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  • richardmitnick 12:13 pm on July 15, 2017 Permalink | Reply
    Tags: , Angular momentum, , , , , Elliptical galaxies, , , Shedding light on galaxies' rotation secrets, Spiral galaxies   

    From EurekaAlert: “Shedding light on galaxies’ rotation secrets” 

    eurekaalert-bloc

    EurekaAlert

    13-Jul-2017

    Media Contact
    Donato Ramani
    ramani@sissa.it
    39-342-802-2237
    http://www.sissa.it/

    Spiral galaxies are strongly rotating whereas the rotation velocity of ellipticals is much lower. A new study investigates the reasons of such a dichotomy revealing that it is imprinted at formation.

    Scuola Internazionale Superiore di Studi Avanzati

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    Spiral galaxies are found to be strongly rotating, with an angular momentum higher by a factor of about 5 than ellipticals. What is the origin of such a difference?
    Credit Wikimedia Common.

    The dichotomy concerns the so-called angular momentum (per unit mass), that in physics is a measure of size and rotation velocity. Spiral galaxies are found to be strongly rotating, with an angular momentum higher by a factor of about 5 than ellipticals. What is the origin of such a difference? An international research team investigated the issue in a study just published in the Astrophysical Journal. The team was led by SISSA Ph.D. student JingJing Shi under the supervision of Prof. Andrea Lapi and Luigi Danese, and in collaboration with Prof. Huiyuan Wang from USTC (Hefei) and Dr. Claudia Mancuso from IRA-INAF (Bologna). The researchers inferred from observations the amount of gas fallen into the central region of a developing galaxy, where most of the star formation takes places.

    The outcome is that in elliptical galaxies only about 40% of the available gas fell into that central region. More relevantly, this gas fueling star formation was characterized by a rather low angular momentum since the very beginning. This is in stark contrast with the conditions found in spirals, where most of the gas ending up in stars had an angular momentum appreciably higher. In this vein, the researchers have traced back the dichotomy in the angular momentum of spiral and elliptical galaxies to their different formation history. Elliptical galaxies formed most of their stars in a fast collapse where angular momentum is dissipated. This process is likely stopped early on by powerful gas outflows from supernova explosions, stellar winds and possibly even from the central supermassive black hole. For spirals, on the other hand, the gas infelt slowly conserving its angular momentum and stars formed steadily along a timescale comparable to the age of the Universe.

    “Till recent years, in the paradigm of galaxy formation and evolution, elliptical galaxies were thought to have formed by the merging of stellar disks in the distant Universe. Along this line, their angular momentum was thought to be the result of dissipative processes during such merging events” say the researchers. Recently, this paradigm had been challenged by far-infrared/sub-millimeter observations brought about by the advent of space observatories like Herschel and ground based interferometers like the Atacama Large Millimeter Array (ALMA).

    ESA/Herschel spacecraft

    ESO/NRAO/NAOJ ALMA Array in Chile in the Atacama at Chajnantor plateau, at 5,000 metres

    These observations have the power of penetrating through interstellar dust and so to unveil the star formation processes in the very distant, dusty galaxies, that constituted the progenitors of local ellipticals. “The net outcome from these observations is that the stars populating present-day ellipticals are mainly formed in a fast dissipative collapse in the central regions of dusty starforming galaxies. After a short timescale of less than 1 billion years the star formation has been quenched by powerful gas outflows”. Despite this change of perspective, the origin of the low angular momentum observed in local ellipticals still remained unclear.

    “This study reconciles the low angular momentum observed in present-day ellipticals with the new paradigm emerging from Herschel and ALMA observations of their progenitors” conclude the scientists. “We demonstrated that the low angular momentum of ellipticals is mainly originated by nature in the central regions during the early galaxy formation process, and not nurtured substantially by the environment via merging events, as envisaged in previous theories”.

    See the full article here .

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  • richardmitnick 4:20 pm on September 2, 2016 Permalink | Reply
    Tags: , , Density waves, , Spiral galaxies   

    From Sky and Telescope: “Why [Some] Galaxies Have Spiral Arms” 

    SKY&Telescope bloc

    Sky & Telescope

    August 29, 2016
    Camille M. Carlisle

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    The galaxy M101 is a “grand design” spiral (meaning it’s dominated by prominent, well-organized arms) of type Sc. Of its estimated trillion stars, many thousands of its brightest supergiants are resolved by Hubble. NASA / ESA / K. Kuntz (JHU) / F. Bresolin (Univ. of Hawaii) / J. Trauger (JPL) / J. Mould (NOAO) / Y.-H. Chu (Univ. of Illinois, Urbana) / STScI

    Arguably the prettiest objects in space are spiral galaxies. Young, bright stars trace the arms of these graceful whorls, and dark dust lanes act like galactic eyeliner to dramatically shade them.

    In principle it’s easy to make a spiral arm. For various reasons, stuff in the disk sometimes clumps together, but the clump won’t stay a clump for long: stars and clouds near the galactic center circle the galaxy faster than the material farther out does, so over time the clump gets stretched into a spiral.

    However, by this reasoning, the arm should quickly wrap itself around the galaxy’s center, destroying the spiral. That generally doesn’t happen. Thus for at least half a century, astronomers have debated why these patterns persist. Maybe, many have suggested, stars don’t actually create the pattern — instead, they’re just passing through it. The arms instead would arise thanks to what are called density waves. Now, observations published in the August 10th Astrophysical Journal Letters provide long-looked-for evidence that these waves do exist.

    Yield to Oncoming Stars

    If you’ve ever been in a slowdown on the highway, you’ve experienced a density wave. Cars whizzing down the road encounter a region where, for whatever reason, they have to decelerate. Once they’ve passed it, they speed up again. Yet even though cars are successfully passing through the jam, the slow stretch persists and keeps propagating along the highway.

    The same thing happens (we think) in spiral galaxies. Even as a clump in the disk stretches into a spiral, all the stars and clouds keep moving through that arm, just as cars continue to pass through a highway choke point. Essentially, clouds and stars slow down and speed up again in a chain reaction — a density wave — that moves through the galaxy.

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    This diagram shows the authors’ scenario for how density waves create spiral arms. The green dashed line is the co-rotation radius, where the density wave (brownish curve, labeled “stellar arm”) and the stars and gas in the galactic disk travel at the same speed. Within that radius, the stars travel faster than the wave; outside the radius, the stars travel slower. In the scenario above, the density wave compresses the gaseous arm (black), which then forms new stars (blue arm) that age as they travel farther from the density wave. Those newborn stars combine with other, old-and-red stars that were already in the disk and were squeezed closer together by the wave (red). Because arms wind up with time, a galaxy’s arms will look tighter or looser depending on which population of stars astronomers observe. Hamed Pour-Imani et al. / Astrophysical Journal Letters 827:L2, 2016 August 10. © AAS

    The reason we can see this spiral pattern is because as it passes through the galaxy the density wave compresses gas clouds, triggering star formation. The youngest, brightest stars will thus be nearest the wave and trace out an arm. As stars move out of the wave and spread out across the disk they will age and these biggest, brightest stars will die off, preventing the arm from totally winding up.

    But that doesn’t mean there’s no winding. An important prediction comes out of this scenario: how tightly wound a spiral’s arms appear depends on which population of stars you observe. As time goes on the stars get farther from the wave, and — because the inner stars move faster and the outer stars move slower — their orbital motions do wind the arm they’re tracing, tightening the spiral over time.

    But because the hot, bluish, live-fast-die-young ones kick the bucket soon after they encounter the density wave, they’ll only trace loosely wound arms. Conversely the older, redder stars will trace more tightly wound arms. So if astronomers look at a galaxy in wavelengths that pick up young stars, they’ll see a more relaxed spiral than if they look in wavelengths that pick up old stars.

    Density Waves Detected

    Until now, astronomers hadn’t conclusively seen this effect. But the new study by Hamed Pour-Imani (University of Arkansas) and colleagues is convincing proof in its favor. The team compiled archival images of 28 spiral galaxies in far-infrared, near-infrared, optical, and ultraviolet wavelengths. The far-infrared and ultraviolet wavelengths pick up star-forming regions, while optical and near-infrared probe older stars.

    The team checked its results three ways and sure enough, it found exactly what’s predicted: arms traced by older stars hug the galactic centers more tightly than those traced by star-forming regions. The result is a neat confirmation that density waves exist.

    See the full article here .

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    Sky & Telescope magazine, founded in 1941 by Charles A. Federer Jr. and Helen Spence Federer, has the largest, most experienced staff of any astronomy magazine in the world. Its editors are virtually all amateur or professional astronomers, and every one has built a telescope, written a book, done original research, developed a new product, or otherwise distinguished him or herself.

    Sky & Telescope magazine, now in its eighth decade, came about because of some happy accidents. Its earliest known ancestor was a four-page bulletin called The Amateur Astronomer, which was begun in 1929 by the Amateur Astronomers Association in New York City. Then, in 1935, the American Museum of Natural History opened its Hayden Planetarium and began to issue a monthly bulletin that became a full-size magazine called The Sky within a year. Under the editorship of Hans Christian Adamson, The Sky featured large illustrations and articles from astronomers all over the globe. It immediately absorbed The Amateur Astronomer.

    Despite initial success, by 1939 the planetarium found itself unable to continue financial support of The Sky. Charles A. Federer, who would become the dominant force behind Sky & Telescope, was then working as a lecturer at the planetarium. He was asked to take over publishing The Sky. Federer agreed and started an independent publishing corporation in New York.

    “Our first issue came out in January 1940,” he noted. “We dropped from 32 to 24 pages, used cheaper quality paper…but editorially we further defined the departments and tried to squeeze as much information as possible between the covers.” Federer was The Sky’s editor, and his wife, Helen, served as managing editor. In that January 1940 issue, they stated their goal: “We shall try to make the magazine meet the needs of amateur astronomy, so that amateur astronomers will come to regard it as essential to their pursuit, and professionals to consider it a worthwhile medium in which to bring their work before the public.”

     
  • richardmitnick 8:08 pm on December 19, 2015 Permalink | Reply
    Tags: , , Spiral galaxies,   

    From Ethan Siegel: “Why Aren’t Spiral Galaxies More Wound Up?” 

    Starts with a bang
    Starts with a Bang

    Ethan Siegel
    12.19.15

    1
    Image credit: ESA/Hubble & NASA.

    NASA Hubble Telescope
    NASA/ESA Hubble

    It’s rare to find a galaxy where the arms wrap around even a full 360 degrees. But after billions of years, why is that?

    “The farther we peer into space, the more we realize that the nature of the universe cannot be understood fully by inspecting spiral galaxies or watching distant supernovas. It lies deeper.” -Robert Lanza

    Think about the grandest objects you’ve ever seen pictures of in the night sky. Sure, there are a whole slew of targets to choose from, including dying stars, supernova remnants, star-forming nebulae and clusters of stars both new and old, but nothing compares to the beauty of a spiral galaxy. Containing between billions and trillions of stars, these “island universes” display a unique structure all their own. A structure, mind you, that’s puzzling if you think about it, as our questioner Greg Rogers did:

    “One thing that has always bothered me about spiral galaxies is that you only see the arms wrapping around about half-way or so. Since the outside is spinning around the core more slowly, I would expect that we should see some galaxies with arms wrapping many times around the core. Is the universe simply not old enough for these more tightly wound spiral galaxies to have formed?”

    You can look at any number of spiral galaxies, but they all have the same apparent structure in common.

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    Images credit: J. Schulman, Ken Crawford of Racho Del Sol Observatory, and Adam Block / Mt. Lemmon Skycenter / University of Arizona

    Radiating out from the central nucleus come any number of spiral arms — usually between two and four — that wrap around the galaxy as they spiral outward. One of the fantastic discoveries we made in the 1970s, quite contrary to our expectations, is that the stars don’t move slower in their orbital speed around the galaxy as you move outward, the way planets orbit our central star more slowly the farther out you go. Instead, the speed remains constant, which is another way of saying that the galactic rotation curves have flat profiles.

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    Image credit: Wikimedia Commons user Stefania.deluca.

    The way we measured this is by looking at edge-on spirals, and seeing how much redshift or blueshift the individual stars displayed relative to their distance from the galactic center. But even though the velocities of the individual stars are roughly constant, a star that’s twice as far from the center as another takes twice as long to go around, while one ten times as distant takes ten times as long to orbit.

    Given that this is the case, we can do a little math: for a galaxy like our Milky Way, based on how fast the Sun and the other stars appear to move, it takes the Sun about 220 million years to make a single orbit around the galaxy. At our distance of roughly 26,000 light years from the galactic center, we’re a little less than halfway to the outskirts. This means that for a ~12 billion year old galaxy like our own: the outer stars should have completed only around 25 orbits; stars where our Sun are should have completed approximately 54 orbits; stars in the inner 10,000 light years should have completed more than 100 orbits. In other words, we’d expect galaxies to “wind up” over time, as the video below shows.


    download mp4 video here.

    But as our images of galaxies show, they don’t wrap around dozens of times; the arms in most cases don’t even wrap around one time! When we first realized this property of galaxies, it meant one thing was for certain: these spiral arms aren’t material, they’re simply a visual effect. This remains true whether galaxies are in isolation or not. But there’s another hint these galaxies offer, if we look closely.

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    Image credit: ESO, taken with the EFOSC instrument, attached to the 3.58-metre New Technology Telescope [NTT] at ESO’s La Silla Observatory in Chile.

    ESO EFOSC2
    EFOSC instrument

    ESO NTT
    ESO/NTT

    ESO LaSilla
    ESO/La Silla

    Do you notice how there are “pink” spots dotted all along the spiral arms here? These appear whenever we have active regions of new star formation; the pink signature is actually an excess of emitted light at a very precise wavelength: 656.3 nanometers. This emission occurs when hot, new stars burn brightly enough to ionize gaseous material, and then when the electrons recombine with the protons, the newly formed hydrogen atoms emit light at very particular frequencies, including the one that turns these regions pink.

    What this indicates to us is that these spiral arms are actually made out of regions where the density of material is higher than the other locations in the galaxy, and that stars are free to move in-and-out of these arms as time goes on.


    download mp4 video here.

    The idea that explains this has been around since 1964, and is known as density wave theory. The theory holds that the arms themselves appear to stay in the same exact spots as time goes on, the same way that traffic jams stay in the same spots. Even though the individual objects (stars in the arms; cars in a traffic jam) are free to move through, the same rough number remain in the “jam” at any given time. This results in the dense pattern maintaining itself over time.


    download mp4 video here.

    The physics behind it is even simpler: stars at different radii all exert the gravitational forces we’re accustomed to, and those forces are what maintain the spiral shape. In other words, if you start with a region where the gas is overdense and you allow your “disk” to rotate, you’ll get an initial series of regions where stars first form: the proto-arms. As the galaxy evolves over time, these arms — and the overdense regions — are maintained by the effects of gravity alone.

    What’s remarkable is that this effect works equally well whether there’s dark matter in a giant halo surrounding your galaxy (below, right) or none at all (below, left).

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    Image credit: Wikimedia Commons user Ingo Berg, turned into a GIF by Forbes staff.

    Even though the premise of your question, Greg, was flawed, since the outer stars in a galaxy move just as fast (speed-wise) as the inner stars, it’s true that the arms will never wind up, no matter how old a galaxy gets, simply due to the physics of galaxies themselves. Much like a traffic jam, the stars, gas and dust that find themselves in the spiral arms at any given time will be in a much busier neighborhood, and once they move out again, they’ll find a great distance from themselves to any other star, just like our Sun experiences today.

    See the full article here .

    Please help promote STEM in your local schools.

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    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

     
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