Wednesday, October 31, 2012

Stars Ancient and Modern?

The globular star cluster NGC 6362

The globular star cluster NGC 6362 in the constellation of Ara (The Altar)

Wide-field view of the sky around the globular cluster NGC 6362

Hubble image of the globular star cluster NGC 6362
Comparison of views of the globular star cluster NGC 6362 from WFI and Hubble
Videos
Zooming in on the globular star cluster NGC 6362

Panning across the globular star cluster NGC 6362

This colourful view of the globular star cluster NGC 6362 was captured by the Wide Field Imager attached to the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory in Chile. This new picture, along with a new image of the central region from the NASA/ESA Hubble Space Telescope, provide the best view of this little-known cluster ever obtained. Globular clusters are mainly composed of tens of thousands of very ancient stars, but they also contain some stars that look suspiciously young. 

Globular star clusters are among the oldest objects in the Universe, and NGC 6362 cannot hide its age in this picture. The many yellowish stars in the cluster have already run through much of their lives and become red giant stars. But globular clusters are not static relics from the past —  some curious stellar activities are still going on in these dense star cities.

For instance, NGC 6362 is home to many blue stragglers — old stars that really do succeed in passing for a younger age. All of the stars in a globular cluster formed over a fairly short period of time, typically about 10 billion years ago for most globulars. Yet blue stragglers are bluer and more luminous — and hence more massive — than they should be after ten billion years of stellar evolution. Blue stars are hot and consume their fuel quickly, so if these stars had formed about ten billion years ago, then they should have fizzled out long ago. How did they survive?

Astronomers are keen to understand the secret of the youthful appearance of blue stragglers. Currently, there are two main theories: stars colliding and merging, and a transfer of material between two companion stars. The basic idea behind both of these options is that the stars were not born as big as we see them today, but that they received an injection of extra material at some point during their lifetimes and this then gave them a new lease of life.

Although less well known than some brighter globular clusters, NGC 6362 holds much that is of interest to astronomers and has been well studied over the years. It was selected as one of the 160 stellar fields for the Pre-FLAMES Survey — a preliminary survey conducted between 1999 and 2002 using the 2.2-metre telescope at La Silla to find suitable stars for follow-up observations with the VLT’s spectroscopic instrument FLAMES. The picture here comes from data collected as part of this survey.

The new image shows the entire cluster against a rich background of the carpet of stars in the Milky Way. The central parts of NGC 6362 have also been studied in detail by the NASA/ESA Hubble Space Telescope. The Hubble view shows a much smaller area of sky in much greater detail. The two views — one wide-angle and one zoomed in — complement each other perfectly.

This brilliant ball of stars lies in the southern constellation of Ara (The Altar). It can be easily seen in a small telescope. It was first spotted in 1826 by the Scottish astronomer James Dunlop using a 22-centimetre telescope in Australia.

More information

The year 2012 marks the 50th anniversary of the founding of the European Southern Observatory (ESO). ESO is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive astronomical observatory. It is supported by 15 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Portugal, Spain, Sweden, Switzerland and the United Kingdom. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world’s most advanced visible-light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world’s largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is the European partner of a revolutionary astronomical telescope ALMA, the largest astronomical project in existence. ESO is currently planning the 39-metre European Extremely Large optical/near-infrared Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.

Links

Contacts


Richard Hook
ESO, La Silla, Paranal, E-ELT and Survey Telescopes Public Information Officer
Garching bei München, Germany
Tel: +49 89 3200 6655
Cell: +49 151 1537 3591
Email:
rhook@eso.org

Tuesday, October 30, 2012

Galactic Thief: "I Would Have Gotten Away With It, If It Weren't for Those Meddling Astronomers"

As the Milky Way rises over the horizon at the European Southern Observatory, its companion galaxies also come into view. The Large Magellanic Cloud (LMC) at far left lies about 160,000 light-years away, while the Small Magellanic Cloud (SMC, above and to the right of the LMC) lies about 200,000 light-years away. New simulations show that the LMC stole stars from the SMC when the two galaxies collided 300 million years ago. Microlensing events that have been observed are due to LMC stars passing in front of a stream of stars pulled from the SMC. Credit: ESO/Y. Beletsky.

Cambridge, MA - One of the closest galaxies to the Milky Way almost got away with theft. However, new simulations convicted the Large Magellanic Cloud (LMC) of stealing stars from its neighbor, the Small Magellanic Cloud (SMC). And the crucial evidence came from surveys looking for something entirely different - dark objects on the outskirts of the Milky Way.

 Astronomers have been monitoring the LMC to hunt for evidence of massive compact halo objects, or MACHOs. MACHOs were thought to be faint objects, roughly the mass of a star, but their exact nature was unknown. Several surveys looked for MACHOs in order to find out if they could be a major component of dark matter - the unseen stuff that holds galaxies together.

 In order for MACHOs to make up dark matter, they must be so faint that they can't be directly detected. Instead, astronomers looked for a phenomenon known as microlensing. During a microlensing event, a nearby object passes in front of a more distant star. The gravity of the closer object bends light from the star like a lens, magnifying it and causing it to brighten.

 By studying the LMC, astronomers hoped to see MACHOs within the Milky Way lensing distant LMC stars. The number of microlensing events observed by various teams was smaller than needed to account for dark matter, but much higher than expected from the known population of stars in the Milky Way. This left the origin of the observed events a puzzle and the existence of MACHOs as exotic objects a possibility.

 "We originally set out to understand the evolution of the interacting LMC and SMC galaxies," explains lead author Gurtina Besla of Columbia University. "We were surprised that, in addition, we could rule out the idea that dark matter is contained in MACHOs."

 "Instead of MACHOs, a trail of stars removed from the SMC is responsible for the microlensing events," says co-author Avi Loeb of the Harvard-Smithsonian Center for Astrophysics.

 "You could say we discovered a crime of galactic proportions," he adds.

 Computer simulations showed that the most likely explanation for the observed microlensing events was an unseen population of stars removed by the LMC from its companion, the SMC. Foreground stars in the LMC are gravitationally lensing the trail of removed stars located behind the LMC from our point of view.

 Only a fast-moving population of stars could yield the observed rate and durations of the microlensing events. The best way to get such a stellar population is a galactic collision, which appears to have occurred in the LMC-SMC system.

 "By reconstructing the scene, we found that the LMC and SMC collided violently hundreds of millions of years ago. That's when the LMC stripped out the lensed stars," says Loeb.

 Their research also supports recent findings suggesting that both Magellanic Clouds are on their first pass by the Milky Way.

 Although the evidence for the trail of lensed stars is persuasive, they haven't been directly observed yet. A number of teams are searching for the signatures of these stars within a bridge of gas that connects the Magellanic Clouds.

 The simulation results will be published in the Monthly Notices of the Royal Astronomical Society and are available online.

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

Christine Pulliam
Public Affairs Specialist
Harvard-Smithsonian Center for Astrophysics
617-495-7463

Monday, October 29, 2012

Fire burn and cauldron bubble

 A giant bubble blown by the massive Wolf-Rayet star HD 50896, the pink star in the centre of the image.

 X-ray data from XMM-Newton’s EPIC camera are shown in blue, while optical images were acquired using the Michigan Curtis Schmidt Telescope at Cerro Tololo Inter-American Observatory (CTIO) and presented in red (H-alpha) and green (OIII).

 The bubble, known as S 308, is about 60 light-years across and is located 5000 light-years away in the constellation of Canis Major.

Credits: ESA, J. Toala & M. Guerrero (IAA-CSIC), Y.-H. Chu & R. Gruendl (UIUC), S. Arthur (CRyA–UNAM), R. Smith (NOAO/CTIO), S. Snowden (NASA/GSFC) and G. Ramos-Larios (IAM) .  HI-RES JPEG (Size
: 1159 kb)

The cosmic cauldron has brewed up a Halloween trick in the form of a ghostly face that glows in X-rays, as seen by ESA’s XMM-Newton space telescope. The eerie entity is a bubble bursting with the fiery stellar wind of a ‘live fast, die young’ star.

The bubble lies 5000 light-years from Earth in the constellation of Canis Major, the ‘greater dog’, and can be imagined to take on a dog- or wolf-like face.

It spans nearly 60 light-years across and was blown by the powerful stellar wind of the Wolf-Rayet star HD 50896 – the pink star near the centre of the image that makes up one of the object’s piercing eyes.

Wolf-Rayet bubbles are the result of a hot, massive star – typically greater than 35 the mass of our Sun – expelling material through a strong stellar wind. This star’s howling wind is a million-degree plasma potion that emits X-rays, represented in blue in this image.

Where this fierce wind ploughs into surrounding material it is lit up in red tones as seen in the ‘cheek’ of the face.

The green halo is a result of a shock wave racing out from the star and colliding with the layers of stellar material already ejected into space.

A ‘blow-out’ of X-ray emission at the top left gives the wolf an ear, and a denser region to the bottom right can be likened to a snout.

The witching hour will soon come for this bubble and its star. The bubble will burst and disperse into the surrounding environment, while the star will end its life in a dramatic supernova explosion.

X-Ray Emission from the Wolf-Rayet Bubble S 308 by J. Toala et al is published in the Astrophysical Journal 755, 77 (2012). 

Source: ESA

Super-massive black hole inflates giant bubble

Click here for a high res image

Using a brand-new radio telescope, astronomers have produced one of the best images ever made at the lowest frequencies of giant bubbles produced by a super-massive black hole. The observations were performed at frequencies ranging from 20 to 160 MHz which are normally used for communications by airplane pilots. The picture shows what looks like a giant balloon filled with radio emitting plasma, which exceeds the size of an entire galaxy.

 Some black holes actively accrete matter. Part of this material does not fall into the black hole but is ejected in a narrow stream of particles, traveling at nearly the speed of light. When the stream slows down, it creates a tenuous balloon that can engulf the entire galaxy. Invisible to optical telescopes, the bubble is very prominent at low radio frequencies. The new International LOFAR Telescope (ILT) , designed and built by ASTRON in an international collaboration, is ideally suited to detect this low frequency emission.

 Click here for a hi-res version

"The result is of great importance", says Francesco de Gasperin, lead author of the study that will be published in the journal Astronomy & Astrophysics. "It shows the enormous potential of LOFAR, and provides compelling evidence of the close ties between black hole, host galaxy, and their surroundings. Like symbiotic species" adds de Gasperin, "a galaxy and its central black hole lead intimately connected lives, the galaxy providing matter to feed the black hole, and the black hole returning energy to the galaxy".

The image was made during the test-phase of LOFAR, and targeted the giant elliptical galaxy Messier 87, at the centre of a galaxy cluster in the constellation of Virgo. This galaxy is 2000 times more massive than our Milky Way and hosts in its centre one of the most massive black holes discovered so far, with a mass six billion times that of our Sun. Every few minutes this black hole swallows an amount of matter similar to that of the whole Earth, converting part of it into radiation and a larger part into powerful jets of ultra-fast particles, which are responsible for the observed radio emission.

"This is the first time such high-quality images are possible at these low frequencies", says professor Heino Falcke, chairman of the board of the ILT and co-author of the study. "This was a challenging observation; we did not expect to get such fantastic results so early in the commissioning phase of LOFAR."
 
 Click here for a hi-res version.

To determine the age of the bubble, the authors added radio observations at different frequencies from the Very Large Array in New Mexico (USA), and the Effelsberg 100-meter radio telescope near Bonn (Germany). The team found that this bubble is surprisingly young, just about 40 million years, which is a mere instant on cosmic time scales. The low frequency observation does not reveal any relic emission outside the well-confined bubble boundaries, this means that the bubble is not just a relic of an activity that happened long ago but is constantly refilled with fresh particles ejected by the central black hole.

 "What is particularly fascinating", says Andrea Merloni from the Max-Planck Institute of Extraterrestrial Physics in Garching, who supervised de Gasperin's doctoral work, "is that the results also provide clues on the violent matter-to-energy conversion that occurs very close to the black hole. In this case the black hole is particularly efficient in accelerating the jet, and much less effective in producing visible emission."

 Francesco de Gasperin performed the study as part of his PhD work at the Max Planck Institute for Astrophysics and at the Excellence Cluster Universe. De Gasperin is now a postdoctoral researcher at the University of Hamburg.
      
For more information, please contact:

ASTRON

Femke Boekhorst, PR & Communication
E-mail: boekhorst@astron.nl
Phone: + 31 521 595 204

Radboud University and ASTRON

Prof. Heino Falcke, Professor of Astroparticle Physics and Radio Astronomy
E-mail: h.falcke@astro.ru.nl
Phone: +31 24-36-52020

Max Planck Institute for Astrophysics

Francesco De Gasperin
E-mail: fdg@hs.uni-hamburg.de
Phone: +49 89 30000 2196 

Caption to the image:
This false colour image shows the galaxy M87. Optical light is shown in white/blue (Credits: SDSS), the radio emission in yellow/orange (LOFAR). At the centre, the radio emission has a very high surface brightness, showing where the jet powered by the supermassive black hole is located. Credits: Francesco de Gasperin, on behalf of the LOFAR collaboration.

The paper online: http://dx.doi.org/10.1051/0004-6361/201220209

  
About LOFAR

The LOFAR telescope, designed and built by ASTRON, is a revolutionary instrument able to detect radio waves with wavelengths up to 30-meter. Radio waves this long are typically generated by human activities as radio broadcasts, radar signals or satellite communications. They are also emitted by exotic objects in deep-space, such as accreting black holes, rotating neutron stars and supernovae. To detect these waves, LOFAR uses thousands of antennas spread all over Europe and combines the signals in a supercomputer located in the Netherlands. The 100 Gigabit per second of data flowing from all antennas are analyzed simultaneously and in real-time to provide the most detailed images ever done at these frequencies.

International LOFAR Telescope operations are coordinated by ASTRON, the Netherlands Institute for Radio Astronomy, on behalf of a consortium consisting of the Netherlands, Germany, France, the UK, and Sweden. Many of the technological solutions developed for LOFAR, in particular the calibration of phased-arrays as well as large-scale data transport and processing, will be highly relevant for future radio telescope projects such as the Square Kilometer Array (SKA).

A New Class of Extragalactic Objects

An artist's conception of a blazar. Astronomers have discovered a gamma-ray source that, although in most ways seeming to be a blazar, has no radio emission -- a feature that makes it unique (so far) and very difficult to understand. Credit: NASA-JPL.  Low Resolution Image (jpg)
 
A blazar is a galaxy with an intensely bright central nucleus containing a supermassive black hole, much like a quasar. The difference is that a blazar can emit light with extremely high energy gamma rays that are sometimes over a hundred million times more energetic than the highest energy X-rays that the Chandra X-ray Observatory studies. The overall emission of a blazar also varies dramatically with time and all known blazars are bright at radio wavelengths.

 Astronomers suspect that the bizarre behavior of blazars results when matter falling onto the vicinity of the massive black hole erupts into powerful, narrow beams of high velocity charged particles. The intense X-ray and gamma ray emission we see, and the strong radio emission and variability as well, are thought to be the results of our fortuitously staring right down the throats of the jets. In most other galaxies, infrared radiation comes from dust heated either by star formation or ultraviolet radiation from the vicinity of the massive black hole, rather than a blazar jet.

 CfA astronomers Allesandro Paggi, Raffaele D'Abrusco, Josh Grindlay, and Howard Smith and their colleagues recently published a new method to find and study blazars. They discovered that the infrared colors of blazars, as measured by the recent NASA WISE survey satellite, are so unusual that objects with these colors are very likely to be blazars. Ninety-seven percent of known blazars were easily picked out from thousands of other WISE sources by their infrared colors.

 There are about 1873 known gamma ray sources. About one-third of them are quite mysterious, however, because their very imprecise spatial locations have not allowed them to be associated with particular galaxies that can be studied with optical telescopes. The CfA astronomers discovered that about half of unknown gamma-ray sources could reasonably be identified with infrared emitting blazars, with the WISE coordinates then allowing detailed follow-up observations.

 One unidentified gamma-ray source recently flared in emission, prompting the team to see if it too had an infrared blazar-like color counterpart consistent with its location. In a new paper in this week's Astrophysical Journal Letters, the astronomers report finding one. The mystery, however, is that the counterpart is not a known blazar: it has no radio emission, it is not known to vary, and although it is an X-ray emitter the rest of its broad distribution of energy is unlike that of most blazars. It is possible that another galaxy nearby is actually the gamma-ray counterpart, but all of the alternate candidates show even greater disparities. If the WISE source is in fact the counterpart to the gamma-ray burst, its absence of radio emission means that it represents a strange new class of extragalactic source. If it is not the counterpart, its lack of radio emission is still a blazar mystery. Further research is needed to sort resolve the mystery, but the work so far illustrates the powerful capabilities of multi-wavelength research.

 

Saturday, October 27, 2012

STEREO Reaches New Milestone At Its Sixth Anniversary

Each of these images was captured from a different perspective by one of NASA's Solar Terrestrial Relations Observatory (STEREO) spacecraft on Oct. 14, 2012. The image on the left, STEREO-B, shows a dark vertical line slightly to the upper left of center. Only by looking at the image on the right, captured by STEREO-A from a different direction, is this feature revealed to be a giant prominence of solar material bursting through the sun's atmosphere. Credit: NASA/STEREO.
View larger - View STEREO B larger - View STEREO A larger

On the evening of Oct. 25, 2006, the twin Solar Terrestrial Relations Observatory (STEREO) spacecraft launched into space, destined for fairly simple orbits: both circle the sun like Earth does, STEREO-A traveling in a slightly smaller and therefore faster orbit, STEREO-B traveling in a larger and slower orbit. Those simple orbits, however, result in interesting geometry. As one spacecraft gained an increasing lead over Earth, the other trailed further and further behind. In February of 2011, each STEREO spacecraft was situated on opposite sides of the sun, and on Sept. 1, 2012, the two spacecraft and and the Solar Dynamics Observatory (at Earth) formed an equal-sided triangle, with each observatory providing overlapping views of the entire sun.


Since its launch in 2006, the STEREO spacecraft have drifted further and further apart to gain different views of the sun. Credit: NASA/GSFC .  View larger
By providing such unique viewpoints, STEREO has offered scientists the ability to see all sides of the sun simultaneously for the first time in history, augmented with a view from Earth's perspective by NASA's Solar Dynamics Observatory (SDO). In addition to giving researchers a view of active regions on the sun before they even come over the horizon, combining two views is crucial for three-dimensional observations of the giant filaments that dance off the sun's surface or the massive eruptions of solar material known as coronal mass ejections (CMEs). Examine the images below to see how a feature on the sun can look dramatically different from two perspectives.


This map of the full sun on Oct. 14, 2012, was created by images from,  in order from left to right, STEREO-A, STEREO-B and SDO. Credit: NASA/STEREO/SDO/GSFC . View larger


Karen C. Fox
NASA's Goddard Space Flight Center

New Study Brings a Doubted Exoplanet 'Back from the Dead'

A second look at data from NASA's Hubble Space Telescope is reanimating the claim that the nearby star Fomalhaut hosts a massive exoplanet. The study suggests that the planet, named Fomalhaut b, is a rare and possibly unique object that is completely shrouded by dust.

 Fomalhaut is the brightest star in the constellation Piscis Austrinus and lies 25 light-years away.

 In November 2008, Hubble astronomers announced the exoplanet, named Fomalhaut b, as the first one ever directly imaged in visible light around another star. The object was imaged just inside a vast ring of debris surrounding but offset from the host star. The planet's location and mass -- no more than three times Jupiter's -- seemed just right for its gravity to explain the ring's appearance.

 Recent studies have claimed that this planetary interpretation is incorrect. Based on the object's apparent motion and the lack of an infrared detection by NASA's Spitzer Space Telescope, they argue that the object is a short-lived dust cloud unrelated to any planet.

 A new analysis, however, brings the planet conclusion back to life.


In 2008, Hubble astronomers announced the detection of a giant planet around the bright star Fomalhaut. Recent studies have questioned this conclusion. Now, a reanalysis of Hubble data has revived the "deceased" exoplanet as a dust-shrouded world with less than twice the mass of Jupiter.  (Credit: NASA's Goddard Space Flight Center).

"Although our results seriously challenge the original discovery paper, they do so in a way that actually makes the object's interpretation much cleaner and leaves intact the core conclusion, that Fomalhaut b is indeed a massive planet," said Thayne Currie, an astronomer formerly at NASA's Goddard Space Flight Center in Greenbelt, Md., and now at the University of Toronto.

 The discovery study reported that Fomalhaut b's brightness varied by about a factor of two and cited this as evidence that the planet was accreting gas. Follow-up studies then interpreted this variability as evidence that the object actually was a transient dust cloud instead.

 In the new study, Currie and his team reanalyzed Hubble observations of the star from 2004 and 2006. They easily recovered the planet in observations taken at visible wavelengths near 600 and 800 nanometers, and made a new detection in violet light near 400 nanometers. In contrast to the earlier research, the team found that the planet remained at constant brightness.

 The team attempted to detect Fomalhaut b in the infrared using the Subaru Telescope in Hawaii, but was unable to do so. The non-detections with Subaru and Spitzer imply that Fomalhaut b must have less than twice the mass of Jupiter.

 Another contentious issue has been the object's orbit. If Fomalhaut b is responsible for the ring's offset and sharp interior edge, then it must follow an orbit aligned with the ring and must now be moving at its slowest speed. The speed implied by the original study appeared to be too fast. Additionally, some researchers argued that Fomalhaut b follows a tilted orbit that passes through the ring plane.

 Using the Hubble data, Currie's team established that Fomalhaut b is moving with a speed and direction consistent with the original idea that the planet's gravity is modifying the ring.

This visible-light image from the Hubble Space Telescope shows the vicinity of the star Fomalhaut, including the location of its dust ring and disputed planet, Fomalhaut b. A coronagraphic mask helped dim the star's brightness. This view combines two 2006 observations that were taken with masks of different sizes (1.8 and 3 arcseconds).  (Credit: NASA/ESA/T. Currie, U. Toronto).

This is an artist's impression of the exoplanet, Fomalhaut b, orbiting its sun, Fomalhaut.  (Credit: ESA; Hubble, M. Kornmesser; and ESO, L. Calçada and L. L. Christensen).   Larger image

"What we've seen from our analysis is that the object's minimum distance from the disk has hardly changed at all in two years, which is a good sign that it's in a nice ring-sculpting orbit," explained Timothy Rodigas, a graduate student in the University of Arizona and a member of the team.
Currie's team also addressed studies that interpret Fomalhaut b as a compact dust cloud not gravitationally bound to a planet. Near Fomalhaut's ring, orbital dynamics would spread out or completely dissipate such a cloud in as little as 60,000 years. The dust grains experience additional forces, which operate on much faster timescales, as they interact with the star's light.

Given what we know about the behavior of dust and the environment where the planet is located, we think that we're seeing a planetary object that is completely embedded in dust rather than a free-floating dust cloud," said team member John Debes, an astronomer at the Space Telescope Science Institute in Baltimore, Md.

A paper describing the findings has been accepted for publication in The Astrophysical Journal Letters.

Because astronomers detect Fomalhaut b by the light of surrounding dust and not by light or heat emitted by its atmosphere, it no longer ranks as a "directly imaged exoplanet." But because it's the right mass and in the right place to sculpt the ring, Currie's team thinks it should be considered a "planet identified from direct imaging."

Fomalhaut was targeted with Hubble most recently in May by another team. Those observations are currently under scientific analysis and are expected to be published soon.

Related Links

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

Friday, October 26, 2012

Violent star formation episodes in dwarf galaxies

NGC 3738
Credit: ESA/Hubble & NASA

The NASA/ESA Hubble Space Telescope has imaged the faint irregular galaxy NGC 3738, a starburst galaxy. The galaxy is in the midst of a violent episode of star formation, during which it is converting reservoirs of hydrogen gas harboured in the galaxy’s centre into stars. Hubble spots this gas glowing red around NGC 3738, one of the most distinctive signs of ongoing star formation.

Lying in the constellation of Ursa Major (The Great Bear), NGC 3738 is located about 12 million light-years from the Sun, and belongs to the Messier 81 group of galaxies. This galaxy — first observed by astronomer William Herschel  back in 1789 — is a nearby example of a blue compact dwarf, the faintest type of starburst galaxy. Blue compact dwarfs are small compared to large spiral galaxies — NGC 3738 is around 10 000 light-years across, just one tenth of the size of the Milky Way.

This type of galaxy is blue in appearance by virtue of containing large clusters of hot, massive stars, which ionise the surrounding interstellar gas with their intense ultraviolet radiation. They are relatively faint and appear to be irregular in shape. Unlike spirals or elliptical galaxies, irregular galaxies do not have any distinctive features, such as a nuclear bulge or spiral arms. Rather, they are extremely chaotic in appearance. These galaxies are thought to resemble some of the earliest that formed in the Universe and may provide clues as to how stars appeared shortly after the Big Bang. 

This image was created by combining visual and infrared images taken with the Wide Field Channel of the Advanced Camera for Surveys aboard the Hubble Space Telescope. The field of view of the Wide Field Channel is approximately 3.4 by 3.4 arcminutes wide.

Source: ESA/Hubble - Space Telescope


Monster Galaxy May Have Been Stirred Up By Black-hole Mischief

 Galaxy Cluster Abell 2261
Credit: NASA, ESA, M. Postman (STScI)
T. Lauer (NOAO), and the CLASH team


Astronomers using NASA's Hubble Space Telescope have obtained a remarkable new view of a whopper of an elliptical galaxy that may have been puffed up by the actions of one or more black holes in its core.

Spanning a little more than one million light-years, the galaxy is about 10 times the diameter of our Milky Way galaxy. The bloated galaxy is a member of an unusual class of galaxies with a diffuse core filled with a fog of starlight where there would normally be a concentrated peak of light around a central black hole. Viewing the core is like seeing a city with no downtown, just houses sprinkled across a vast landscape.

Astronomers used Hubble's Advanced Camera for Surveys and Wide Field Camera 3 to measure the amount of starlight across the galaxy, dubbed A2261-BCG. The Hubble observations revealed that the galaxy's puffy core, measuring about 10,000 light-years, is the largest yet seen.

A galaxy's core size typically is correlated to the dimensions of its host galaxy, but in this case, the central region is much larger than astronomers would expect for the galaxy's size. In fact, the bloated core is more than three times larger than the center of other very luminous galaxies. Located three billion light-years away, the galaxy is the most massive and brightest galaxy in the Abell 2261 cluster.

Astronomers have proposed two possibilities for the puffy core. One scenario is that a pair of merging black holes gravitationally stirred up and scattered the stars. Another idea is that the merging black holes were ejected from the core. Left without an anchor, the stars began spreading out even more, creating the puffy-looking core.

Previous Hubble observations have revealed that supermassive black holes, weighing millions or billions times more than the Sun, reside at the centers of nearly all galaxies and may play a role in shaping those central regions.

"Expecting to find a black hole in every galaxy is sort of like expecting to find a pit inside a peach," explained astronomer Tod Lauer of the National Optical Astronomy Observatory in Tucson, Ariz., a co-author of the Hubble study. "With this Hubble observation, we cut into the biggest peach and we can't find the pit. We don't know for sure that the black hole is not there, but Hubble shows that there's no concentration of stars in the core."

Team leader Marc Postman of the Space Telescope Science Institute in Baltimore, Md., said the galaxy stood out in the Hubble image. "When I first saw the image of this galaxy, I knew right away it was unusual," Postman explained. "The core was very diffuse and very large. The challenge was then to make sense of all the data, given what we knew from previous Hubble observations, and come up with a plausible explanation for the intriguing nature of this particular galaxy."

The paper describing the results appeared in the Sept. 10 issue of The Astrophysical Journal. The astronomers expected to see a slight cusp of light in the galaxy's center, marking the location of the black hole and attendant stars. Instead, the starlight's intensity remained fairly even across the galaxy.

One possibility for the puffy core may be due to two central black holes orbiting each other. These black holes collectively could have been as massive as several billion suns. Though one of the black holes would be native to the galaxy, a second black hole could have been added from a smaller galaxy that was gobbled up by the massive elliptical.

In this scenario, stars circling in the giant galaxy's center came close to the twin black holes. The stars were then given a gravitational boot out of the core. Each gravitational slingshot robbed the black holes of momentum, moving the pair ever closer together, until finally they merged, forming one supermassive black hole that still resides in the galaxy's center.

Another related possibility is that the black-hole merger created gravity waves, which are ripples in the fabric of space. According to the theory of general relativity, a pair of merging black holes produce ripples of gravity that radiate away. If the black holes are of unequal mass, then some of the energy may radiate more strongly in one direction, producing the equivalent of a rocket thrust. The imbalance of forces would have ejected the merged black hole from the center at speeds of millions of miles an hour, resulting in the rarity of a galaxy without a central black hole. "The black hole is the anchor for the stars," Lauer explained. "If you take it out, all of a sudden you have a lot less mass. The stars don't get held down very well and they expand out, enlarging the core even more."

The team admits that the ejected black-hole scenario may sound far-fetched, "but that's what makes observing the universe so intriguing — sometimes you find the unexpected," said Postman.

Added Lauer: "This is a system that's interesting enough that it pushes against a lot of questions. We have thought an awful lot about what black holes do. But we haven't been able to test our theories. This is an interesting place where a lot of the ideas we've had can come together and can be tested, fairly exotic ideas about how black holes may interact with each other dynamically and how they would affect the surrounding stellar population."

The team is now conducting follow-up observations with the Very Large Array radio telescope (VLA) in New Mexico. The astronomers expect material falling onto a black hole to emit radio waves, among other types of radiation. They will compare the VLA data with the Hubble images to more precisely pin down the location of the black hole, if it indeed exists.

The Abell 2261 cluster is part of a multi-wavelength survey, led by Postman, called the Cluster Lensing And Supernova survey with Hubble (CLASH). The survey probes the distribution of dark matter in 25 massive galaxy clusters.

CONTACT

Ray Villard / Donna Weaver

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

Marc Postman
Space Telescope Science Institute, Baltimore, Md.
410-338-4340
postman@stsci.edu

Tod LauerNational
Optical Astronomy Observatory, Tucson, Ariz.
520-318-8920
lauer@noao.edu

Thursday, October 25, 2012

Saturn's giant storm reveals the planet's churning atmosphere

VLT image of Saturn's giant vortex at mid-infrared wavelengths.
Image courtesy of L.N. Fletcher, University of Oxford, UK, and ESO

Saturn's 'Great Springtime Storm' in visible light.
Credit: NASA/JPL-Caltech/Space Science Institute
Hi-Res [jpg]     212.87 kb
 
 
This animation shows the evolution of Saturn's 'Great Springtime Storm' in the planet's stratosphere. It is based on observations performed at mid-infrared wavelengths.  Credit: ESA/C. Carreau


Evolution of Saturn's storm in the stratosphere.
Credit: ESA/C. Carreau  Click here for more information on this video.

A recent study of the giant storm whirling on Saturn for the past two years, which became known as the "Great Springtime Storm", has given planetary scientists new clues about the planet's weather. Using a combination of data from the Cassini orbiter and ground-based telescopes, the scientists traced the storm's development from deep within the churning clouds in Saturn's lower atmosphere to altitudes hundreds of kilometres above the cloud decks, in the planet's stratosphere. There, two large pockets of warm air formed and later merged into one gigantic hot vortex that has been travelling around Saturn's northern hemisphere since mid-2011. The study of this storm and its associated vortex, which occurred unusually early in Saturn's 30-year-long weather cycle, suggests that waves play an important role in the energy transfer across the planet's atmosphere.

Storms are large disturbances in a planetary atmosphere. A common phenomenon on Earth, storms are not unique to our planet's weather and may arise on any planet that is surrounded by a thick atmosphere. Astronomical records report similar events on several planets in the Solar System, and recent data hint at possible storms on exoplanets.

A new study, based on data from the NASA/ESA/ASI Cassini-Huygens mission and ground-based telescopes, has looked into one of the largest storms recorded in the Solar System, which started whirling over Saturn's mid-northern latitudes about two years ago. The storm originated in the planet's lower atmosphere, where it was first seen in December 2010, and later grew to encircle the entire planet. The disturbance also propagated to higher atmospheric layers, where its aftermath can still be detected. It is known as the 'Great Springtime Storm' because it took place during the spring season in the planet's northern hemisphere, which started in August 2009 and lasts about seven years.
 
"Giant storms on Saturn occur regularly and have been observed for over a century, but this is the first time we could follow the temporal evolution of such an event in great detail," notes Leigh Fletcher from the University of Oxford, UK. Fletcher has led an extensive study of the Great Springtime Storm using data gathered in the infrared portion of the electromagnetic spectrum by the Cassini spacecraft, which has been orbiting Saturn since 2004, as well as ESO's Very Large Telescope and NASA's Infrared Telescope Facility.

 "The storm was first detected in the planet's lower atmosphere – the troposphere – via optical and radio observations. Then we looked for its signature at mid-infrared wavelengths,"
explains Fletcher.

"When we look at Saturn's atmosphere in optical wavelengths, we see the sunlight that is reflected by a haze layer located deep down in the troposphere. In the mid-infrared, instead, we directly measure the temperature of the atmosphere for many kilometres above the clouds. This allows us to peer through the three-dimensional structure of the atmosphere," he adds.
Observing at these longer wavelengths provided a drastically different view, and allowed Fletcher and his collaborators to probe how the storm had infiltrated the upper part of the atmosphere – the stratosphere upwards from the troposphere. The presence of Cassini in the saturnian system and its ability to perform mid-infrared observations has allowed the astronomers to monitor the evolution of this unique meteorological event in unprecedented detail.
Mid-infrared images from January 2011 showed that two large pockets of warm air had formed over the storm, in the stratosphere. These warm air masses, also referred to as 'beacons', were both moving westwards, although with different speeds, and remained clearly separated for a few months. Between April and June 2011, the two beacons merged and gave rise to a giant vortex of clockwise-swirling air – an anti-cyclone – with temperatures up to 221 K, hotter than the surrounding air by 70-80 K.
The huge anti-cyclone in Saturn's stratosphere had fully detached from the tropospheric disturbance that caused it in the first place. At its biggest, in late June 2011, the vortex covered about 62 000 km – almost one quarter of the planet's circumference at the mid-northern latitudes affected by the storm. At the same time, the storm in the troposphere, only visible at optical wavelengths, had almost ceased.
"We kept monitoring Saturn during the storm with the help of many small, ground-based telescopes operated by professional and amateur astronomers alike, and found no sign of the giant vortex in the optical data. Although the tropospheric storm was the underlying cause of this enormous vortex, the vortex subsequently evolved independently of events happening deeper down, and was still present long after the tropospheric storm was over," he adds.

Since July 2011, the giant hot vortex has been shrinking and cooling at a very slow pace. It is still present in Saturn's stratosphere, where it has shrunk to less than half of its greatest extent, and is expected to disappear completely in a couple of years.

The data analysed by Fletcher and his collaborators showed how the temperature, wind velocity and chemical composition varied within and around the giant vortex. This allowed them to unveil how the storm had evolved over several months, and to investigate the energy transfer mechanisms at play among the various layers of Saturn's atmosphere.

"We suspected that the weather in the lower atmosphere has an impact on what happens at much higher layers, hundreds of kilometres upwards, just as happens in Earth's atmosphere. Now we have evidence for this on Saturn," says Fletcher.

In Earth's atmosphere, storm-generated waves are known to transport air and energy across the atmosphere, including upwards to the stratosphere. It is possible that a similar mechanism has taken place on Saturn, too: wave-like perturbations, induced by the tropospheric storm, might have made their way upwards to the stratosphere, where they released their energy and caused the formation of the two beacons.

"What is unusual in this particular case is that the two beacons interacted with one another up in the stratosphere, giving rise to the giant vortex. How exactly this happened remains an open question that needs to be tackled via numerical simulations," comments Fletcher.

The timing of the storm is also quite puzzling. Since 1876, large disturbances have been observed on Saturn with striking regularity: once every 'saturnian' year, which lasts about 30 years, and always during the northern hemisphere's summer season. The last such storm on record dates back to 1990, and the next one was expected in 2020.

"The Great Springtime Storm is definitely ahead of schedule with respect to Saturn's standard storm cycle. It is still unclear whether this is an isolated event or a signal that the storm season on the planet started earlier than expected," comments Nicolas Altobelli, Cassini-Huygens Project Scientist at ESA.

"Cassini will keep monitoring Saturn's atmosphere from its vantage point. The mission will be operating until the northern summer solstice, which will take place in May 2017. The storm season on Saturn's northern hemisphere may not be over yet, and in this case we might be able to see other spectacular events in the next few years," Altobelli adds.

"If storms are detected on Saturn in the upcoming future, it will be important to verify whether these will also produce dramatic aftereffects such as the stratospheric vortex from 2011," Fletcher concludes.

Notes for editors
The study presented here is based on data gathered with the Composite Infrared Spectrometer (CIRS) on board the Cassini orbiter from the NASA/ESA/ASI Cassini-Huygens mission. The data were complemented by ground-based observations from the VLT Imager and Spectrometer for the mid-Infrared (VISIR) on ESO's Very Large Telescope, located in Chile, and from the Mid-Infrared Spectrometer and Imager (MIRSI) on NASA's Infrared Telescope Facility, located at Mauna Kea, Hawaii, USA.

Related publication
 
L. N. Fletcher, et al., "The origin and evolution of Saturn's 2011-2012 stratospheric vortex", 2012, Icarus, Volume 221, Issue 2, November-December 2012, Pages 560-586, DOI:10.1016/j.icarus.2012.08.024

Contact

Leigh N. Fletcher
Atmospheric, Oceanic & Planetary Physics
University of Oxford, UK
Phone: +44-1865272089

Nicolas Altobelli
ESA Cassini-Huygens Project Scientist
Directorate of Science and Robotic ExplorationEuropean Space Agency
Tel: +34-918131201

NASA's Spitzer Sees Light of Lonesome Stars

New research from scientists using NASA's Spitzer Space Telescope suggests that a mysterious infrared glow across our whole sky is coming from stray stars torn from galaxies. When galaxies grow, they merge and become gravitationally tangled in a violent process that results in streams of stars being ripped away from the galaxies. Such streams, called tidal tails, can be seen in this artist's concept. Scientists say that Spitzer is picking up the collective glow of stars such as these, which linger in the spaces between galaxies. Image credit: NASA/JPL-Caltech/UC Irvine .  Full image and caption


The image on the left shows a portion of our sky, called the Boötes field, in infrared light, while the image on the right shows a mysterious, background infrared glow captured by NASA's Spitzer Space Telescope in the same region of sky. Using Spitzer, researchers were able to detect this background glow, which spreads across the whole sky, by masking out light from galaxies and other known sources of light (the masks are the gray, blotchy marks). Image credit: NASA/JPL-Caltech.


PASADENA, Calif. - A new study using data from NASA's Spitzer Space Telescope suggests a cause for the mysterious glow of infrared light seen across the entire sky. It comes from isolated stars beyond the edges of galaxies. These stars are thought to have once belonged to the galaxies before violent galaxy mergers stripped them away into the relatively empty space outside of their former homes.

 "The infrared background glow in our sky has been a huge mystery," said Asantha Cooray of the University of California at Irvine, lead author of the new research published in the journal Nature. "We have new evidence this light is from the stars that linger between galaxies. Individually, the stars are too faint to be seen, but we think we are seeing their collective glow."

 The findings disagree with another theory explaining the same background infrared light observed by Spitzer. A group led by Alexander "Sasha" Kashlinsky of NASA's Goddard Space Flight Center in Greenbelt, Md., proposed in June this light, which appears in Spitzer images as a blotchy pattern, is coming from the very first stars and galaxies.

 In the new study, Cooray and colleagues looked at data from a larger portion of the sky, called the Bootes field, covering an arc equivalent to 50 full Earth moons. These observations were not as sensitive as those from the Kashlinsky group's studies, but the larger scale allowed researchers to analyze better the pattern of the background infrared light.

 "We looked at the Bootes field with Spitzer for 250 hours," said co-author Daniel Stern of NASA's Jet Propulsion Laboratory in Pasadena, Calif. "Studying the faint infrared background was one of the core goals of our survey, and we carefully designed the observations in order to directly address the important, challenging question of what causes the background glow."

 The team concluded the light pattern of the infrared glow is not consistent with theories and computer simulations of the first stars and galaxies. Researchers say the glow is too bright to be from the first galaxies, which are thought not to have been as large or as numerous as the galaxies we see around us today. Instead, the scientists propose a new theory to explain the blotchy light, based on theories of "intracluster" or "intrahalo" starlight.

 Theories predict a diffuse smattering of stars beyond the halos, or outer reaches, of galaxies, and in the spaces between clusters of galaxies. The presence of these stars can be attributed to two phenomena. Early in the history of our universe as galaxies grew in size, they collided with other galaxies and gained mass. As the colliding galaxies became tangled gravitationally, strips of stars were shredded and tossed into space. Galaxies also grow by swallowing smaller dwarf galaxies, a messy process that also results in stray stars.

 "A light bulb went off when reading some research papers predicting the existence of diffuse stars," Cooray said. "They could explain what we are seeing with Spitzer."

 More research is needed to confirm this sprinkling of stars makes up a significant fraction of the background infrared light. For instance, it would be necessary to find a similar pattern in follow-up observations in visible light. NASA's upcoming James Webb Space Telescope (JWST) might finally settle the matter for good.

 "The keen infrared vision of the James Webb Telescope will be able to see some of the earliest stars and galaxies directly, as well as the stray stars lurking between the outskirts of nearby galaxies," said Eric Smith, JWST's deputy program manager at NASA Headquarters in Washington. "The mystery objects making up the background infrared light may finally be exposed."

 Other authors include Joseph Smidt, Francesco De Bernardis, Yan Gong and Christopher C. Frazer of UC Irvine; Matthew L. N. Ashby of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass; Peter R. Eisenhardt of JPL; Anthony H. Gonzalez of the University of Florida in Gainesville; Christopher S. Kochanek of Ohio State University in Columbus; Szymon Koz?owski of Ohio State and the Warsaw University Observatory in Poland; and Edward L. Wright of the University of California, Los Angeles.

 JPL manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate in Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology in Pasadena. Data are archived at the Infrared Science Archive housed at the Infrared Processing and Analysis Center at Caltech. Caltech manages JPL for NASA.

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

Alan Buis 
818-354-0474
Jet Propulsion Laboratory, Pasadena, Calif.
Alan.buis@jpl.nasa.gov

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

Wednesday, October 24, 2012

NGC 4178: Revealing a Mini-Supermassive Black Hole

 NGC 4178
 Credit:     X-ray: NASA/CXC/George Mason Univ/N.Secrest et al; 
Optical: SDSS


One of the lowest mass supermassive black holes ever observed in the middle of a galaxy has been identified, thanks to NASA's Chandra X-ray Observatory and several other observatories. The host galaxy is of a type not expected to harbor supermassive black holes, suggesting that this black hole, while related to its supermassive cousins, may have a different origin.

The black hole is located in the middle of the spiral galaxy NGC 4178, shown in this image from the Sloan Digital Sky Survey. The inset shows an X-ray source at the position of the black hole, in the center of a Chandra image. An analysis of the Chandra data, along with infrared data from NASA's Spitzer Space Telescope and radio data from the NSF's Very Large Array suggests that the black hole is near the extreme low-mass end of the supermassive black hole range.

These results were published in the July 1, 2012 issue of The Astrophysical Journal by Nathan Secrest, from George Mason University in Fairfax, Virginia, and collaborators.

The properties of the X-ray source, including its brightness and spectrum - the amount of X-rays at different wavelengths - and its brightness at infrared wavelengths, suggest that a black hole in the center of NGC 4178 is rapidly pulling in material from its surroundings. The same data also suggest that light generated by this infalling material is heavily absorbed by gas and dust surrounding the black hole.

A known relationship between the mass of a black hole and the amount of X-rays and radio waves it generates was used to estimate the mass of the black hole. This method gives a black hole mass estimate of less than about 200,000 times that of the sun. This agrees with mass estimates from several other methods employed by the authors, and is lower than the typical values for supermassive black holes of millions to billions of times the mass of the sun.

NGC 4178 is a spiral galaxy located about 55 million light years from Earth. It does not contain a bright central concentration, or bulge, of stars in its center. Besides NGC 4178, four other galaxies without bulges are currently thought to contain supermassive black holes. Of these four black holes, two have masses that may be close to that of the black hole in NGC 4178. XMM-Newton observations of an X-ray source discovered by Chandra in the center of the galaxy NGC 4561 indicate that the mass of this black hole is greater than 20,000 times the mass of the sun, but the mass could be substantially higher if the black hole is pulling in material slowly, causing it to generate less X-ray emission. A paper describing these results was published in the October 1st, 2012 issue of The Astrophysical Journal by Araya Salvo and collaborators.

The mass of the black hole in the galaxy NGC 4395 is estimated to be about 360,000 times the mass of the sun, as published by Peterson and collaborators in the October 20, 2005 issue of the Astrophysical Journal.

Previously, astronomers have found that observations of a large number of galaxies are consistent with a close correlation between the mass of a supermassive black hole and the mass of the bulge of its host galaxy. Theoretical models developed to explain these results invoke mergers of galaxies, and predict that galaxies without bulges are unlikely to host supermassive black holes. The results found for NGC 4178 and the four other galaxies mentioned run counter to these predictions, and may suggest that more than one mechanism is at work in forming supermassive black holes.

Three other X-ray sources were found in the Chandra image. If they are located in NGC 4178 they are likely to be binary systems  containing a black hole or neutron star. The brightest of the three sources may be an intermediate-mass black hole with a mass that is about 6,000 times that of the sun.

The co-authors of the paper describing these results are Shobita Satyapal, and Mario Gliozzi, from George Mason University; Teddy Cheung, from the National Academy of Science in Washington DC; Anil Seth, from the University of Utah in Salt Lake City, UT, and Torsten Böker from ESA/ESTEC in the Netherlands.

NASA's Marshall Space Flight Center in Huntsville, Ala., manages the Chandra program for NASA's Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory controls Chandra's science and flight operations from Cambridge, Mass.

Fast Facts for NGC 4178:

Release Date:     October 24, 2012
Scale:     Main image is 10 arcmin across (about 160,000 light years)
Category:     Quasars & Active Galaxies
Coordinates (J2000):     RA 12h 12m 46.40s | Dec +10° 51' 57.00"
Constellation:    Virgo
Observation Date:      19 Feb 2011
Observation Time:      10 hours
Obs. ID:      12748
Instrument:     ACIS
References:     Secrest, N et al, 2012 ApJ 753:38; arXiv:1205.0230
Color Code:     X-ray (Purple); Optical (Red, Green, Blue)

84 Million Stars and Counting

 PR Image eso1242a
VISTA gigapixel mosaic of the central parts of the Milky Way

PR Image eso1242b
Wide-field view of the Milky Way, showing the extent of a new VISTA gigapixel image

  PR Image eso1242c
Optical/infrared comparison of the central parts of the Milky Way

 PR Image eso1242d
Colour–magnitude diagram of the Galactic bulge

 PR Image eso1242e
Annotated map of VISTA’s view of the centre of the Milky Way

Video

 PR Video eso1242a
Infrared/visible light comparison of VISTA’s gigapixel view of the centre of the Milky Way

 VISTA creates largest ever catalogue of centre of our galaxy

Using a whopping nine-gigapixel image from the VISTA infrared survey telescope at ESO’s Paranal Observatory, an international team of astronomers has created a catalogue of more than 84 million stars in the central parts of the Milky Way. This gigantic dataset contains more than ten times more stars than previous studies and is a major step forward for the understanding of our home galaxy. The image gives viewers an incredible, zoomable view of the central part of our galaxy. It is so large that, if printed with the resolution of a typical book, it would be 9 metres long and 7 metres tall.

“By observing in detail the myriads of stars surrounding the centre of the Milky Way we can learn a lot more about the formation and evolution of not only our galaxy, but also spiral galaxies in general,” explains Roberto Saito (Pontificia Universidad Católica de Chile, Universidad de Valparaíso and The Milky Way Millennium Nucleus, Chile), lead author of the study.

Most spiral galaxies, including our home galaxy the Milky Way, have a large concentration of ancient stars surrounding the centre that astronomers call the bulge. Understanding the formation and evolution of the Milky Way’s bulge is vital for understanding the galaxy as a whole. However, obtaining detailed observations of this region is not an easy task.

“Observations of the bulge of the Milky Way are very hard because it is obscured by dust,” says Dante Minniti (Pontificia Universidad Catolica de Chile, Chile), co-author of the study. “To peer into the heart of the galaxy, we need to observe in infrared light, which is less affected by the dust.”

The large mirror, wide field of view and very sensitive infrared detectors of ESO’s 4.1-metre Visible and Infrared Survey Telescope for Astronomy (VISTA) make it by far the best tool for this job. The team of astronomers is using data from the VISTA Variables in the Via Lactea programme (VVV) [1], one of six public surveys carried out with VISTA. The data have been used to create a monumental 108 200 by 81 500 pixel colour image containing nearly nine billion pixels. This is one of the biggest astronomical images ever produced. The team has now used these data to compile the largest catalogue of the central concentration of stars in the Milky Way ever created [2].

To help analyse this huge catalogue the brightness of each star is plotted against its colour for about 84 million stars to create a colour–magnitude diagram. This plot contains more than ten times more stars than any previous study and it is the first time that this has been done for the entire bulge. Colour–magnitude diagrams are very valuable tools that are often used by astronomers to study the different physical properties of stars such as their temperatures, masses and ages [3].

“Each star occupies a particular spot in this diagram at any moment during its lifetime. Where it falls depends on how bright it is and how hot it is. Since the new data gives us a snapshot of all the stars in one go, we can now make a census of all the stars in this part of the Milky Way,”
explains Dante Minniti.

The new colour–magnitude diagram of the bulge contains a treasure trove of information about the structure and content of the Milky Way. One interesting result revealed in the new data is the large number of faint red dwarf stars. These are prime candidates around which to search for small exoplanets using the transit method [4].

“One of the other great things about the VVV survey is that it’s one of the ESO VISTA public surveys. This means that we’re making all the data publicly available through the ESO data archive, so we expect many other exciting results to come out of this great resource," concludes Roberto Saito.

Notes

[1] The VISTA Variables in the Via Lactea (VVV) survey is an ESO public survey dedicated to scanning the southern plane and bulge of the Milky Way through five near-infrared filters. It started in 2010 and was granted a total of 1929 hours of observing time over a five-year period. Via Lactea is the Latin name for the Milky Way.

[2] The image used in this work covers about 315 square degrees of the sky (a bit less than 1% of the entire sky) and observations were carried out using three different infrared filters. The catalogue lists the positions of the stars along with their measured brightnesses through the different filters. It contains about 173 million objects, of which about 84 million have been confirmed as stars. The other objects were either too faint or blended with their neighbours or affected by other artefacts, so that accurate measurements were not possible. Others were extended objects such as distant galaxies.

The image used here required a huge amount of data processing, which was performed by Ignacio Toledo at the ALMA OSF. It corresponds to a pixel scale of 0.6 arcseconds per pixel, down-sampled from the original pixel scale of 0.34 arcseconds per pixel.

[3] A colour–magnitude diagram is a graph that plots the apparent brightnesses of a set of objects against their colours. The colour is measured by comparing how bright objects look through different filters. It is similar to a Hertzsprung-Russell (HR) diagram but the latter plots luminosity (or absolute magnitude) rather than just apparent brightness and a knowledge of the distances of the stars plotted is also needed.

[4] The transit method for finding planets searches for the small drop in brightness of a star that occurs when a planet passes in front of it and blocks some of its light. The small size of the red dwarf stars, typically with spectral types K and M, gives a greater relative drop in brightness when low-mass planets pass in front of them, making it easier to search for planets around them.

More information

This research was presented in a paper “Milky Way Demographics with the VVV Survey I. The 84 Million Star Colour–Magnitude Diagram of the Galactic Bulge“ by R. K. Saito et al., which was published in the journal Astronomy & Astrophysics (A&A, 544, A147).

The team is composed of R. K. Saito (Pontificia Universidad Católica de Chile, Santiago, Chile; Universidad de Valparaíso, Chile; The Milky Way Millennium Nucleus, Chile), D. Minniti (Pontificia Universidad Católica de Chile; Vatican Observatory), B. Dias (Universidade de São Paulo, Brazil), M. Hempel (Pontificia Universidad Católica de Chile), M. Rejkuba (ESO, Garching, Germany), J. Alonso-García (Pontificia Universidad Católica de Chile), B. Barbuy (Universidade de São Paulo), M. Catelan (Pontificia Universidad Católica de Chile), J. P. Emerson (Queen Mary University of London, United Kingdom), O. A. Gonzalez (ESO, Garching, Germany), P. W. Lucas (University of Hertfordshire, Hatfield, United Kingdom) and M. Zoccali (Pontificia Universidad Católica de Chile).

The year 2012 marks the 50th anniversary of the founding of the European Southern Observatory (ESO). ESO is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive ground-based astronomical observatory by far. It is supported by 15 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Portugal, Spain, Sweden, Switzerland and the United Kingdom. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world’s most advanced visible-light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world’s largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is the European partner of a revolutionary astronomical telescope ALMA, the largest astronomical project in existence. ESO is currently planning a 40-metre-class European Extremely Large optical/near-infrared Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.

Links

Contacts

 Roberto Saito
 Pontificia Universidad Católica de Chile
 Santiago, Chile
 Tel: +56 2 354 5767
 Email:
rsaito@astro.puc.cl

 Dante Minniti
 Pontificia Universidad Católica de Chile
 Santiago, Chile
 Tel: +56 2 463 3267
 Email:
dante@astro.puc.cl

 Richard Hook
 ESO, La Silla, Paranal, E-ELT and Survey Telescopes Public Information Officer
 Garching bei München, Germany
 Tel: +49 89 3200 6655
 Cell: +49 151 1537 3591
 Email:
rhook@eso.org