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  • richardmitnick 8:48 am on October 8, 2017 Permalink | Reply
    Tags: , , , , , Dark galaxy Dragonfly 44, , U Toronta Dragon Fly Telescope Array, UDGs-“ultra-diffuse galaxies”   

    From Quanta: “Strange Dark Galaxy Puzzles Astrophysicists” 

    Quanta Magazine
    Quanta Magazine

    September 27, 2016
    Joshua Sokol

    The surprising discovery of a massive, Milky Way-size galaxy that is made of 99.99 percent dark matter has astronomers dreaming up new ideas about how galaxies form.

    Astronomers have long known of small dark-matter dominated galaxies. None were supposed to be as big as ordinary spiral galaxies such as NGC 3810, seen here in negative. Photo illustration by Olena Shmahalo/Quanta Magazine. Source: NASA/ESA Hubble.

    NASA/ESA Hubble Telescope

    Among the thousand-plus galaxies in the Coma cluster, a massive clump of matter some 300 million light-years away, is at least one — and maybe a few hundred — that shouldn’t exist.

    Coma cluster via NASA/ESA Hubble

    Dragonfly 44 is a dim galaxy, with one star for every hundred in our Milky Way.

    The ultra-diffuse galaxy Dragonfly 44. Image credit: Pieter van Dokkum / Roberto

    But it spans roughly as much space as the Milky Way.

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

    In addition, it’s heavy enough to rival our own galaxy in mass, according to results published in The Astrophysical Journal Letters at the end of August. That odd combination is crucial: Dragonfly 44 is so dark, so fluffy, and so heavy that some astronomers believe it will either force a revision of our theories of galaxy formation or help us understand the properties of dark matter, the mysterious stuff that interacts with normal matter via gravity and not much else. Or both.

    The discovery came almost by accident. The astronomers Pieter van Dokkum of Yale University and Roberto Abraham of the University of Toronto were interested in testing theories of how galaxies form by searching for objects that have been invisible to even the most advanced telescopes: faint, wispy and extended objects in the sky. So their team built the Dragonfly Telephoto Array, a collection of modified Canon lenses that focus light onto commercial camera sensors.

    U Toronta Dragon Fly Telescope Array

    This setup cut down on any scattered light inside the system that might hide a dim object.

    The plan was to study the faint fringes of nearby galaxies. But the famous Coma cluster — the collection of galaxies that long ago inspired astronomer Fritz Zwicky’s conjecture that such a thing as dark matter might exist — beckoned. “Partway through, we just could not resist looking at Coma,” Abraham said. “You could argue that this discovery emerged from a lack of discipline.” They planned to study the Coma cluster’s intracluster light — the faint glow of loose stars floating between the cluster’s galaxies.

    Instead, they found 47 faint smudges that wouldn’t go away. These smudges seemed to have diameters roughly the same size as the Milky Way. Yet according to the commonly accepted models of galaxy formation, anything that big shouldn’t be so dim.

    In these theories, clumps of dark matter seed the universe with light. First, clouds of dark matter coalesce into relatively dense dark-matter haloes.

    Dark matter halo Image credit: Virgo consortium / A. Amblard / ESA

    Then gas and fragments of other galaxies, drawn by the halo’s gravity, collect at the center. They spin out into a disk and collapse into luminous stars to form something we can see through telescopes. The whole process seems to be reasonably predictable for big galaxies such as our Milky Way. Having measured either a galaxy’s dark-matter halo or its assortment of stars, you should be able to predict the other to within a factor of two.

    The dark galaxy Dragonfly 44. The scale bar represents a distance of 10 kiloparsecs, or about 33,000 light years. Pieter van Dokkum, Roberto Abraham, Gemini Observatory/AURA.

    “It’s not just dogma. It’s basically that there are no exceptions that we knew of,” said Jeremiah Ostriker, an astrophysicist at Columbia University.

    After Abraham and van Dokkum realized that they appeared to be looking at 47 exceptions, they did a search through the literature. They found that similar fuzzy blobs have been on the edge of discovery since the 1970s. Van Dokkum thinks astronomy’s transition from photographic plates — which were perhaps better suited to picking up extended, diffuse objects — to modern digital sensors may actually have hid them from further attention.

    Abraham and van Dokkum first noticed their smudges in the spring of 2014. Since then, similar “ultra-diffuse galaxies,” or UDGs, have been discovered in other galaxy groupings like the Virgo and Fornax clusters. And in the Coma cluster, one study suggested [The Astrophysical Journal Letters], there may be a thousand more of them, including 332 that are about as large as the Milky Way.

    Meanwhile, the Dragonfly team has been advancing the case that these new dim galaxies really are oddballs that challenge current theory. They’re failed galaxies, this argument holds. Dark matter planted the seeds of a spiral disk and stars, but somehow the luminous structure didn’t sprout.

    That argument has convinced outside experts like Ostriker, who finds van Dokkum’s prior record highly credible. “There are many, many other people who could have ‘discovered’ this where I’d be much more skeptical,” Ostriker said. “The simplest way of putting it is: His papers aren’t wrong.”

    Not everyone is so convinced. While these UDGs may be large, they’re not necessarily massive, argue some astronomers. One idea is that UDGs might be lightweight galaxies that look puffy because they are in the process of being torn apart by gravitational tides from the rest of the Coma cluster.

    Michelle Collins, an astronomer at the University of Surrey, argues that “the only other place we’ve seen things that are that extreme or more extreme are a handful of galaxies around the Local Group,” referring to small, dim “dwarf galaxies” that frequently orbit larger galaxies such as our Milky Way. “They are all things that are currently being ripped apart.” That would make most UDGs just large dwarf galaxies in the process of being ripped to shreds.

    Another possibility hinges on the idea that galaxies can “breathe.” At the end of 2015, Kareem El-Badry, who was at the time an undergraduate student at Yale University, proposed that galaxies can swell out and then collapse in size by over a factor of two. In this process, gas first falls into the galaxy, forming massive stars — the breathing in. The stars quickly end their lives in supernova explosions that blast the gas outside the galaxy — the breathing out. The gas eventually cools, and gravity pulls it back toward the galactic center. In a lone galaxy, this rhythm can continue indefinitely. But in the harsh environment of the Coma cluster, where hot gas fills the space between galaxies, the gas after the galaxy exhales could be stripped away, leaving the whole galaxy stuck in a puffy state.

    Lucy Reading-Ikkanda for Quanta Magazine

    Yet another interpretation, suggested in March 2016 by Harvard University astrophysicists Nicola Amorisco and Avi Loeb, is that UDGs are ordinary galaxies that are just spinning fast. “In our scenario, it’s very natural,” Loeb said.

    That idea piggybacks on standard theories of galaxy formation, in which gas pours into a dark-matter halo to build a galaxy. As the material falls, it begins to rotate. The amount of rotation determines the size of the final galaxy. Without much spin, gravity pulls the galaxy into a compact shape. But galaxies that get a big rotational push can spin themselves out into large, lightweight disks.

    It could be, according to this model, that the UDGs are natural examples of the very fastest spinners. If so, their stretched-out disks wouldn’t be dense enough to form as many stars as a slower rotator like the Milky Way, explaining why they look so faint.

    These ideas may well explain some of the UDG population, according to Abraham. “Probably this is going to evolve into a mixed bag of things,” he said. But according to his team’s latest data, obtained from observations that spanned a total of 33.5 hours on the 10-meter Keck II telescope in Hawaii, there is no evidence that the Dragonfly 44 galaxy is rotating.

    Keck Observatory, Maunakea, Hawaii, USA.4,207 m (13,802 ft) above sea level

    In addition, they argue that the total mass of the galaxy is around a trillion suns — massive enough to prevent it being ripped apart like a dwarf galaxy, and heavier than the galaxies thought to periodically puff up.

    That mass measurement is the real sticking point, said Philip Hopkins, a theoretical astrophysicist at the California Institute of Technology who is preparing several papers on UDGs. It comes from two observations of different parts of Dragonfly 44. First, the motions of stars in the galaxy’s inner regions suggest that the area is massive, filled with dark matter. Second, the outskirts of the galaxy are home to a number of globular clusters — tight, ancient balls of stars. Just as the number of stars in a galaxy is ordinarily linked to the amount of dark matter, observations show that the more globular clusters a galaxy has, the higher the mass of its dark-matter halo. Dragonfly 44 has Milky Way-level clusters. Other UDGs seem to have lots of globular clusters, too.

    Because of this, even if these UDGs don’t have heavy dark-matter haloes, researchers will still be left to explain why they have far more globular clusters than the known relationship suggests they should. “Something is weird about these things,” Hopkins said. “Either way, it’s really cool.”

    The discovery has generated enough interest to earn the team precious time on the Hubble Space Telescope to study Dragonfly 44’s globular clusters. “The thing I find hilarious is we’re using humanity’s most powerful telescope in space to follow up a bunch of telephoto lenses,” Abraham said. To fully understand the relationship between dark matter and the globular clusters, though, they have to measure the motions of the clusters — for which they’ll need to wait until the James Webb Space Telescope launches in 2018 [revised to 2019].

    NASA/ESA/CSA Webb Telescope annotated

    In parallel, they’re looking to find and characterize more Dragonfly 44s, preferably a few located both outside of a cluster — and thus free of the harsh cluster environment — and closer to us. It’s an open question as to whether they exist elsewhere and, if so, what form they take. “The resolution of whether the UDGs are what we argue they are, or something else, would come from finding them outside of clusters of galaxies and seeing how they look there,” Loeb said. A few candidates have emerged, van Dokkum said, and they are now being followed up with Keck and Hubble.

    For theorists like Ostriker, that’s an exciting prospect. If the motion of stars in a galaxy like Dragonfly 44 can be studied up close, it would be a make-or-break test for current dark-matter theories, which make different predictions about how the missing mass should be distributed. The leading theory, called cold dark matter, suggests dark matter should surge at the heart of a galaxy. Right now, though, the dark-matter-dominated galaxies we have to study are nearby dwarf galaxies, and they don’t exhibit that characteristic. “Many of the properties that dark matter is supposed to have … these little galaxies don’t show,” Ostriker said. “But we say, ‘We don’t really know how these things were formed anyway,’ and we just change the subject.”

    By contrast, an otherwise normal-but-dark Milky Way would eliminate that loophole. In the universe’s other Milky Way-size galaxies, stars and gas can outweigh dark matter in the central regions by a factor of five to one. That makes disentangling the gravitational pull of dark matter alone tricky. But the center of Dragonfly 44’s disk is 98 percent dark matter, meaning a map of its central mass would give unprecedented insight into dark matter’s properties, Ostriker said.

    The way forward to understand UDGs isn’t clear yet, Abraham said, but hopefully at least some of the ideas now being proposed will persist through the next few years of observations. “In astronomy, it’s still valid to be just an explorer. In the case of Dragonfly, we’re like Leif Eriksson,” he said. “You’ve been on the ship for months, and suddenly somebody said, ‘Land ho!’ And it’s not on the map.”

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    Formerly known as Simons Science News, Quanta Magazine is an editorially independent online publication launched by the Simons Foundation to enhance public understanding of science. Why Quanta? Albert Einstein called photons “quanta of light.” Our goal is to “illuminate science.” At Quanta Magazine, scientific accuracy is every bit as important as telling a good story. All of our articles are meticulously researched, reported, edited, copy-edited and fact-checked.

  • richardmitnick 1:57 pm on August 25, 2016 Permalink | Reply
    Tags: , , Dark galaxy Dragonfly 44, , , ,   

    From Keck: “Scientists Discover Massive Galaxy Made of 99.99 Percent Dark Matter” 

    Keck Observatory

    August 25, 2016

    Pieter van Dokkum
    Yale University
    New Haven, Connecticut, USA
    Tel: +1-203-432-3000
    E-mail: pieter.vandokkum@yale.edu


    Steve Jefferson
    W. M. Keck Observatory
    (808) 881-3827

    Keck Observatory.
    Keck, with Subaru and IRTF (NASA Infrared Telescope Facility). Vadim Kurland

    Keck Observatory

    The dark galaxy Dragonfly 44. The image on the left is a wide view of the galaxy taken with the Gemini North telescope using the Gemini Multi-Object Spectrograph (GMOS). The close-up on the right is from the same very deep image, revealing the large, elongated galaxy, and halo of spherical clusters of stars around the galaxy’s core, similar to the halo that surrounds our Milky Way Galaxy. Dragonfly 44 is very faint for its mass, and consists almost entirely of Dark Matter. Credit: Pieter van Dokkum, Roberto Abraham, Gemini; Sloan Digital Sky Survey.

    Using the world’s most powerful telescopes, an international team of astronomers has discovered a massive galaxy that consists almost entirely of Dark Matter. Using the W. M. Keck Observatory and the Gemini North telescope – both on Maunakea, Hawaii – the team found a galaxy whose mass is almost entirely Dark Matter. The findings are being published in The Astrophysical Journal Letters today.

    Gemini/North telescope at Manua Kea, Hawaii, USA
    Gemini/North telescope at Manua Kea, Hawaii, USA; GEMINI/North GMOS

    Even though it is relatively nearby, the galaxy, named Dragonfly 44, had been missed by astronomers for decades because it is very dim. It was discovered just last year when the Dragonfly Telephoto Array observed a region of the sky in the constellation Coma.

    U Toronto Dunlap Dragonfly telescope Array
    U Toronto Dunlap Dragonfly telescope Array

    Upon further scrutiny, the team realized the galaxy had to have more than meets the eye: it has so few stars that it quickly would be ripped apart unless something was holding it together.

    To determine the amount of Dark Matter in Dragonfly 44, astronomers used the DEIMOS instrument installed on Keck II to measure the velocities of stars for 33.5 hours over a period of six nights so they could determine the galaxy’s mass.


    The team then used the Gemini Multi-Object Spectrograph (GMOS) on the 8-meter Gemini North telescope on Maunakea in Hawaii to reveal a halo of spherical clusters of stars around the galaxy’s core, similar to the halo that surrounds our Milky Way Galaxy.

    “Motions of the stars tell you how much matter there is, van Dokkum said. “They don’t care what form the matter is, they just tell you that it’s there. In the Dragonfly galaxy stars move very fast. So there was a huge discrepancy: using Keck Observatory, we found many times more mass indicated by the motions of the stars, then there is mass in the stars themselves.”

    The mass of the galaxy is estimated to be a trillion times the mass of the Sun – very similar to the mass of our own Milky Way galaxy. However, only one hundredth of one percent of that is in the form of stars and “normal” matter; the other 99.99 percent is in the form of dark matter. The Milky Way has more than a hundred times more stars than Dragonfly 44.

    Finding a galaxy with the mass of the Milky Way that is almost entirely dark was unexpected. “We have no idea how galaxies like Dragonfly 44 could have formed,” Roberto Abraham, a co-author of the study, said. “The Gemini data show that a relatively large fraction of the stars is in the form of very compact clusters, and that is probably an important clue. But at the moment we’re just guessing.”

    “This has big implications for the study of Dark Matter,” van Dokkum said. “It helps to have objects that are almost entirely made of Dark Matter so we don’t get confused by stars and all the other things that galaxies have. The only such galaxies we had to study before were tiny. This finding opens up a whole new class of massive objects that we can study.

    “Ultimately what we really want to learn is what Dark Matter is,” van Dokkum said. “The race is on to find massive dark galaxies that are even closer to us than Dragonfly 44, so we can look for feeble signals that may reveal a Dark Matter particle.”

    Additional co-authors are Shany Danieli, Allison Merritt, and Lamiya Mowla of Yale, Jean Brodie of the University of California Observatories, Charlie Conroy of Harvard, Aaron Romanowsky of San Jose State University, and Jielai Zhang of the University of Toronto.

    The W. M. Keck Observatory operates the largest, most scientifically productive telescopes on Earth. The two, 10-meter optical/infrared telescopes near the summit of Maunakea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrographs and world-leading laser guide star adaptive optics systems.

    DEIMOS (DEep Imaging Multi-Object Spetrograph) boasts the largest field of view (16.7 arcmin by 5 arcmin) of any of the Keck Observatory instruments, and the largest number of pixels (64 Mpix). It is used primarily in its multi-object mode, obtaining simultaneous spectra of up to 130 galaxies or stars. Astronomers study fields of distant galaxies with DEIMOS, efficiently probing the most distant corners of the universe with high sensitivity.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    To advance the frontiers of astronomy and share our discoveries with the world.

    The W. M. Keck Observatory operates the largest, most scientifically productive telescopes on Earth. The two, 10-meter optical/infrared telescopes on the summit of Mauna Kea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrometer and world-leading laser guide star adaptive optics systems. Keck Observatory is a private 501(c) 3 non-profit organization and a scientific partnership of the California Institute of Technology, the University of California and NASA.

    Today Keck Observatory is supported by both public funding sources and private philanthropy. As a 501(c)3, the organization is managed by the California Association for Research in Astronomy (CARA), whose Board of Directors includes representatives from the California Institute of Technology and the University of California, with liaisons to the board from NASA and the Keck Foundation.
    Keck UCal

    Keck NASA

    Keck Caltech

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