Posted on by Anne Minard

Touching the Tarantula: Hubble Gets in Close

Credit: NASA, ESA

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Hubble has edged in close to the Tarantula Nebula, peering into its bright center of ionized gases, dust and still-forming stars. The Tarantula is already a go-to celestial marvel, because its hydrogen-fueled young stars shine with such intense ultraviolet light that they ionize and redden the surrounding gas — making the nebula visible without a telescope for Earth-bound observers 170,000 light-years away. The new image may make this popular beacon, in our neighboring galaxy the Large Magellanic Cloud, even more famous.

 

Credit: NASA, ESA

The wispy arms of the Tarantula Nebula (RA 05h 38m 38s dec -69° 05.7?) were originally thought to resemble spindly spider legs, giving the nebula its unusual name. The part of the nebula visible in the new image is criss-crossed with tendrils of dust and gas churned up by recent supernovae. These remnants include NGC 2060, visible above and to the left of the center of the image, which contains the brightest known pulsar.

The tarantula’s bite goes beyond NGC 2060. Near the edge of the nebula, outside the frame, below and to the right, lie the remains of supernova SN 1987a, the closest supernova to Earth to be observed since the invention of telescopes in the 17th century. Hubble and other telescopes have been returning to spy on this stellar explosion regularly since it blew up in 1987, and each subsequent visit shows an expanding shockwave lighting up the gas around the star, creating a pearl necklace of glowing pockets of gas around the remains of the star. SN 1987a is visible in wide field images of the nebula, such as that taken by the MPG/ESO 2.2-meter telescope.

A compact and extremely bright star cluster called RMC 136 lies above and to the left of this field of view, providing much of the radiation that powers the multi-coloured glow. Until recently, astronomers debated whether the source of the intense light was a tightly bound cluster of stars, or perhaps an unknown type of super-star thousands of times bigger than the sun. It is only in the last 20 years, with the fine detail revealed by Hubble and the latest generation of ground-based telescopes, that astronomers have been able to conclusively prove that it is, indeed, a star cluster.

But even if the Tarantula Nebula doesn’t contain this hypothetical super-star, it still hosts some extreme phenomena, making it a popular target for telescopes. Within the bright star cluster lies star RMC 136a1, which was recently found to be the heaviest ever discovered: the star’s mass when it was born was around 300 times that of the sun. This heavyweight is challenging astronomers’ theories of star formation, smashing through the upper limit they thought existed on star mass.

Source: ESA press release at the Hubble site. See also previous releases on the Large Magellanic Cloud and RMC 136.

Posted on by Anne Minard

‘Armada of Telescopes’ Captures Most Distant Galaxy Cluster Ever Seen

Hubble infrared image showing CL J1449+0856, the most distant mature cluster of galaxies found. Color data was added from ESO’s Very Large Telescope and the NAOJ’s Subaru Telescope. Credit: NASA, ESA, R. Gobat (Laboratoire AIM-Paris-Saclay, CEA/DSM-CNRS–)

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The galaxies above are among the oldest objects astronomers have ever laid eyes — er, telescopes — on, formed when the Universe was less than a quarter of its current age. In a new study out in the journal Astronomy & Astrophysics, a team of researchers has announced that they’ve used a fleet of the world’s most powerful telescopes to measure the distance from here to there.

And things look awfully familiar.

“The surprising thing is that when we look closely at this galaxy cluster it doesn’t look young — many of the galaxies have settled down and don’t resemble the usual star-forming galaxies seen in the early Universe,” said lead author Raphael Gobat of Université Paris Diderot in France.

The Very Large Telescope (VLT) at ESO's Cerro Paranal observing site in the Atacama Desert of Chile, consisting of four Unit Telescopes with main mirrors 8.2-m in diameter and four movable 1.8-m diameter Auxiliary Telescopes. The telescopes can work together, in groups of two or three, to form a giant interferometer, allowing astronomers to see details up to 25 times finer than with the individual telescopes. Credit: Iztok Boncina/ESO

Clusters of galaxies are the largest structures in the Universe that are held together by gravity. Astronomers expect these clusters to grow over time so that massive clusters would be rare in the early Universe. Although even more distant clusters have been seen, they appear to be young clusters in the process of formation, not settled mature systems.

The international team of astronomers used the powerful VIMOS and FORS2 instruments on ESO’s Very Large Telescope (VLT) to measure the distances to some of the blobs in a curious patch of very faint red objects first observed with the Spitzer space telescope. This grouping, named CL J1449+0856  for its position in the sky, had all the hallmarks of being a very remote cluster of galaxies. The results showed that we are indeed seeing a galaxy cluster as it was when the Universe was about three billion years old.

Once the team knew the distance to this very rare object, they looked carefully at the component galaxies using both Hubble and ground-based telescopes, including the VLT. They found evidence suggesting that most of the galaxies in the cluster were not forming stars, but were composed of stars that were already about one billion years old. This makes the cluster a mature object, similar in mass to the Virgo Cluster, the nearest rich galaxy cluster to the Milky Way.

Further evidence that this is a mature cluster comes from observations of X-rays coming from CL J1449+0856 made with ESA’s XMM-Newton space observatory. The cluster is giving off X-rays that must be coming from a very hot cloud of tenuous gas filling the space between the galaxies and concentrated towards the center of the cluster. This is another sign of a mature galaxy cluster, held firmly together by its own gravity, as very young clusters have not had time to trap hot gas in this way.

As Gobat concludes, “These new results support the idea that mature clusters existed when the Universe was less than one quarter of its current age. Such clusters are expected to be very rare according to current theory, and we have been very lucky to spot one. But if further observations find many more then this may mean that our understanding of the early Universe needs to be revised.”

Source: ESO press release. The research appears in a paper, “A mature cluster with X-ray emission at z = 2.07,” by R. Gobat et al., published in the journal Astronomy & Astrophysics. (see also arxiv). Lead author’s affiliation page: Université Paris Diderot.

Posted on by Anne Minard

New Look at Messier 82 Reveals Superwind Source, Young Star Clusters

False color mosaic showing the Subaru COMICS image (red), a Hubble Space Telescope near-infrared image of stars (green) and a Chandra satellite X-ray image (blue) dominated by extremely hot gas and black holes. Credit: JAXA

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Messier 82’s galactic windstorms emanate from many young star clusters, rather than any single source, say astronomers who released this new image today.

The international team of scientists, led by Poshak Gandhi of the Japan Aerospace Exporation Agency (JAXA), has used the Subaru Telescope to produce a new view of M 82 at infrared wavelengths that are 20 times longer than those visible to the human eye.

M 82 (09h 55m 52.2s, +69° 40′ 47″) is located close to the ladle of the Big Dipper in the constellation Ursa Major and is the nearest starburst galaxy, at a distance of about 11 million light years from Earth.

The combination of Subaru Telescope’s large 8.2 m primary mirror and its Cooled Mid-Infrared Camera and Spectrometer (COMICS) allowed the team to obtain a sharp, magnified view of the inner area of the galaxy.

Images of M 82. The bottom image from Subaru shows the superwind crossing the disk structure. Courtesy of JAXA.

Previous observations of M 82 with infrared telescopes, including the middle and bottom image in the three-part series, have found a very strong wind emanating from it — a ‘superwind’ that is composed of dusty gas and extends over many hundreds of thousands of light years. This high-powered windstorm ejects material from the galaxy at a speed of about a half a million miles per hour, sweeping it up from the central regions and depositing it far and wide over the galaxy and beyond. The contents of this material are seeds for solar systems like our own, and perhaps for life itself. The dusty superwind glows brightly in the infrared, because billions of bright, newly-formed stars heat it up.

With the new Subaru image, scientists have gained insight about the sources of the superwind.

“The wind is found to originate from multiple ejection sites spread over hundreds of light years rather than emanating from any single cluster of new stars. We can now distinguish ‘pillars’ of fast gas, and even a structure resembling the surface of a ‘bubble’ about 450 light years wide,” Gandhi explained.

COMICS has detectors particularly adept at indicating the presence of warm dust, which it found was more than 100 degrees hotter than the bulk of material filling the rest of the galaxy. The widespread, continuous flow of energy from young stars into the galactic expanse keeps the dust hot.

Further insights from the Subaru image emerge when it’s combined with previous images from Hubble and Chandra. Their integration produces a beautiful mosaic, represented in the lead image, that provides the first opportunity to isolate M 82’s infrared properties. Supported by these data, scientists can study the broad spectrum of radiation of different kinds of objects spread over the galaxy’s plane, including supernovae, star clusters, and black holes.

Many questions remain, such as how many more stars the galaxy contains — many could still be obscured by the dust of star formation — and whether or not M 82 hosts an actively growing supermassive black hole.

The results are reported in the article “Diffraction-limited Subaru imaging of M82: sharp mid-infrared view of the starburst core” by P. Gandhi, N. Isobe, M. Birkinshaw, D.M. Worrall, I. Sakon, K. Iwasawa & A. Bamba, in the Publications of the Astronomical Society of Japan, v. 63 (2011), in press.

Source: Subaru press release

Posted on by Nancy Atkinson

Spitzer Captures a Pink Sunflower in Space

Classifying Galaxies
This image from NASA's Spitzer Space Telescope shows infrared light from the Sunflower galaxy, otherwise known as Messier 63. Spitzer's view highlights the galaxy's dusty spiral arms. Image credit: NASA/JPL-Caltech

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Looking out my own window this morning provides a gloomy overcast view, so this new image from the Spitzer Space Telescope provides a day-brightener: a pink sunflower! This is the Sunflower galaxy, also known as Messier 63, and with Spitzers’ infrared eyes, the arms of the galaxy show up vividly. Infrared light is sensitive to the dust lanes in spiral galaxies, which appear dark in visible-light images. Spitzer’s view reveals complex structures that trace the galaxy’s spiral arm pattern.

Source: JPL
This galaxy is about 37 million-light years away from Earth, and lies close to the well-known Whirlpool galaxy and the associated Messier 51 group of galaxies.

Posted on by Anne Minard

Dusty Neighbor NGC 247 is a Million Light-Years Closer Than Thought

Spiral galaxy NGC 247, shot with the Wide Field Imager at ESO’s La Silla Observatory in Chile. Credit: ESO

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One of our celestial neighbors, the spiral galaxy NGC 247, just moved about a million light-years closer.

Well, not really. But astronomers are retooling estimates of the distance to it, which was overestimated in the past partly because of the nearly edge-on tilt, shown above. The just-released image, from the Wide Field Imager on the MPG/ESO 2.2-metre telescope in Chile, shows large numbers of the galaxy’s component stars and glowing pink clouds of hydrogen, marking regions of active star formation, in the loose and ragged spiral arms. Numerous other galaxies can be seen in the distance.

Through a moderate-sized amateur telescope, the Cetus galaxy appears large but dim, and is seen best in a dark sky. Credit: ESO, IAU and Sky & Telescope

NGC 247 (RA 00h 47′ 14″  – 20deg 52′ 04″) is one of the closest spiral galaxies of the southern sky, now believed to lie about 11 million light-years away in the constellation Cetus (The Whale). It’s part of the Sculptor Group, a collection of galaxies associated with the Sculptor Galaxy (NGC 253, shown in previous releases here and here). This is the nearest group of galaxies to our Local Group, which includes the Milky Way.

To measure the distance from the Earth to a nearby galaxy, astronomers have to rely on a type of variable star called a Cepheid to act as a distance marker. Cepheids are very luminous stars, whose brightness varies at regular intervals. The time taken for the star to brighten and fade can be plugged into a simple mathematical relation that gives its intrinsic brightness. When compared with the measured brightness this gives the distance. However, this method isn’t foolproof, as astronomers think this period–luminosity relationship depends on the composition of the Cepheid.

Another problem arises from the fact that some of the light from a Cepheid may be absorbed by dust en route to Earth, making it appear fainter, and therefore further away than it really is. This is a particular problem for NGC 247 with its highly inclined orientation, as the line of sight to the Cepheids passes through the galaxy’s dusty disc.

However, a team of astronomers is currently looking into the factors that influence these celestial distance markers in a study called the Araucaria Project. The team has already reported that NGC 247 is more than a million light-years closer to the Milky Way than was previously thought, bringing its distance down to just over 11 million light-years.

More information about the lead image: It was created from a large number of monochrome exposures taken through blue, yellow/green and red filters taken over many years. In addition, exposures through a filter that isolates the glow from hydrogen gas have also been included and colored red. The total exposure time per filter was 20 hours, 19 hours, 25 minutes and 35 minutes, respectively.

Source: ESO press release. The paper appears here. See also the website for the Araucaria Project.

Posted on by Anne Minard

Halt, Black Hole! Gemini Captures Explosions That Deprive Black Holes of Mass

Artist’s rendering of the environment around the supermassive black hole at the center of Mrk 231. The broad outflow seen in the Gemini data is shown as the fan-shaped wedge at the top of the accretion disk around the black hole, in side view. A similar outflow is probably present under the disk as well. The total amount of material entrained in the broad flow is at least 400 times the mass of the sun per year. Credit: Gemini Observatory/AURA, artwork by Lynette Cook

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Astronomers have long suspected that something must stymie actively growing black holes, because most galaxies in the local universe don’t have them. Now, the Gemini Observatory has captured a galactic check-and-balance — a large-scale quasar outflow in the galaxy Markarian 231 that appears to be depriving a supermassive black hole its diet of gas and dust.

The work is a collaboration between David Rupke of Rhodes College in Tennessee and the University of Maryland’s Sylvain Veilleux. The results are to be published in the March 10 issue of The Astrophysical Journal Letters.

Markarian 231 (12h56’14.23″ +56d52’25.24″) is located about 600 million light-years away in the direction of the constellation of Ursa Major. Although its mass is uncertain, some estimates indicate that Mrk 231 has a mass in stars about three times that of the Milky Way, and its central black hole is estimated to have a mass of at least 10 million solar masses or also about three times that of the supermassive black hole in the Milky Way.

Theoretical modeling specifically points to quasar outflows as the counterbalance to black hole growth. In this negative feedback loop, while the black hole is actively acquiring mass as a quasar, the outflows carry away energy and material, suppressing further growth. Small-scale outflows had been observed before, but none sufficiently powerful to account for this predicted and fundamental aspect of galaxy evolution. The Gemini observations provide the first clear evidence for outflows powerful enough to support the process necessary to starve the galactic black hole and quench star formation by limiting the availability of new material.

This extraction from the data cube shows the large-scale, fast outflow of neutral sodium at the center of the quasar Markarian 231. We are looking down onto the material that moves toward us relative to the galaxy, so the measured velocities are negative. The large black circle marks the location of the black hole, and red lines show the location of a radio jet. In addition to the quasar outflow, the jet pushes the material at the top right, resulting in even greater speeds. Part of the starburst is located at the position of the box at the lower left, and it is likely responsible for the gas motion in this region.

Study author Veilleux says Mrk 231 is an ideal laboratory for studying outflows caused by feedback from supermassive black holes: “This object is arguably the closest and best example that we know of a big galaxy in the final stages of a violent merger and in the process of shedding its cocoon and revealing a very energetic central quasar. This is really a last gasp of this galaxy; the black hole is belching its next meals into oblivion!” As extreme as Mrk 231’s eating habits appear, Veilleux adds that they are probably not unique: “When we look deep into space and back in time, quasars like this one are seen in large numbers, and all of them may have gone through shedding events like the one we are witnessing in Mrk 231.”

Although Mrk 231 is extremely well studied, and known for its collimated jets, the Gemini observations exposed a broad outflow extending in all directions for at least 8,000 light-years around the galaxy’s core. The resulting data reveal gas (characterized by sodium, which absorbs yellow light) streaming away from the galaxy center at speeds of over 1,000 kilometers per second. At this speed, the gas could go from New York to Los Angeles in about 4 seconds. This outflow is removing gas from the nucleus at a prodigious rate — more than 2.5 times the star formation rate. The speeds observed eliminate stars as the possible “engine” fueling the outflow. This leaves the black hole itself as the most likely culprit, and it can easily account for the tremendous energy required.

The energy involved is sufficient to sweep away matter from the galaxy. However, “when we say the galaxy is being blown apart, we are only referring to the gas and dust in the galaxy,” notes Rupke. “The galaxy is mostly stars at this stage in its life, and the outflow has no effect on them. The crucial thing is that the fireworks of new star formation and black hole feeding are coming to an end, most likely as a result of this outflow.”

Source: Gemini press release. The paper appears here. See also some galactic merger animations, courtesy of the Harvard-Smithsonian Center for Astrophysics.

Posted on by Anne Minard

Continent-Wide Telescope Array Now Seeing 450 Million Light-Years Into Space

Artist's conception of Milky Way, showing locations of star-forming regions whose distances were recently measured. CREDIT: M. Reid, Harvard-Smithsonian CfA; R. Hurt, SSC/JPL/Caltech, NRAO/AUI/NSF

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Kitt Peak. Los Alamos. St. Croix. Pie Town.

What do these places have in common? They each house one of 10 giant telescopes in the Very Large Baseline Array, a continent-spanning collection of telescopes that’s flexing its optical muscles, reaching farther into space — with more precision — than any other telescope in the world.

And today, at the 177th annual meeting of the American Association for the Advancement of Science in Washington, DC, VLBA researchers announced an amazing feat: They’ve used the VLBA to peer, with stunning accuracy, three times as far into the universe as they had just two years ago. New measurements with the VLBA have placed a galaxy called NGC 6264 (coordinates below) at a distance of 450 million light-years from Earth, with an uncertainty of no more than 9 percent. This is the farthest distance ever directly measured, surpassing a measurement of 160 million light-years to another galaxy in 2009.

VLBA telescope locations, courtesy of NRAO/AUI

Previously, distances beyond our own Galaxy have been estimated through indirect methods. But the direct seeing power of the VLBA scraps the need for assumptions, noted James Braatz, of the National Radio Astronomy Observatory.

The VLBA provides the greatest ability to see fine detail, called resolving power, of any telescope in the world. It can produce images hundreds of times more detailed than those from the Hubble Space Telescope, at a power equivalent to sitting in New York and reading a newspaper in Los Angeles. VLBA sites include Kitt Peak, Arizona; Los Alamos and Pie Town, New Mexico; St. Croix in the Virgin Islands, Mauna Kea, Hawaii; Brewster, Washington; Fort Davis, Texas; Hancock, New Hampshire; North Liberty, Iowa; and Owens Valley in California. Sure, I could include pictures of the scopes in Hawaii or the Virgin Islands. But Pie Town, besides hosting the Very Large Array, also has two fun restaurants (the Daily Pie and the Pie-O-Neer) with really amazing pie. And an annual pie-eating festival. So it wins:

The VLBA site at Pie Town, N.M., courtesy of NRAO/AUI.

Tripling the visible “yardstick” into space bears favorably on numerous areas of astrophysics, including determining the nature of dark energy, which constitutes 70 percent of the Universe. The VLBA is also redrawing the map of the Milky Way and is poised to yield tantalizing new information about extrasolar planets, the NRAO points out.

Fine-tuning the measurement of ever-greater distances is vital to determining the expansion rate of the Universe, which helps theorists narrow down possible explanations for the nature of dark energy. Different models of Dark Energy predict different values for the expansion rate, known as the Hubble Constant.

“Solving the Dark Energy problem requires advancing the precision of cosmic distance measurements, and we are working to refine our observations and extend our methods to more galaxies,” Braatz said. Measuring more-distant galaxies is vital, because the farther a galaxy is, the more of its motion is due to the expansion of the Universe rather than to random motions.

As for the map of our own galaxy, the direct VLBA measurements are improving on earlier estimates by as much as a factor of two. The clearer observations have already revealed the Milky Way has four spiral arms, not two as previously thought.

Mark Reid, of the Harvard-Smithsonian Center for Astrophysics led an earlier VLBA study revealing that the Milky Way is also rotating faster than previously believed — and that it’s as massive as Andromeda.

Reid’s team is now observing the Andromeda Galaxy in a long-term project to determine the direction and speed of its movement through space. “The standard prediction is that the Milky Way and Andromeda will collide in a few billion years. By measuring Andromeda’s actual motion, we can determine with much greater accuracy if and when that will happen,” Reid said.

The VLBA is also being used for a long-term, sensitive search of 30 stars to find the subtle gravitational tug that will reveal orbiting planets. That four-year program, started in 2007, is nearing its completion. The project uses the VLBA along with NRAO’s Green Bank Telescope in West Virginia, the largest fully-steerable dish antenna in the world. Early results have ruled out any companions the size of brown dwarfs for three of the stars, and the astronomers are analyzing their data as the observations continue.

Ongoing upgrades in electronics and computing have enhanced the VLBA’s capabilities. With improvements now nearing completion, the VLBA will be as much as 5,000 times more powerful as a scientific tool than the original VLBA of 1993.

NGC 6264 Coordinates, from DOCdb: 16<sup>h</sup> 57<sup>m</sup> 16.08<sup>s</sup>; +27° 50′ 58.9″

Source: A press release from the National Radio Astronomy Observatory, via the American Astronomical Society (AAS). Not to be confused with the American Association for the Advancement of Science (AAAS), which now conducting its annual meeting in Washington, DC — and where the VLBA results were presented.

Posted on by Anne Minard

Hubble Zeroes in on Hot, Young Stars

Flocculent Spiral NGC 2841, in the constellation Ursa Major. Credit: NASA, ESA and Hubble

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The Flocculent Spiral NGC 2841, shown above, is known for its profusion of young, blue stars. And yet, until recently, astronomers haven’t been able to use those stars as windows into the still-mysterious phenomenon of star formation.

Hubble’s most recent wide-field camera upgrade is changing that.

The new Wide Field Camera 3 (WFC3) was installed on Hubble in May 2009 during Servicing Mission 4, and replaces the Wide Field and Planetary Camera 2. The new camera is optimized to observe in the infrared and ultraviolet wavelengths emitted by newborn stars, shown by the bright blue clumps in the lead image. Thus, it can peer behind the veil of dust that would otherwise hide those stars from view.

The image shows a lot of hot, young stars in the disc of NGC 2841, but in reality there are just a few sites of current star formation where hydrogen gas is collapsing into new stars. It is likely that these fiery youngsters destroyed the star-forming regions in which they were formed.

Image from NASA's Galaxy Evolution Explorer (GALEX), via the NASA/IPAC Extragalactic Database

NGC 2841 is about 46 million light years away in the constellation Ursa Major. It’s part of a common group of galaxies called flocculent spirals; flocculent means fluffy or wooly-looking. Rather than boasting well-defined spiral arms, these galaxies display patchy stellar distribution.

Star formation is one of the most important processes shaping the Universe; it plays a pivotal role in the evolution of galaxies and it is also in the earliest stages of star formation that planetary systems first appear. Yet there is still much that astronomers don’t understand, such as how the properties of stellar nurseries vary according to the composition and density of the gas present, and what triggers star formation in the first place. The driving force behind star formation is particularly unclear for flocculent spirals.

An international team of astronomers is using Hubble’s WFC3 to study a sample of nearby, but wildly differing, locations where stars are forming. The observational targets include both star clusters and galaxies, and star formation rates range from the baby-booming starburst galaxy Messier 82 to the much more sedate star producer NGC 2841.

Source: Eurekalert. See also this NASA description and image of flocculent spiral NGC 4414.

Detailed credit information for the lead image: NASA, ESA and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration Acknowledgment: M. Crockett and S. Kaviraj (Oxford University, UK), R. O’Connell (University of Virginia), B. Whitmore (STScI) and the WFC3 Scientific Oversight Committee.

Posted on by Anne Minard

Galaxy Size Matters … And This is Not a Rorschach Test

False color image of the Lockman-hole area of the sky at infrared wavelengths as imaged by the Herschel Space Observatory. Credit: ESA/SPIRE Consortium/HerMES Consortium

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When it comes to forming stars, the size of a galaxy does matter, according to research out today in the online version of Nature.

But it doesn’t have to be as massive as we once thought.

Alexandre Amblard, an astrophysicist at the University of California, Irvine, and his colleagues used new data from the Herschel Space Observatory to peer into Lockman Hole area of the sky, where extragalactic light comes from star-forming galaxies out of reach for even the world’s most powerful telescopes.

The Lockman Hole is a patch of the sky, 15 square degrees, lying roughly between the pointer stars of the Big Dipper.

Called submillimetre galaxies, the study subjects emit light at wavelengths between the radio and infrared parts of the spectrum, so studying them requires novel approaches borrowing from both radio and optical astronomy. The galaxies by themselves are too blurry to be resolved with individual far-infrared telescopes – but their average properties can be observed and analyzed, which is exactly what Amblard and his colleagues did.

The authors measured variations in the intensity of extragalactic light at far-infrared wavelengths, and derived statistics for the level of clustering of light halos. They assume that the clustering reflects the underlying distribution of dark matter, and fit the data to a halo model of galaxy formation, which connects the spatial distribution of galaxies in the Universe to that of dark matter.

Distribution of dark matter when the Universe was about 3 billion years old, obtained from a numerical simulation of galaxy formation. The left panel displays the continuous distribution of dark matter particles, showing the typical wispy structure of the cosmic web, with a network of sheets and filaments, while the right panel highlights the dark matter halos representing the most efficient cosmic sites for the formation of star-bursting galaxies with a minimum dark matter halo mass of 300 billion times that of the Sun. Credit: VIRGO Consortium/Alexandre Amblard/ESA

Amblard and his colleagues discovered an enormous fact: the ‘haloes’ of dark matter that surround the Universe’s most active star-forming galaxies are each more massive than about 300 billion solar masses.

What’s even more interesting is that the new threshold for star formation is actually smaller than some previous estimates.

“I think there was one prediction that put the number around 5000 billion times that of the sun, but that was just a prediction from a theory of galaxy formation.“ said Asantha Cooray, also an astrophysicist at UC Irvine and second author on the new paper. “The general consensus was that it may be between 100 to 1000 billion times the sun. We now have a more precise answer from this work.”

Cooray said he’s most excited “that we can look at a detailed image of the sky showing distant, star-forming galaxies and infer not only details about the stars and gas in those galaxies but also about the amount of dark matter needed to form such galaxies. Beyond inferring the presence, we still don’t know exactly what dark matter is.”

The results appear online ahead of print today on Nature’s website.

Posted on by Nancy Atkinson

Thick Stellar Disk Isolated in Andromeda

Schematic representation of a thick disc structure. The thick disc is formed of stars that are typically much older than those in the thin disc, making it an ideal probe of galactic evolution (Credit: Amanda Smith, IoA graphics officer)

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From the Institute of Astronomy at Cambridge University press release:

A team of astronomers from the UK, the US and Europe have identified a thick stellar disc in the nearby Andromeda galaxy for the first time. The discovery and properties of the thick disc will constrain the dominant physical processes involved in the formation and evolution of large spiral galaxies like our own Milky Way.

By analyzing precise measurements of the velocities of individual bright stars within the Andromeda galaxy using the Keck telescope in Hawaii, the team have managed to separate out stars tracing out a thick disc from those comprising the thin disc, and assess how they differ in height, width and chemistry.

Optical image of The Andromeda galaxy (M31) (credit Robert Gendler)

Spiral structure dominates the morphology of large galaxies at the present time, with roughly 70% of all stars contained in a flat stellar disc. The disc structure contains the spiral arms traced by regions of active star formation, and surrounds a central bulge of old stars at the core of the galaxy. “From observations of our own Milky Way and other nearby spirals, we know that these galaxies typically possess two stellar discs, both a ‘thin’ and a ‘thick’ disc,” explains the leader of the study, Michelle Collins, a PhD student at Cambridge’s Institute of Astronomy. The thick disc consists of older stars whose orbits take them along a path that extends both above and below the more regular thin disc. “The classical thin stellar discs that we typically see in Hubble imaging result from the accretion of gas towards the end of a galaxy’s formation, whereas thick discs are produced in a much earlier phase of the galaxy’s life, making them ideal tracers of the processes involved in galactic evolution.”

Currently, the formation process of the thick disc is not well understood. Previously, the best hope for comprehending this structure was by studying the thick disc of our own Galaxy, but much of this is obscured from our view. The discovery of a similar thick disk in Andromeda presents a much cleaner view of spiral structure. Andromeda is our nearest large spiral neighbor — close enough to be visible to the unaided eye — and can be seen in its entirety from the Milky Way. Astronomers will be able to determine the properties of the disk across the full extent of the galaxy and look for signatures of the events connected to its formation. It requires a huge amount of energy to stir up a galaxy’s stars to form a thick disc component, and theoretical models proposed include accretion of smaller satellite galaxies, or more subtle and continuous heating of stars within the galaxy by spiral arms.

Ages and orientations of the stellar components of disc galaxies. The halo (or spheroid) contains the oldest populations, followed by the thick stellar disc. The thin disc typically contains the youngest generations of stars. (Credit: RAVE collaboration)

“Our initial study of this component already suggests that it is likely older than the thin disc, with a different chemical composition” commented UCLA Astronomer, Mike Rich. “Future more detailed observations should enable us to unravel the formation of the disc system in Andromeda, with the potential to apply this understanding to the formation of spiral galaxies throughout the Universe.”

“This result is one of the most exciting to emerge from the larger parent survey of the motions and chemistry of stars in the outskirts of Andromeda,” said fellow team member, Dr. Scott Chapman, also at the Institute of Astronomy. “Finding this thick disc has afforded us a unique and spectacular view of the formation of the Andromeda system, and will undoubtedly assist in our understanding of this complex process.”

This study was published in Monthly Notices of the Royal Astronomical Society by Michelle Collins, Scott Chapman and Mike Irwin from the Institute of Astronomy, together with Rodrigo Ibata from L’Observatoire de Strasbourg, Mike Rich from University of California, Los Angeles, Annette Ferguson from the Institute for Astronomy in Edinburgh, Geraint Lewis from the University of Sydney, and Nial Tanvir and Andreas Koch from the University of Leicester.

This study is published in Monthly Notices of the Royal Astronomical Society:
* http://arxiv.org/abs/1010.5276
* http://www.ast.cam.ac.uk/~mlmc2/M31thickdisc.html