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  • richardmitnick 8:14 am on October 12, 2020 Permalink | Reply
    Tags: "Milky Way’s shredded companion provides clues about dark matter", , , , , Sagittarius dwarf galaxy,   

    From Science Magazine: “Milky Way’s shredded companion provides clues about dark matter” 

    From Science Magazine

    Oct. 9, 2020
    Adam Mann

    An artist’s illustration shows streams of stars pulled from a companion galaxy circling the Milky Way. Similar streams originating from the Sagittarius dwarf galaxy can help reveal the shape of dark matter in our cosmic vicinity. Credit: NASA/JPL-Caltech/R. Hurt. (SSC/Caltech.)

    The Milky Way hasn’t been kind to the Sagittarius dwarf galaxy. Located some 70,000 light-years away, the bundle of stars has been shredded and stretched into a filamentous stream by the gravity of the Milky Way. Now, scientists have mapped Sagittarius in exquisite detail, and they’ve used that map to provide a long-sought picture of the mysterious dark matter halo in which our Galaxy resides.

    First spotted in 1994, Sagittarius is one of the Milky Way’s closest companions.

    Across the ages, gravitational forces have ripped it apart, scattering stars into a stream that now completely encircles the Milky Way. That makes Sagittarius a sensitive scale for measuring the distribution of mass in our Galaxy, which includes not just the visible disk of stars, but also an unseen halo of dark matter, thought to comprise up to 90% of the total mass.

    In principle, researchers could monitor the orbits of nearby star clusters and galaxies and use the laws of physics to calculate how much matter is tugging on them. But their motion across the sky is too slow to help within human lifetimes. The Sagittarius stream, on the other hand, already embodies those motions. “It’s essentially like an orbit drawn for you on the sky,” says Vasily Belokurov, an astronomer at the University of Cambridge UK.

    For the past quarter-century, astronomers have tried to use maps of Sagittarius to calculate the shape of the Milky Way’s dark matter halo. But identifying the stream from our vantage in the Milky Way’s disk is challenging, and astronomers have come up with halo shapes as varied as eggs and rugby footballs.

    Then along came the European Space Agency’s Gaia satellite.

    ESA (EU)/GAIA satellite .

    Two years ago, the probe began to release its ultraprecise maps of the stars in the Milky Way—and stars in the surrounding streams. With the data, Belokurov and his colleagues could tell that the Sagittarius stream was being yanked indirectly by another gravitational player: that of the galaxy’s largest companion, the Large Magellanic Cloud (LMC), which weighs between one-fifth and one-third as much as the Milky Way itself.

    Large Magellanic Cloud. Adrian Pingstone December 2003

    Rewinding the clock, the researchers modeled the pas de trois over 3 billion years—and found that both the LMC and Sagittarius swooped close to the Milky Way , as recently as 50 million years ago. The LMC’s significant heft pulled our Galaxy, which then induced a force affecting Sagittarius. That helps explain a peculiar sideways tug on the Sagittarius stream, say Belokurov and his colleagues, who report the results in a paper posted to the preprint server arXiv [Tango for three: Sagittarius, LMC, and the Milky Way]. Solving this puzzle made it easier to use the Sagittarius stream as a scale and to infer the shape of the galaxy’s dark matter halo. “It’s the lock you need before you can unlock the main lock,” Belokurov says.

    The team’s results suggest the distribution of dark matter around the Milky Way is complex. Closer to the disk of our Galaxy, where the dark matter is expected to be most dense, the halo takes the shape of a squashed sphere—a bit like a pumpkin, with the pumpkin’s top pointing out of the galactic plane. But farther out, about 65,000 light-years from the galactic center, the shape of the halo changes: The pumpkin tips over on its side, so that its stem is aligned with the disk of the galaxy.

    The twists and turns of this convoluted shape could provide hints as to how the Milky Way’s halo is connected to the local network of dark matter filaments, called the cosmic web, that strings together neighboring large galaxies, Belokurov says.

    Kathryn Johnston, an astronomer at Columbia University who was not involved in the work, agrees. “We’ve never been able to see anything beyond the simplest shape of the dark matter halo,” she says. “This is a hint of large-scale global deformation, and that’s very exciting.”

    Gaining even this limited view of the Milky Way’s dark matter halo is important, Belokurov says, because it’s the closest halo we have access to: It could help researchers understand how light or heavy dark matter particles might be, and improve models that trace the evolution of the cosmic web from the big bang to today.

    See the full article here.


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  • richardmitnick 12:55 pm on September 24, 2018 Permalink | Reply
    Tags: , , , , , , , Sagittarius dwarf galaxy   

    From COSMOS Magazine: “A galactic near-miss set stars on an unexpected path around the Milky Way” 

    Cosmos Magazine bloc

    From COSMOS Magazine

    24 September 2018
    Ben Lewis

    A close pass from the Sagittarius dwarf galaxy sent ripples through the Milky Way that are still visible today.

    Image Credit: R. Ibata (UBC), R. Wyse (JHU), R. Sword (IoA)

    Milky Way NASA/JPL-Caltech /ESO R. Hurt

    Tiny galaxy; big trouble. Gaia imaging shows the Sagittarius galaxy, circled in red. ESA/Gaia/DPAC

    ESA/GAIA satellite

    Between 300 and 900-million years ago the Sagittarius dwarf galaxy made a close pass by the Milky Way, setting millions of stars in motion, like ripples on a pond. The after-effects of that galactic near miss are still visible today, according to newly published findings.

    The unique pattern of stars left over from the event was detected by the European Space Agency’s star mapping mission, Gaia. The details are contained in a paper written by Teresa Antoja and colleagues from the Universitat de Barcelona in Spain, and published in the journal Nature.

    The movements of over six million stars in the Milky Way were tracked by Gaia to reveal that groups of them follow different courses as they orbit the galactic centre.

    In particular, the researchers found a pattern that resembled a snail shell in a graph that plotted star altitudes above or below the plane of the galaxy, measured against their velocity in the same direction. This is not to say that the stars themselves are moving in a spiral, but rather that the roughly circular orbits correlate with up-and-down motion in a pattern that has never been seen before.

    While some perturbations in densities and velocities had been seen previously, it was generally assumed that the movement of the disk’s stars is largely in dynamic equilibrium and symmetry about the galactic plane. Instead, Antoja’s team discovered something had knocked the disk askew.

    “It is a bit like throwing a stone in a pond, which displaces the water as ripples and waves,” she explains.

    Whereas water will eventually settle out after being disturbed, a star’s motion carries signatures from the change in movement. While the ripples in the distribution caused by Sagittarius passing by has evened out, the motion of the stars themselves still carry the pattern.

    “At the beginning the features were very weird to us,” says Antoja. “I was a bit shocked and I thought there could be a problem with the data because the shapes are so clear.”

    The new revelations came about because of a huge increase in quality of the Gaia data, compared to what had been captured previously. The new information provided, for the first time, a measurement of three-dimensional speeds for the stars. This allowed the study of stellar motion using the combination of position and velocity, known as “phase space”.

    “It looks like suddenly you have put the right glasses on and you see all the things that were not possible to see before,” says Antoja.

    Computer models suggest the disturbance occurred between 300 and 900 million years ago – a point in time when it’s known the Sagittarius galaxy came near ours.

    In cosmic terms, that’s not very long ago, which also came as a surprise. It was known that the Milky Way had endured some much earlier collisions – smashing into a dwarf galaxy some 10 billion years ago, for instance – but until now more recent events had not been suspected. The Gaia results have changed that view.

    See the full article here .

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  • richardmitnick 10:57 am on January 11, 2017 Permalink | Reply
    Tags: , , , , , Farthest Stars in Milky Way Might Be Ripped from Another Galaxy, Sagittarius dwarf galaxy   

    From CfA: “Farthest Stars in Milky Way Might Be Ripped from Another Galaxy” 

    Harvard Smithsonian Center for Astrophysics

    Center For Astrophysics

    January 11, 2017
    Christine Pulliam
    Media Relations Manager
    Harvard-Smithsonian Center for Astrophysics

    In this computer-generated image, a red oval marks the disk of our Milky Way galaxy and a red dot shows the location of the Sagittarius dwarf galaxy. The yellow circles represent stars that have been ripped from the Sagittarius dwarf and flung far across space. Five of the 11 farthest known stars in our galaxy were probably stolen this way. Marion Dierickx / CfA

    The 11 farthest known stars in our galaxy are located about 300,000 light-years from Earth, well outside the Milky Way’s spiral disk. New research by Harvard astronomers shows that half of those stars might have been ripped from another galaxy: the Sagittarius dwarf. Moreover, they are members of a lengthy stream of stars extending one million light-years across space, or 10 times the width of our galaxy.

    “The star streams that have been mapped so far are like creeks compared to the giant river of stars we predict will be observed eventually,” says lead author Marion Dierickx of the Harvard-Smithsonian Center for Astrophysics (CfA).

    The Sagittarius dwarf is one of dozens of mini-galaxies that surround the Milky Way.

    Stars from the Sagittarius dwarf galaxy are being spread all around the Milky Way, contributing to the lumpiness of the distribution of stars found in SDSS. In this illustration, the Milky Way is shown in yellow and the sun is a green star. The white dots show the expected positions of the stars from a simulation done by Rensselaer graduate student Ben Willett. The blue dots show the positions of actual stars extracted from the SDSS database by graduate student Nate Cole. The red dashed line shows the general direction all of the stars are going as they orbit around the center of the Milky Way. http://www.rpi.edu/about/inside/issue/v2n19/galaxy.html

    Over the age of the universe it made several loops around our galaxy. On each passage, the Milky Way’s gravitational tides tugged on the smaller galaxy, pulling it apart like taffy.

    Dierickx and her PhD advisor, Harvard theorist Avi Loeb, used computer models to simulate the movements of the Sagittarius dwarf over the past 8 billion years. They varied its initial velocity and angle of approach to the Milky Way to determine what best matched current observations.

    “The starting speed and approach angle have a big effect on the orbit, just like the speed and angle of a missile launch affects its trajectory,” explains Loeb.

    At the beginning of the simulation, the Sagittarius dwarf weighed about 10 billion times the mass of our Sun, or about one percent of the Milky Way’s mass. Dierickx’s calculations showed that over time, the hapless dwarf lost about a third of its stars and a full nine-tenths of its dark matter. This resulted in three distinct streams of stars that reach as far as one million light-years from the Milky Way’s center. They stretch all the way out to the edge of the Milky Way halo and display one of the largest structures observable on the sky.

    Moreover, five of the 11 most distant stars in our galaxy have positions and velocities that match what you would expect of stars stripped from the Sagittarius dwarf. The other six do not appear to be from Sagittarius, but might have been removed from a different dwarf galaxy.

    Mapping projects like the Sloan Digital Sky Survey have charted one of the three streams predicted by these simulations, but not to the full extent that the models suggest.

    SDSS Telescope at Apache Point, NM, USA
    SDSS Telescope at Apache Point, NM, USA

    Future instruments like the Large Synoptic Survey Telescope, which will detect much fainter stars across the sky, should be able to identify the other streams.

    LSST/Camera, built at SLAC
    LSST/Camera, built at SLAC
    LSST Interior
    LSST telescope, currently under construction at Cerro Pachón Chile
    LSST telescope, currently under construction at Cerro Pachón Chile, a 2,682-meter-high mountain in Coquimbo Region, in northern Chile, alongside the existing Gemini South and Southern Astrophysical Research Telescopes.

    “More interlopers from Sagittarius are out there just waiting to be found,” says Dierickx.

    These findings have been accepted for publication in The Astrophysical Journal and are available online.

    See the full article here .

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    The Center for Astrophysics combines the resources and research facilities of the Harvard College Observatory and the Smithsonian Astrophysical Observatory under a single director to pursue studies of those basic physical processes that determine the nature and evolution of the universe. The Smithsonian Astrophysical Observatory (SAO) is a bureau of the Smithsonian Institution, founded in 1890. The Harvard College Observatory (HCO), founded in 1839, is a research institution of the Faculty of Arts and Sciences, Harvard University, and provides facilities and substantial other support for teaching activities of the Department of Astronomy.

  • richardmitnick 8:14 am on July 28, 2016 Permalink | Reply
    Tags: , , , Sagittarius dwarf galaxy   

    From New Scientist: “Torn-apart galaxy may be exacting revenge on the Milky Way” 


    New Scientist

    27 July 2016
    Ken Croswell

    The Milky Way is pulling the Sagittarius dwarf galaxy into long streams. David Martinez-Delgado (MPIA) & Gabriel Perez (IAC)

    It’s poetic justice on a cosmic scale. The same galaxy that the Milky Way is tearing apart may be responsible for a mysterious warp in our galaxy’s disc.

    Approximately 50 galaxies orbit our own. The two brightest, the Large and Small Magellanic Clouds, were once thought to be the nearest as well. But in 1994 astronomers spotted a galaxy on the far side of the Milky Way in the constellation Sagittarius that’s only 80,000 light years from Earth – half the distance to the Large Magellanic Cloud.

    The Sagittarius dwarf galaxy is so close that the Milky Way’s gravitational pull is tearing it apart, spilling its stars and star clusters into long streams. But now it seems that this victimised galaxy is striking back.


    In 1956, radio observations of the Milky Way’s atomic hydrogen gas revealed that the outer disc is warped: parts of it lie above the plane of the galaxy while other parts lie below. The Magellanic Clouds are too far away to warp the disc, and a 2010 estimate of the Sagittarius dwarf’s mass suggested it was too light, only 600 million solar masses. So the source of this warp was a mystery.

    Strapping Sagittarius

    Now, Simon Gibbons of the University of Cambridge and his colleagues have analysed the motion of stars in the Sagittarius streams to estimate that the galaxy originally possessed 60 billion solar masses.

    “This new more massive Sagittarius, if it’s right, could be the cause of the warp,” Gibbons says. Since the 1990s, astronomers have recognised that Sagittarius could warp the disc – if the satellite was massive enough.

    David Law of the Space Telescope Science Institute in Baltimore, Maryland – one of the astronomers who did the earlier mass estimate – says the new study uses a more realistic model of the Sagittarius dwarf and does a good job of reproducing its history. In contrast, his work focused on using the path of the Sagittarius stream to probe our galaxy’s dark matter halo.

    But the Milky Way may have the last word. In 2011, other astronomers suggested that such a massive Sagittarius would actually add to our galaxy’s beauty by enhancing its spiral structure.

    Journal reference: Monthly Notices of the Royal Astronomical Society, submitted. ArXiv: arxiv.org/abs/1607.00803

    See the full article here .

    Please help promote STEM in your local schools.

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