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Posts Tagged ‘Galaxies

Barred Spiral Galaxy Swirls in the Night Sky

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This image shows the swirling shape of galaxy NGC 2217, in the constellation of Canis Major (The Great Dog). In the central region of the galaxy is a distinctive bar of stars within an oval ring. Further out, a set of tightly wound spiral arms almost form a circular ring around the galaxy. NGC 2217 is therefore classified as a barred spiral galaxy, and its circular appearance indicates that we see it nearly face-on.

The outer spiral arms have a bluish colour, indicating the presence of hot, luminous, young stars, born out of clouds of interstellar gas. The central bulge and bar are yellower in appearance, due to the presence of older stars. Dark streaks can also be seen in places against the galaxy’s arms and central bulge, where lanes of cosmic dust block out some of the starlight.

The majority of spiral galaxies in the local Universe — including our own Milky Way — are thought to have a bar of some kind, and these structures play an important role in the development of a galaxy. They can, for example, funnel gas towards the centre of the galaxy, helping to feed a central black hole, or to form new stars.
Credit: ESO

Written by physicsgg

January 23, 2012 at 1:12 pm

Posted in ASTRONOMY, ASTROPHYSICS

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Hubble Zooms in on Double Nucleus in Andromeda Galaxy

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A new Hubble Space Telescope image centers on the 100-million-solar-mass black hole at the hub of the neighboring spiral galaxy M31, or the Andromeda galaxy, the only galaxy outside the Milky Way visible to the naked eye and the only other giant galaxy in the local group

This is a Hubble image of the 100-million-solar-mass black hole at the hub of the neighboring spiral galaxy M31, or the Andromeda galaxy. The compact cluster of blue stars is surrounded by the larger “double nucleus” of M31. The double nucleus is actually an elliptical ring of old reddish stars in orbit around the black hole but more distant than the blue stars. Credit: NASA, ESA, and T. Lauer (National Optical Astronomy Observatory)

This is the sharpest visible-light image ever made of the nucleus of an external galaxy…
Read more:www.nasa.gov

Written by physicsgg

January 12, 2012 at 7:52 am

Large peculiar motion of the solar system from the dipole anisotropy

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…. in sky brightness due to distant radio sources

Ashok K. Singal

The distribution of strong NVSS sources (S>300 mJy) in galactic co-ordinates

According to the cosmological principle, the Universe should appear isotropic, without any preferred directions, to an observer whom we may consider to be fixed in the co-moving co-ordinate system of the expanding Universe. Such an observer is stationary with respect to the average distribution of the matter in the Universe and the sky brightness at any frequency should appear uniform in all directions to such an observer. However a peculiar motion of such an observer, due to a combined effect of Doppler boosting and aberration, will introduce a dipole anisotropy in the observed sky brightness; in reverse an observed dipole anisotropy in the sky brightness could be used to infer the peculiar velocity of the observer with respect to the average Universe. We determine the peculiar velocity of the solar system relative to the frame of distant radio sources, by studying the anisotropy in the sky brightness from discrete radio sources, i.e., an integrated emission from discrete sources per unit solid angle. Our results give a direction of the velocity vector in agreement with the Cosmic Microwave Background Radiation (CMBR) value, but the magnitude (~ 1600± 400 km/s) is ~4  times the CMBR value ($369 ± 1 km/s) at a statistically significant (~ 3σ) level. A genuine difference between the two dipoles would imply anisotropic Universe, with the anisotropy changing with the epoch. This would violate the cosmological principle where the isotropy of the Universe is assumed for all epochs, and on which the whole modern cosmology is based upon…….
Read more: arxiv.org/PS

Written by physicsgg

October 31, 2011 at 2:03 pm

Posted in ASTRONOMY, ASTROPHYSICS

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Astronomers Pin Down Galaxy Collision Rate

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Galactic Wrecks Far from Earth: These images from NASA's Hubble Space Telescope's ACS in 2004 and 2005 show four examples of interacting galaxies far away from Earth. The galaxies, beginning at far left, are shown at various stages of the merger process. The top row displays merging galaxies found in different regions of a large survey known as the AEGIS. More detailed views are in the bottom row of images

A new analysis of Hubble surveys, combined with simulations of galaxy interactions, reveals that the merger rate of galaxies over the last 8 billion to 9 billion years falls between the previous estimates.

The galaxy merger rate is one of the fundamental measures of galaxy evolution, yielding clues to how galaxies bulked up over time through encounters with other galaxies. And yet, a huge discrepancy exists over how often galaxies coalesced in the past. Measurements of galaxies in deep-field surveys made by NASA’s Hubble Space Telescope generated a broad range of results: anywhere from 5 percent to 25 percent of the galaxies were merging.

The study, led by Jennifer Lotz of the Space Telescope Science Institute in Baltimore, Md., analyzed galaxy interactions at different distances, allowing the astronomers to compare mergers over time. Lotz’s team found that galaxies gained quite a bit of mass through collisions with other galaxies. Large galaxies merged with each other on average once over the past 9 billion years. Small galaxies were coalescing with large galaxies more frequently. In one of the first measurements of smashups between dwarf and massive galaxies in the distant universe, Lotz’s team found these mergers happened three times more often than encounters between two hefty galaxies.

“Having an accurate value for the merger rate is critical because galactic collisions may be a key process that drives galaxy assembly, rapid star formation at early times, and the accretion of gas onto central supermassive black holes at the centers of galaxies,” Lotz explains.

The team’s results are accepted for publication appeared in The Astrophysical Journal.

The problem with previous Hubble estimates is that astronomers used different methods to count the mergers.

“These different techniques probe mergers at different ‘snapshots’ in time along the merger process,” Lotz says. “It is a little bit like trying to count car crashes by taking snapshots. If you look for cars on a collision course, you will only see a few of them. If you count up the number of wrecked cars you see afterwards, you will see many more. Studies that looked for close pairs of galaxies that appeared ready to collide gave much lower numbers of mergers than those that searched for galaxies with disturbed shapes, evidence that they’re in smashups.”

To figure out how many encounters happen over time, Lotz needed to understand how long merging galaxies would look like “wrecks” before they settle down and begin to look like normal galaxies again.

That’s why Lotz and her team turned to highly detailed computer simulations to help make sense of the Hubble photographs. The team made simulations of the many possible galaxy collision scenarios and then mapped them to Hubble images of galaxy interactions.

Creating the computer models was a time-consuming process. Lotz’s team tried to account for a broad range of merger possibilities, from a pair of galaxies with equal masses joining together to an interaction between a giant galaxy and a puny one. The team also analyzed different orbits for the galaxies, possible collision impacts, and how galaxies were oriented to each other. In all, the group came up with 57 different merger scenarios and studied the mergers from 10 different viewing angles. “Viewing the simulations was akin to watching a slow-motion car crash,” Lotz says.

The simulations followed the galaxies for 2 billion to 3 billion years, beginning at the first encounter and continuing until the union was completed, about a billion years later.

“Our simulations offer a realistic picture of mergers between galaxies,” Lotz says.

In addition to studying the smashups between giant galaxies, the team also analyzed encounters among puny galaxies. Spotting collisions with small galaxies are difficult because the objects are so dim relative to their larger companions.

“Dwarf galaxies are the most common galaxy in the universe,” Lotz says. “They may have contributed to the buildup of large galaxies. In fact, our own Milky Way galaxy had several such mergers with small galaxies in its recent past, which helped to build up the outer regions of its halo. This study provides the first quantitative understanding of how the number of galaxies disturbed by these minor mergers changed with time.”

Lotz compared her simulation images with pictures of thousands of galaxies taken from some of Hubble’s largest surveys, including the All-Wavelength Extended Groth Strip International Survey (AEGIS), the Cosmological Evolution Survey (COSMOS), and the Great Observatories Origins Deep Survey (GOODS), as well as mergers identified by the DEEP2 survey with the W.M. Keck Observatory in Hawaii. She and other groups had identified about a thousand merger candidates from these surveys but initially found very different merger rates.

“When we applied what we learned from the simulations to the Hubble surveys in our study, we derived much more consistent results,” Lotz says.

Her next goal is to analyze galaxies that were interacting around 11 billion years ago, when star formation across the universe peaked, to see if the merger rate rises along with the star formation rate. A link between the two would mean galaxy encounters incite rapid star birth.

www.nasa.gov

Written by physicsgg

October 27, 2011 at 6:20 pm

Posted in ASTRONOMY, ASTROPHYSICS

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Light from galaxy clusters confirm theory of relativity

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Researchers have analyzed measurements of the light from galaxies in approximately 8,000 galaxy clusters. Galaxy clusters are accumulations of thousands of galaxies (every light in the image is a galaxy), which are held together by their own gravity. This gravity affects the light that is sent out into space from the galaxies

All observations in astronomy are based on light emitted from stars and galaxies and, according to the general theory of relativity, the light will be affected by gravity. At the same time all interpretations in astronomy are based on the correctness of the theory of relatively, but it has never before been possible to test Einstein’s theory of gravity on scales larger than the solar system. Now astrophysicists at the Dark Cosmology Centre at the Niels Bohr Institute have managed to measure how the light is affected by gravity on its way out of galaxy clusters. The observations confirm the theoretical predictions. The results have been published in the journal Nature.
Observations of large distances in the universe are based on measurements of the redshift, which is a phenomenon where the wavelength of the light from distant galaxies is shifted more and more towards the red with greater distance. The redshift indicates how much the universe has expanded from when the light left until it was measured on Earth. Furthermore, according to Einstein’s general theory of relativity, the light and thus the redshift is also affected by the gravity from large masses like galaxy clusters and causes a gravitational redshift of the light. But the gravitational influence of light has never before been measured on a cosmological scale.
“It is really wonderful. We live in an era with the technological ability to actually measure such phenomena as cosmological gravitational redshift”, says astrophysicist Radek Wojtak, Dark Cosmology Centre under the Niels Bohr Institute at the University of Copenhagen….. Read the rest of this entry »

Written by physicsgg

September 28, 2011 at 6:17 pm

Posted in ASTRONOMY, RELATIVITY

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Black holes act as galactic thermostats

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Do AGNs act as galactic thermostats?

Τhe supermassive black hole at the centre of a massive galaxy or galaxy cluster acts as a furnace, pumping heat into its surroundings. But astronomers have struggled to understand how a steady temperature is maintained throughout the whole galaxy when the black hole only appears to interact with nearby gas. Now, researchers in Canada and Australia believe the answer could be a feedback loop in which gravity causes gas to accumulate around the black hole until its density reaches a tipping point. Then, the gas rushes into the black hole, temporarily turning up the heat….. Read the rest of this entry »

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September 15, 2011 at 2:20 pm

Posted in ASTROPHYSICS

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Star Formation in Molecular Clouds

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Star cluster formation in a turbulent molecular cloud core

Stars and star clusters form by gravoturbulent fragmentation of interstellar gas clouds. The supersonic turbulence ubiquitously observed in Galactic molecular gas generates strong density fluctuations with gravity taking over in the densest and most massive regions. Collapse sets in to build up stars. Turbulence plays a dual role. On global scales it provides support, while at the same time it can promote local collapse. Stellar birth is thus intimately linked to the dynamical behavior of parental gas cloud, which governs when and where protostars form, and how they contract and grow in mass via accretion from the surrounding cloud material. The thermodynamic behavior of the star forming gas plays a crucial part in this process and influences the stellar mass function as well as the dynamic properties of the nascent stellar cluster. This lecture provides a critical review of our current understand- ing of stellar birth and compares observational data with competing theoretical models….
Read more: http://arxiv.org/abs/1109.0467

Written by physicsgg

September 5, 2011 at 9:52 am

Posted in ASTRONOMY, COSMOLOGY

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Chaotic Spiral Galaxies

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George Contopoulos, Mirella Harsoula
Abstract: We study the role of asymptotic curves in supporting the spiral structure of a N-body model simulating a barred spiral galaxy. Chaotic orbits with initial conditions on the unstable asymptotic curves of the main unstable periodic orbits follow the shape of the periodic orbits for an initial interval of time and then they are diffused outwards supporting the spiral structure of the galaxy. Chaotic orbits having small deviations from the unstable periodic orbits, stay close and along the corresponding unstable asymptotic manifolds, supporting the spiral structure for more than 10 rotations of the bar. Chaotic orbits of different Jacobi constants support different parts of the spiral structure. We also study the diffusion rate of chaotic orbits outwards and find that chaotic orbits that support the outer parts of the galaxy are diffused outwards more slowly than the orbits supporting the inner parts of the spiral structure…..

Conclusions
The main conclusions of our paper are the following:
1) Stickiness of chaotic orbits close to the unstable asymptotic manifolds of various periodic orbits delays the diffusion of these orbits outwards and therefore modulates the shape of the spiral structure of the galaxy for more than 10 rotations of the bar, corresponding to 1/3 of the Hubble time.
2) Chaotic orbits that are limited outside corotation modulate the outer parts of the spiral structure for smaller values of Jacobi constant while orbits with greater values of Jacobi constant modulate the inner parts of the spiral structure. Moreover, in our N-body model, stickiness to resonances for smaller values of Jacobi constants lasts for longer times than stickiness for greater values of Jacobi constants.
3) Asymptotic orbits (having initial conditions on the unstable asymptotic curve of an unstable periodic orbit) stay located close to the periodic orbit for an initial interval of time, following the shape of this specific orbit, before diffusing from it and supporting the spiral structure. Chaotic orbits having initial conditions inside corotation modulate the envelope of the bar and the innermost spiral structure during a time interval of fast diffusion (≈ 1/3 of the Hubble time) and then they are diffused outwards with much slower rates.
4) Using a sample of sticky chaotic orbits close to a number of unstable periodic orbits inside and outside corotation, in different energy levels, we are able to reproduce quite well the outer envelope of the bar and the spiral structure of the galaxy.

Read more: http://arxiv.org/abs/1108.5958

Written by physicsgg

August 31, 2011 at 11:07 am

Posted in ASTRONOMY

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