A Herschel PACS (Photodetector Array Camera and Spectrometer) image of the center of the Milky Way. The dark line of cool gas is thought to be an elliptical ring surrounding the galactic center. The galaxy’s central supermassive black hole Sagittarius A* (Sgr A*) is labelled. The differential velocity of clouds in the ring may result from interaction with Sgr A*. Credit: ESA/Herschel/NASA/Molinari et al.
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The Herschel Space Observatory scanned the center of the galaxy in far-infrared and found a cool (in all senses of the word) twisting ring of rapidly orbiting gas clouds. The ring is estimated to have dimensions of 100 parsecs by 60 parsecs (or 326 by 196 light years) – with a composite mass of 30 million solar masses.
The ring is proposed to oscillate twice about the galactic mid-plane for each orbit it makes of the galactic center – giving it the apparent shape of an infinite symbol when viewed from the side.
The research team speculate that the ring may be conforming to the shape of a standing wave – perhaps caused by the spin of the central galactic bulge and the lateral movement of gas across the galaxy’s large central bar. The researchers suggest that the combination of these forces may produce some kind of gravitational ‘sloshing’ effect, which would account for the unusual movement of the ring.
The estimated shape of the 100 by 60 parsec ring. Note the oscillating shape from a lateral perspective – and from above, note the ring encircles the supermassive black hole Sagittarius A*, but the black hole is not at its center. Credit: Molinari et al.
Although the ring is estimated to have an average orbital velocity of 10 to 20 kilometers a second, an area of dense cloud coming in close to the galaxy’s central supermassive black hole, Sagittarius A*, was clocked at 50 kilometers a second – perhaps due to its close proximity to Sagittarius A*.
However, the researchers also estimate that Sagittarius A* is well off-centre of the gas ring. Thus, the movement of the ring is dominated by the dynamics of the galactic bulge – rather than Sagittarius A*, which would only exert a significant gravitational influence within a few parsecs of itself.
Astronomers using the Hubble Space Telescope to peer deep into the central bulge of our galaxy have found a population of rare and unusual stars. Dubbed “blue stragglers”, these stars seem to defy the aging process, appearing to be much younger than they should be considering where they are located. Previously known to exist within ancient globular clusters, blue stragglers have never been seen inside our galaxy’s core – until now.
The stars were discovered following a seven-day survey in 2006 called SWEEPS – the Sagittarius Window Eclipsing Extrasolar Planet Search – that used Hubble to search a section of the central portion of our Milky Way galaxy, looking for the presence of Jupiter-sized planets transiting their host stars. During the search, which examined 180,000 stars, Hubble spotted 42 blue stragglers.
Of the 42 it’s estimated that 18 to 37 of them are genuine.
What makes blue stragglers such an unusual find? For one thing, stars in the galactic hub should appear much older and cooler… aging Sun-like stars and old red dwarfs. Scientists believe that the central bulge of the Milky Way stopped making new stars billions of years ago. So what’s with these hot, blue, youthful-looking “oddballs”? The answer may lie in their formation.
Artist's concept of a blue straggler pair. NASA, ESA, and G. Bacon (STScI)
A blue straggler may start out as a smaller member of a binary pair of stars. Over time the larger star ages and gets even bigger, feeding material onto the smaller one. This fuels fusion in the smaller star which then grows hotter, making it shine brighter and bluer – thus appearing similar to a young star.
However they were formed, just finding the blue stragglers was no simple task. The stars’ orbits around the galactic core had to be determined through a confusing mix of foreground stars within a very small observation area. The region of the sky Hubble studied was no larger than the width of a fingernail held at arm’s length! Still, within that small area Hubble could see over 250,000 stars. Incredible.
“Only the superb image quality and stability of Hubble allowed us to make this measurement in such a crowded field.”
– Lead author Will Clarkson, Indiana University in Bloomington and the University of California in Los Angeles
The discovery of these rare stars will help astronomers better understand star formation in the Milky Way’s hub and thus the evolution of our galaxy as a whole.
Some sixteen decades ago, Lord Rosse was the first to point out spiral structure in distant “nebula”… and today astrophysicists Thomas Dame and Patrick Thaddeus are discovering it closer to home. Our Milky Way Galaxy was believed to only have six spiral arms, but their research has revealed an outer extension of the Scutum-Centaurus arm from the inner galaxy.
“We have identified a spiral arm lying beyond the Outer Arm in the first Galactic quadrant ~15 kpc from the Galactic center.” says Dame and Thaddeus. “One of the detections was fully mapped to reveal a large molecular cloud with a radius of 47 pc and a molecular mass of ~50,000 M. At a mean distance of 21 kpc, the molecular gas in this arm is the most distant yet detected in the Milky Way. The new arm appears to be the continuation of the Scutum–Centaurus Arm in the outer Galaxy, as a symmetric counterpart of the nearby Perseus Arm.”
Over the last 50 years, many models of our galaxy have been proposed – revealing a pleasing, duo-symmetry. However, finding evidence to prove these theories has been a bit more elusive. Since we cannot observe ourselves, seeing spiral structure on the far side of the galaxy is problematic – hidden by near-side emission at the same velocity. But these researchers didn’t stop. The new arm was found as a result of attempts to follow the Sct–Cen Arm past its tangent.
“The new arm was largely overlooked in existing 21 cm surveys probably because it lies mainly out of the Galactic plane, its Galactic latitude steadily increasing with longitude as it follows the warp in the distant outer Galaxy.” says Dame. “In the first quadrant the only prominent HI spiral feature in the outer Galaxy is the well-known Outer Arm, a feature also well traced by CO. However, at 3 degrees above the plane one sees instead the new arm as a prominent linear feature running roughly parallel to the locus of the Outer Arm but shifted to more negative velocities.”
Is our smoothly constructed galaxy indeed a mirror image of itself? This new evidence suggests the Scutum-Centaurus arm embraces the entire Milky Way – forming a symmetrical, star-forming counterpart to the galaxy’s other arm, Perseus. “Confirmation of the present feature as the ”Outer Sct-Cen Arm” will require a great deal of new data from several telescopes and much observing time over an extended period.” says Thaddeus. “Key steps toward confirming the proposal include, as mentioned, tracking Sct–Cen in the fourth quadrant and, even harder, tracking the Perseus Arm from the point where it passes inside the solar circle near longitude 50 degrees to its putative origin at the far end of the bar.”
Mapping the findings of galactic data on atomic hydrogen gas isn’t going to happen overnight… and even more discoveries and clarifications could be revealed in the future. “The Galactic symmetry suggested by the present work and clearly demonstrated by the identification of the Far 3-kpc Arm a few years ago, coupled with evidence for a global two-armed spiral pattern in the old stars, and, indeed, with the discovery of the bar itself, all hint at Galactic spiral structure that is both simpler and more amenable to study than had long been assumed. As emphasized here, much work remains, but aided by greatly improved distances from forthcoming astrometric surveys, a reasonably complete picture of our Galaxy’s spiral pattern may be achieved over the next decade.”
Spiral galaxy arms may carry stars along with them, suggests new study
There’s a new concept in the works regarding the evolution of galactic arms and how they move across the structure of spiral galaxies. Robert Grand, a postgraduate student at University College London’s Mullard Space Science Laboratory, used new computer modeling to suggest that these signature features of spiral galaxies – including our own Milky Way – evolve in different ways than previously thought.
The currently accepted theory is as spiral galaxies rotate, the “arms” are actually transient structures that move across the flattened disc of stars surrounding the galactic bulge, yet don’t directly affect the movement of the individual stars themselves. This would work in much the same way as a “wave” goes across a crowd at a stadium event. The wave moves, but the individual people do not move along with it – rather, they stay seated after it has passed.
However when Grand researched this suggested motion using computer models of galaxies, he and his colleagues found that this was not what tended to happen. Instead the stars actually moved along with the arms, rather than maintaining their positions.
Also it was observed in these models that the arms themselves are not permanent features, but rather break up and reform over the course of 80 to 100 million years. Grand suggests that this may be due to the powerful gravitational shear forces generated by the spinning of the galaxy.
“We simulated the evolution of spiral arms for a galaxy with five million stars over a period of 6 billion years. We found that stars are able to migrate much more efficiently than anyone previously thought. The stars are trapped and move along the arm by their gravitational influence, but we think that eventually the arm breaks up due to the shear forces.”
– Robert Grand
Snapshots of face-on view of a simulated disc galaxy.
The computer models also showed that the stars along the leading edge of the arms tended to move inwards toward the galactic center while the stars lining the trailing ends were carried to the outer edge of the galaxy.
Since it takes hundreds of millions of years for a spiral galaxy to complete even just one single rotation, observing their evolution and morphology is impossible to do in real time. Researchers like Grand and his simulations are key to our eventual understanding of how these islands of stars formed and continue to shape themselves into the vast, varied structures we see today.
“This research has many potential implications for future observational astronomy, like the European Space Agency’s next corner stone mission, Gaia, which MSSL is also heavily involved in. As well as helping us understand the evolution of our own galaxy, it may have applications for regions of star formation.”
– Robert Grand
The results were presented at the Royal Astronomical Society’s National Astronomy Meeting in Wales on April 20. Read the press release on the Royal Astronomical Society’s website here.
Top image: M81, a spiral galaxy similar to our own Milky Way, is one of the brightest galaxies that can be seen from Earth. The spiral arms wind all the way down into the nucleus and are made up of young, bluish, hot stars formed in the past few million years, while the central bulge contains older, redder stars. Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA)
The Milky Way as seen near the Very Large Telescope in the Atacama Desert. Credit: ESO/Y. Beletsky
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The Chilean Atacama Desert boasts some of the darkest skies on Earth – which is why it is home to several telescopes, including the Very Large Telescope. This beautiful panoramic image was taken there, showing the VLT’s Unit Telescope 1, and across on the other side of the image are the Large and Small Magellanic Clouds glowing brightly. Like an arch in between is plane of our Milky Way galaxy. This awe-inspiring image was taken by ESO Photo Ambassador Yuri Beletsky. These photographers specialize in taking images of not only the night sky, but also the large telescopes that give us eyes to see across the great distances of our Universe.
If you are new to astronomy, you may ask “what is a Messier Marathon and how do I do one?”
Basically a Messier Marathon is an all night (Dusk til Dawn) observing session held around mid March/ early April every year, where an observer attempts to see all, or as many of the 110 Messier objects as listed by Charles Messier.
The Messier list includes: Nebulae, Galaxies, Star clusters, Supernovae and many other deep sky objects. All of the objects in the Messier list are observable with small amateur telescopes and many of the objects are observable with binoculars.
The reason why Messier marathons take place from mid March to early April is because this is when all of the objects are visible in one evening. Other times of the year aren’t suitable as some of the objects will be in daylight or below the horizon etc.
You don’t have to be an astronomy ace or a seasoned astronomer to do a Messier marathon, but you will need a good telescope to see all of the objects. You don’t even need to do a full Messier marathon as many people do half marathons and depending on your location, or when you observe, you may not be able to see all 110 objects as there is a very tight window of opportunity and higher latitude observers do lose a couple of objects below the horizon.
Timing is key to enable you to see as many of 110 messier objects as possible. Many astronomers put tables and even star charts on the internet to help observers see as many objects as possible.
Observing starts at dusk and ends after dawn and on average each object gets about 5 minutes of observing time before you have to move onto the next one. There can be a short respite half way through the observing session for food and rest, but this depends on the order and success of the objects you are viewing?
Before starting your night of viewing Charles Messier’s wonders, make sure you have all your equipment ready, are dressed warm as it will get cold, have all your charts and viewing tables ready. It also helps to have a hot drink and something nice to eat.
The best dates this year for doing a Messier Marathon have passed and the sky was drenched with the glow of the full moon, but we still have early April. Good luck.
Charles Messier (26 June 1730 – 12 April 1817) was a French astronomer most notable for publishing an astronomical catalogue consisting of deep sky objects such as nebulae and star clusters that came to be known as the 110 “Messier objects”. The purpose of the catalogue was to help astronomical observers, in particular comet hunters such as himself, distinguish between permanent and transient objects in the sky.
The original Hubble Ultra-Deep Field (Credit NASA, ESA, and S. Beckwith (STScI) and the HUDF Team).
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We live on a planet which orbits a star, and along with a hundred billion other stars, our Sun orbits the centre of our Milky Way galaxy. It doesn’t just stop there; our galaxy is one of hundreds of billions of galaxies in our Universe that gravitationally clump together in groups or clusters.
Throughout Spring in the northern hemisphere, astronomers and people interested in the night sky are going to be in for a galactic treat, as this is the time of year we can see the Coma/Virgo Super cluster or “Realm of Galaxies”.
Galaxies are massive islands of stars, gas and dust in the Universe; they are where stars and planets are born and eventually die. Galaxies are cosmic factories of creation — where it all happens on a very grand scale. To give you an idea of size, it would take you roughly 100,000 years to travel across the disc of the Milky Way at the speed of light!
Andromeda Galaxy.
The Milky Way is the second largest member of our local group of galaxies with Andromeda being the largest. Other members of our local group include the Triangulum galaxy and large and small Magellanic Clouds.
Virgo Galaxy Cluster - NOAO/AURA/NSF
The Coma/ Virgo Super cluster dominates our intergalactic neighbourhood; it represents the physical centre of our Local Super cluster and influences all the galaxies and galaxy groups by the gravitational attraction of its enormous mass.
Unfortunately galaxies are almost impossible to see with the naked eye, so you will need powerful binoculars or a large telescope, such as a Dobsonian to see most of the brighter galaxies in this region.
The cluster contains approximately 2,000 elliptical and spiral galaxies of which approximately 20 or more are observable using amateur equipment. This includes 16 Messier objects such as the Black eye spiral Galaxy M64, and elliptical galaxies, M86 with its plume, massive M87 at its centre and beautiful spiral M88, to name just a few.
From Left to Right M64, M86 and M88 (Credit NASA)
To find the approximate location of the Realm of Galaxies, first find the constellation of Leo – the lion — easily found in the South East this time of year with the backwards question mark overhis head. Go past Leo’s rear end and you will be in the bowl asterism of Virgo, to the bottom left of Leo and the faint constellation of Coma Berenices (Berenices hair) top left of Leo. This is the Realm of Galaxies!
Star Chart to help you find the Realm of Galaxies (Credit Adrian West)
Download a map of this region or use a star atlas to find your way around this area and try and spot as many galactic delights (faint fuzzies) as you can. As a bonus, the ringed Planet Saturn is just below this area too at the moment!
Give yourself plenty of time, wrap up warm and just think, you are looking for the largest structures in the Universe, hundreds of millions of light years away from Earth.
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″
MESSENGER's new solar system portrait, from the inside out
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Say cheese! The MESSENGER spacecraft has captured the first portrait of our Solar System from the inside looking out. The images, captured Nov. 3 and 16, 2010, were snapped with the Wide Angle Camera (WAC) and Narrow Angle Camera (NAC) of MESSENGER’s Mercury Dual Imaging System (MDIS).
All of the planets are visible except for Uranus and Neptune, which at distances of 3.0 and 4.4 billion kilometers were too faint to detect with even the longest camera exposure time of 10 seconds. Their positions are indicated. The dwarf-planet Pluto, smaller and farther away, would have been even more difficult to observe.
Earth’s Moon and Jupiter’s Galilean satellites (Callisto, Ganymede, Europa, and Io) can be seen in the NAC image insets. Our Solar System’s perch on a spiral arm provided a beautiful view of part of the Milky Way galaxy, bottom center.
The following is a graphic showing the positions of the planets when the graphic was acquired:
The new mosaic provides a complement to the Solar System portrait – that one from the outside looking in – taken by Voyager 1 in 1990.
These six narrow-angle color images were made from the first ever 'portrait' of the solar system taken by Voyager 1, which was more than 4 billion miles from Earth and about 32 degrees above the ecliptic. The spacecraft acquired a total of 60 frames for a mosaic of the solar system which shows six of the planets. Mercury is too close to the sun to be seen. Mars was not detectable by the Voyager cameras due to scattered sunlight in the optics, and Pluto was not included in the mosaic because of its small size and distance from the sun. These blown-up images, left to right and top to bottom are Venus, Earth, Jupiter, and Saturn, Uranus, Neptune. The background features in the images are artifacts resulting from the magnification. The images were taken through three color filters -- violet, blue and green -- and recombined to produce the color images. Jupiter and Saturn were resolved by the camera but Uranus and Neptune appear larger than they really are because of image smear due to spacecraft motion during the long (15 second) exposure times. Earth appears to be in a band of light because it coincidentally lies right in the center of the scattered light rays resulting from taking the image so close to the sun. Earth was a crescent only 0.12 pixels in size. Venus was 0.11 pixel in diameter. The planetary images were taken with the narrow-angle camera (1500 mm focal length). Credit: NASA/JPL
“Obtaining this portrait was a terrific feat by the MESSENGER team,” says Sean Solomon, MESSENGER principal investigator and a researcher at the Carnegie Institution. “This snapshot of our neighborhood also reminds us that Earth is a member of a planetary family that was formed by common processes four and a half billion years ago. Our spacecraft is soon to orbit the innermost member of the family, one that holds many new answers to how Earth-like planets are assembled and evolve.”
In 2010 Nidever et al found the Magellanic Stream was much longer than previously realised - maybe 2.5 billion years old. The stream trails behind the Small and Large Magellanic Clouds - visible below the Milky Ways galactic disk, to the right. Ahead of the Clouds is another structure called the Leading Arm. This is a false colour image - the Stream, Arm and Magellanic Bridge between the two Clouds are only visible in radio light.
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Most people agree that the Magellanic Clouds are in orbit around the Milky Way. What’s not clear is whether it is a bound orbit or just a temporary ‘ships passing in the night’ arrangement. Something which could clarify the relationship is the Magellanic Stream, a 600,000 light year long string of gas dragged through and beyond the Small and Large Magellanic Clouds.
For the complete picture, note that there is also a shorter trail of gas drawn out ahead of the Clouds, known as the Leading Arm – and the gas flow between the Clouds is known as the Magellanic Bridge. The Bridge is an indication that the Clouds are gravitationally bound in a binary pair – at least for now. The Large Magellanic Cloud may dragging the Small Magellanic Cloud behind it, since the Magellanic Stream ‘skid mark’ is most chemically similar to the contents of the Small Magellanic Cloud.
What remains unresolved is whether the Clouds are in a bound orbit around the Milky Way – or are they just passing by? The level of uncertainty about the dynamics of objects that are relatively close to us, and are easily visible to the naked eye, may seem surprising.
Firstly, it is tricky to gain an accurate estimation of each Cloud’s velocity relative to the Milky Way – partly because we, the observers, have our own independent movement and we need to find a reference frame that we can reliably measure the Clouds’ velocity against.
Estimates derived from Hubble Space Telescope observations by Kallivayalil and colleagues in 2006, measured the Clouds’ velocities against a background of distant quasars, which are visible through the Clouds. These data were then used by Besla and colleagues to propose that the Clouds’ velocities were too fast to be in bound orbits around the Milky Way and so must be just passing by.
But there is another area of uncertainty, where – even with the Clouds’ velocity determined – you still need to decide what escape velocity they need to avoid being caught in a bound orbit of the Milky Way. While we can estimate the Milky Way’s mass, there is the issue of dark matter – which we can’t see and hence can’t locate accurately – so there is some uncertainty about how the combined mass of the Milky Way’s visible and dark matter is distributed.
If, like the visible matter, the dark matter is centralized around the galactic hub, the Clouds won’t need so much velocity to escape. But if the dark matter is more evenly distributed with the galactic disk of visible matter being surrounded by a spherical halo of dark matter, then it’s less clear as to whether the Cloud’s could escape (a scenario that was acknowledged by Besla et al).
A spherical halo of dark matter is the generally preferred model for the Milky Way’s total mass distribution – since, without it, the outer edges of the Milky Way’s visible disk are rotating so fast that they should fly off into space.
Diaz and Bekki have run with this idea by computer-modeling a Milky Way with a circular velocity of 250 kilometres a second (a recent new estimate), which hence requires a more substantial dark matter halo than was assumed by Besla et al. Otherwise, they still use the same Cloud velocities determined from the 2006 Hubble Space Telescope observations.
Left: The neutral hydrogen Magellanic Stream stretching upwards past the Large (red) and Small (green) Magellanic Clouds Right: A computer-modeled scenario in which both clouds are in bound orbits around the Milky Way. Most of the Stream, Bridge and Leading Arm structures are replicated - and are found to originate from the substance of the Small Magellanic Cloud. Credit: Diaz and Bekki.
Their model, when wound back in time, suggests the Clouds have been locked in bound orbits around the Milky Way for more than 5 billion years – with the Magellanic Stream and Leading Arm arising more recently, following a close encounter between the two Clouds (an idea also proposed in Besla et al’s unbound orbit model).
Diaz and Bekki suggest that the Clouds began separate orbits, but passed close to each other around 1.25 billion years ago and then became the binary pair we observe today. The Leading Arm is freed gas being drawn into the Milky Way’s halo – an indication that both Clouds may eventually be assimilated.
