понедельник, 2 декабря 2013 г.

"Галактическая сага". Часть 4. Формирование ядра Млечного пути


    У звезд нашли «арахисовые» орбиты

Вид на Млечный Путь со стороны. Рисунок на основе астрономических данных о строении Галактики. (Изображение: ESO/NASA/JPL-Caltech/M. Kornmesser/R. Hurt
    «Международная группа ученых выяснила, что некоторые звезды должны двигаться не по эллиптическим орбитам, а по траектории, которая своей формой напоминает арахис. Подробности со ссылкой на статью ученых в журнале «Monthly Notices of the Royal Astronomical Society» (также доступна в виде препринтаприводит официальный сайт университета Рочестера.
    В работе исследователей из Австралии, Германии, Китая, США и Франции рассматривается множество звезд, которое ведет себя далеко не так же, как планеты в Солнечной системе. В случае с движением планет вокруг звезд массой планет в первом приближении можно пренебречь, но в случае с центральной областью Галактики на гравитационное поле сверхмассивной черной дыры в центре накладываются поля звезд и облаков газа.
    Физики построили модель с 320 тысячами частиц в диске галактики, к которым добавили еще 160 тысяч частиц для имитации темной материи. Эта модель показала, что взаимное влияние частиц друг на друга приводит к передвижению значительной их части по орбитам в виде арахисового ореха. Такие орбиты позволяют сформироваться ядру Галактики достаточно похожим на реальное. Кроме того, из модели следует то, что близкие к центру звезды имеют большую вертикальную (перпендикулярную диску) скорость, чем расположенные на периферии.
    Проверить модель ученые намерены при помощи данных с телескопа «Гайя», запуск которого недавно был отложен и перенесен на декабрь 2013 года. По словам одной из авторов исследования, Алисы Квиллен, «Гайя» позволит получить данные об орбитах миллиардов звезд. Определение траектории звезд вблизи центра Галактики позволяет уточнить представления ученых об эволюции Млечного пути, а также оценить долю в массе галактики темной материи». (29 ноября 2013 года, 13:16). http://lenta.ru/news/2013/11/29/peanutorbit/

    Figures of Eight and Peanut Shells: How Stars Move at the Center of the Galaxy

    Photo: «An artist’s impression showing how the Milky Way galaxy would look seen from almost edge on and from a very different perspective than we get from the Earth. The central bulge shows up as a peanut shaped glowing ball of stars and the spiral arms and their associated dust clouds form a narrow band. Credit: ESO/NASA/JPL-Caltech/M. Kornmesser/R. Hurt».

    «Two months ago astronomers created a new 3D map of stars at the centre of our Galaxy (the Milky Way), showing more clearly than ever the bulge at its core. Previous explanations suggested that the stars that form the bulge are in banana-like orbits, but a paper published this week in Monthly Notices of the Royal Astronomical Society suggests that the stars probably move in peanut-shell or figure of eight-shaped orbits instead.
    The difference is important; astronomers develop theories of star motions to not only understand how the stars in our galaxy are moving today but also how our galaxy formed and evolves. The Milky Way is shaped like a spiral, with a region of stars at the centre known as the "bar," because of its shape. In the middle of this region, there is a "bulge" that expands out vertically.
    In the new paper Alice Quillen, professor of astronomy at the University of Rochester, and her collaborators created a mathematical model of what might be happening at the centre of the Milky Way. Unlike the Solar System where most of the gravitational pull comes from the Sun and is simple to model, it is much harder to describe the gravitational field near the centre of the Galaxy, where millions of stars, vast clouds of dust, and even dark matter swirl about. In this case, Quillen and her colleagues considered the forces acting on the stars in or near the bulge.
    As the stars go round in their orbits, they also move above or below the plane of the bar. When stars cross the plane they get a little push, like a child on a swing. At the resonance point, which is a point a certain distance from the center of the bar, the timing of the pushes on the stars is such that this effect is strong enough to make the stars at this point move up higher above the plane. It is like when the child on the swing has been pushed a little everytime he comes round and eventually he is swinging higher. These stars that are pushed out form the edge of the bulge.
    The resonance at this point means that stars undergo two vertical oscillations for every orbital period. But what is the most likely shape of the orbits in between? The researchers showed through computer simulations that peanut-shell shaped orbits are consistent with the effect of this resonance and could give rise to the observed shape of the bulge, which is also like a peanut-shell.
    Next month the European Space Agency will launch the Gaia spacecraft, which is designed to create a 3D map of the stars in the Milky Way and their motions. This 3D map will help astronomers better understand the composition, formation and evolution of our Galaxy.
    "It is hard to look back into the past of our galaxy and know what was there, but simulations can give us clues," explained Quillen. "Using my model I saw that, over time, the resonance with the bar, which is what leads to these peculiarly shaped orbits, moves outwards. This may be what happened in our galaxy."
    "Gaia will generate huge amounts of data – on billions of stars," said Quillen. This data will allow Quillen and her colleagues to finesse their model further. "This can lead to a better understanding of how the Milky Way might have evolved into the shape it has today."
Quillen explained that there are different models as to how the galactic bulge was formed. Astronomers are interested in finding out how much the bar has slowed down over time and whether the bulge "puffed up all at once or slowly." Understanding the distributions of speeds and directions of motion (velocities) of the stars in the bar and the bulge might help determine this evolution.

Simulations 


Two movies of N-body simulations, one showing a bar that buckles (top), the other without buckling. These movies show face-on and edge-on views of barred galaxies. Both movies show that the peanut shape becomes more extended as the bar slows down.

    "One of the predictions of my model is that there is a sharp difference in the velocity distributions inside and outside the resonance," Quillen said. "Inside – closer to the galactic centre – the disk should be puffed up and the stars there would have higher vertical velocities. Gaia will measure the motions of the stars and allow us to look for variations in velocity distributions such as these."
    To be able to generate a model for the orbits of stars in the bulge, Quillen needed to factor in different variables. She first needed to understand what happens at the region of the resonance, which depends on the speed of the rotating bar and the mass density of the bar.
"Before I could model the orbits, I needed the answer to what I thought was a simple question: what is the distribution of material in the inner galaxy?" Quillen said. "But this wasn't something I could just look up. Luckily my collaborator Sanjib Sharma was able to help out."
    Sharma worked out how the speed of circular orbits changed with distance from the galactic centre (called the rotation curve). Using this information, Quillen could compute a mass density at the location of the resonance, which she needed for her model.
    Quillen was also able to combine the new orbit models with the speed of the bar (which is rotating) to get a more refined estimate of the mass density 3000 light years from the Galaxy centre (about one eighth of the distance from the centre of the Galaxy to Earth), which is where the edge of the bulge is.
    And there is not long now to wait now for Gaia to start collecting data. Gaia's launch time is set for December 19, and will be streamed live on the ESA Portal.
    Quillen's co-authors in this paper are Sanjib Sharma, Sydney Institute for Astronomy, Australia; Ivan Minchev, Astronomy Institute of Potsdam, Germany; Yu-Jing Qin, Shanghai Astronomical Observatory, China; and Paola Di Matteo, Paris-Meudon Observatory, France». (November 27, 2013). http://rochester.edu/news/show.php?id=7882  

    A Vertical Resonance Heating Model for X- or Peanut-Shaped Galactic Bulges

    «We explore a second order Hamiltonian vertical resonance model for X-shaped or peanut-shaped galactic bulges. The X-shape is caused by the 2:1 vertical Lindblad resonance with the bar, with two vertical oscillation periods per orbital period in the bar frame. We examine N-body simulations and find that due to the bar slowing down and disk thickening during bar buckling, the resonance and associated peanut-shape moves outward. The peanut-shape is consistent with the location of the vertical resonance, independent of whether the bar buckled or not. We estimate the resonance width from the potential m=4 Fourier component and find that the resonance is narrow, affecting orbits over a narrow range in the angular momentum distribution, dL/L ~ 0.05. As the resonance moves outward, stars originally in the mid plane are forced out of the mid plane into orbits just within the resonance separatrix. The height of the separatrix orbits, estimated from the Hamiltonian model, is approximately consistent with the peanut-shape height. The X-shape is comprised of stars in the vicinity of the resonance separatrix. The velocity distributions from the simulations illustrate that low inclination orbits are depleted within resonance. Within resonance, the vertical velocity distribution is broad, consistent with resonant heating caused by the passage of the resonance through the disk. In the Milky Way bulge we relate the azimuthally averaged mid-plane mass density near the vertical resonance to the rotation curve and bar pattern speed. At an estimated vertical resonance galactocentric radius of ~1.3 kpc, we confirm a mid-plane density of ~5x10^8 Msol/kpc^3, consistent with recently estimated mass distributions. We find that the rotation curve, bar pattern speed, 2:1 vertical resonance location, X-shape tips, and mid-plane mass density, are all self-consistent in the Milky Way galaxy bulge». (Alice C. QuillenIvan MinchevSanjib SharmaYu-Jing QinPaola Di Matteo. Submitted on 31 Jul 2013)http://arxiv.org/abs/1307.8441

    Федор Дергачев

    К вопросу о твердотельном вращении галактического балджа

    В вышеприведенной статье «У звезд нашли «арахисовые» орбиты» меня заинтересовала попытка теоретиков создать не только кинематическую, но и динамическую модель вращения ядра Галактики (см. об этом в посте «Гравитационная неустойчивость в Галактике»). Любопытно, что о твердотельном вращении галактического балджа в публикации даже не упоминается. Непонятно, правда, была ли получена соответствующая реальная картина вращения при данном математическом моделировании…  

    «Галактическая сага». Часть 5. «Формирование диска Млечного пути». http://artefact-2007.blogspot.ru/2014/01/5.html

    На эту тему:
    «Гравитационная неустойчивость в Галактике». (14 ноября 2012 года). http://artefact-2007.blogspot.ru/2012/11/15.html

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