They occur at a fast and furious pace, lighting up a region for a short time before winking out.

But these fleeting starbursts are only pieces of the story, astronomers say. An analysis of archival images of small, or dwarf, galaxies taken by NASA's Hubble Space Telescope suggests that starbursts, intense regions of star formation, sweep across the whole galaxy and last 100 times longer than astronomers thought. The longer duration may affect how dwarf galaxies change over time, and therefore may shed light on galaxy evolution.

"Our analysis shows that starburst activity in a dwarf galaxy happens on a global scale," explains Kristen McQuinn of the University of Minnesota in Minneapolis and leader of the study. "There are pockets of intense star formation that propagate throughout the galaxy, like a string of firecrackers going off." According to McQuinn, the duration of all the starburst events in a single dwarf galaxy would total 200 million to 400 million years.

These longer timescales are vastly more than the 5 million to 10 million years proposed by astronomers who have studied star formation in dwarf galaxies. "They were only looking at individual clusters and not the whole galaxy, so they assumed starbursts in galaxies lasted for a short time," McQuinn says.

Dwarf galaxies are considered by many astronomers to be the building blocks of the large galaxies seen today, so the length of starbursts is important for understanding how galaxies evolve.

"Astronomers are really interested to find out the steps of galaxy evolution," McQuinn says. "Exploring these smaller galaxies is important because, according to popular theory, large galaxies are created from the merger of smaller, dwarf galaxies. So understanding these smaller pieces is an important part of filling in that scenario."

McQuinn's team analyzed archival Advanced Camera for Surveys data of three dwarf galaxies, NGC 4163, NGC 4068, and IC 4662. Their distances range from 8 million to 14 million light-years away. The trio is part of a survey of starbursts in 18 nearby dwarf galaxies.

Hubble's superb resolution allowed McQuinn's team to pick out individual stars in the galaxies and measure their brightness and color, two important characteristics astronomers use to determine stellar ages. By determining the ages of the stars, the astronomers could reconstruct the starburst history in each galaxy.

Two of the galaxies, NGC 4068 and IC 4662, show active, brilliant starburst regions in the Hubble images. The most recent starburst in the third galaxy, NGC 4163, occurred 200 million years ago and has faded from view.

The team looked at regions of high and low densities of stars, piecing together a picture of the starbursts. The galaxies were making a few stars, when something, perhaps an encounter with another galaxy, pushed them into high star-making mode. Instead of forming eight stars every thousand years, the galaxies started making 40 stars every thousand years, which is a lot for a small galaxy, McQuinn says. The typical dwarf is 10,000 to 30,000 light-years wide. By comparison, a normal-sized galaxy such as our Milky Way is about 100,000 light-years wide.

About 300 million to 400 million years ago star formation occurred in the outer areas of the galaxies. Then it began migrating inward as explosions of massive stars triggered new star formation in adjoining regions. Starbursts are still occurring in the inner parts of NGC 4068 and IC 4662.

The total duration of starburst activity depends on many factors, including the amount of gas in a galaxy, the distribution and density of the gas, and the event that triggered the starburst. A merger or an interaction with a large galaxy, for example, could create a longer starburst event than an interaction with a smaller system.

McQuinn plans to expand her study to a larger sample of more than 20 galaxies. "Studying nearby dwarf galaxies, where we can see the stars in great detail, will help us interpret observations of galaxies in the distant universe, where starbursts were much more common because galaxies had more gas with which to make stars," McQuinn explains.

McQuinn's results appeared in the April 10 issue of The Astrophysical Journal.

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

Kristen McQuinn
University of Minnesota, Minneapolis, Minn.
612-626-1819
kmcquinn@astro.umn.edu

Rogue Black Holes May Roam the Milky Way

This artist's conception shows a rogue black hole floating near a globular star cluster on the outskirts of the Milky Way. New calculations by Ryan O'Leary and Avi Loeb suggest that hundreds of massive black holes, left over from the galaxy-building days of the early universe, may wander the Milky Way. Fortunately, the closest rogue black hole should reside thousands of light-years from Earth.Credit: David A. Aguilar (CfA)

High Resolution Image (jpg)
Low Resolution Image (jpg)

It sounds like the plot of a sci-fi movie: rogue black holes roaming our galaxy, threatening to swallow anything that gets too close. In fact, new calculations by Ryan O'Leary and Avi Loeb (Harvard-Smithsonian Center for Astrophysics) suggest that hundreds of massive black holes, left over from the galaxy-building days of the early universe, may wander the Milky Way.

Good news, however: Earth is safe. The closest rogue black hole should reside thousands of light-years away. Astronomers are eager to locate them, though, for the clues they will provide to the formation of the Milky Way.

"These black holes are relics of the Milky Way's past," said Loeb. "You could say that we are archaeologists studying those relics to learn about our galaxy's history and the formation history of black holes in the early universe."

According to theory, rogue black holes originally lurked at the centers of tiny, low-mass galaxies. Over billions of years, those dwarf galaxies smashed together to form full-sized galaxies like the Milky Way.

Each time two proto-galaxies with central black holes collided, their black holes merged to form a single, "relic" black hole. During the merger, directional emission of gravitational radiation would cause the black hole to recoil. A typical kick would send the black hole speeding outward fast enough to escape its host dwarf galaxy, but not fast enough to leave the galactic neighborhood completely. As a result, such black holes would still be around today in the outer reaches of the Milky Way halo.

Hundreds of rogue black holes should be traveling the Milky Way's outskirts, each containing the mass of 1,000 to 100,000 suns. They would be difficult to spot on their own because a black hole is visible only when it is swallowing, or accreting, matter.

One telltale sign could mark a rogue black hole: a surrounding cluster of stars yanked from the dwarf galaxy when the black hole escaped. Only the stars closest to the black hole would be tugged along, so the cluster would be very compact.

Due to the cluster's small size on the sky, appearing to be a single star, astronomers would have to look for more subtle clues to its existence and origin. For example, its spectrum would show that multiple stars were present, together producing broad spectral lines. The stars in the cluster would be moving rapidly, their paths influenced by the gravity of the black hole.

"The surrounding star cluster acts much like a lighthouse that pinpoints a dangerous reef," explained O'Leary. "Without the shining stars to guide our way, the black holes would be all but impossible to find."

The number of rogue black holes in our galaxy depends on how many of the proto-galactic building blocks contained black holes at their cores, and how those proto-galaxies merged to form the Milky Way. Finding and studying them will provide new clues about the history of our galaxy.

Locating the star cluster signposts may turn out to be relatively straightforward.

"Until now, astronomers were not searching for such a population of highly compact star clusters in the Milky Way's halo," said Loeb. "Now that we know what to expect, we can examine existing sky surveys for this new class of objects."

Loeb and O'Leary's journal paper will be published in the Monthly Notices of the Royal Astronomical Society and is available online at http://arxiv.org/abs/0809.4262.

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

For more information, contact:

David A. Aguilar
Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics
617-495-7462
daguilar@cfa.harvard.edu

Christine Pulliam
Public Affairs Specialist
Harvard-Smithsonian Center for Astrophysics
617-495-7463
cpulliam@cfa.harvard.edu