- 3001 - GALAXIES - collide. The Milky Way is not a static object. Things are changing rapidly everywhere. To peer back to the galaxy’s earliest days, astronomers seek stars that were around back then. These stars were fashioned only from hydrogen and helium, the universe’s rawest materials. Fortunately, the smaller stars from this early stock are also slow to burn, so many are still shining after 13 billion years-
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- When the Khoisan hunter-gatherers of sub-Saharan Africa gazed upon the meandering trail of stars and dust that split the night sky, they saw the “embers of a campfire“. Polynesian sailors perceived a “cloud-eating shark“. The ancient Greeks saw a “stream of milk“, gala, which would eventually give rise to the modern term “galaxy.”
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- In the 20th century, astronomers discovered that our silver river is just one piece of a vast island of stars, and they penned their own galactic origin story. In the simplest telling, it held that our Milky Way galaxy came together nearly 14,000,000,000 years ago when enormous clouds of gas and dust coalesced under the force of gravity.
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- Over time, two structures emerged: first, a vast spherical “halo,” and later, a dense, bright “disk‘. Billions of years after that, our own solar system spun into being inside this disk, so that when we look out at night, we see spilt milk, an edge-on view of the disk splashed across the sky. Enjoy those starry nights!
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- On April 25, 2018, a European spacecraft by the name of “Gaia” released a staggering quantity of information about this very sky. Critically, Gaia’s years-long data set described the detailed motions of roughly 1 billion stars. Previous surveys had mapped the movement of just thousands.
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- The data brought a previously static swath of the galaxy to life. Astronomers raced to download the dynamic star map, and a flurry of discoveries followed. They found that parts of the disk appeared impossibly ancient. They also found evidence of epic collisions that shaped the Milky Way’s violent youth, as well as new signs that the galaxy continues to churn in an unexpected way.
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- After decades of surveys, researchers had assembled a catalog of 42 such ancient stars, known as “ultra metal-poor” stars, any atom bulkier than helium qualifies as metallic. According to the standard story of the Milky Way, these stars should be swarming throughout the halo, the first part of the galaxy to form.
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- By contrast, stars in the disk, which was thought to have taken perhaps an additional billion years to spin itself flat, should be contaminated with heaver elements such as carbon and oxygen.
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- The stars are long-term residents of the disk, and not just tourists passing through. From two recent surveys amassed a library of roughly 5,000 ‘metal-poor’ stars. A few hundred of them appear to be permanent residents of the disk.
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- Another group sifted through about 500 stars identified by another survey, finding that about 1 in 10 of these stars lie flat in circular, sunlike orbits.
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- And a third research group found stars of various metallic ties, and therefore various ages, moving in flat disk orbits.
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- How did they get to their respective locations? Perhaps pockets of pristine gas managed to dodge all the metals expelled from supernovas for eons, then collapsed to form stars that looked deceptively old. Or, the disk may have started taking shape when the halo did, nearly 1 billion years ahead of schedule.
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- How did those ancient stars get into the disk? Simply put, they were stellar immigrants. Some of them were born in clouds that predated the Milky Way. Then the clouds just happened to deposit some of their stars into orbits that would eventually form part of the galactic disk. Other stars came from small “dwarf” galaxies that slammed into the Milky Way and aligned with an emerging disk.
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- With Gaia, astronomers have found direct evidence of these cataclysmic collisions. The galactic wreckage was everywhere. Perhaps half of all the stars in the inner 60,000 light-years of the halo, which extends hundreds of thousands of light-years in every direction, came from this lone collision, which may have boosted the young Milky Way’s mass by as much as 10%.
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- The astronomers named the incoming galaxy “Gaia-Enceladus“, after the Greek goddess Gaia, one of the primordial deities, and her Titan son Enceladus. When the Milky Way and Gaia-Enceladus collided, perhaps 10 billion years ago, the Milky Way’s delicate disk may have suffered widespread damage.
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- Astronomers debate why our galactic disk seems to have two parts: a thin disk, and a thicker one where stars bungee up and down while orbiting the galactic center. Evidence now suggests that Gaia-Enceladus exploded much of the disk, puffing it up during the collision.
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- All these mergers have led some astronomers to suggest that the halo may be made almost exclusively of immigrant stars. Models from the 1960s and ’70s predicted that most Milky Way halo stars should have formed in place. But as more and more stars have been identified as galactic interlopers, astronomers may not need to assume that many, if any, stars are natives.
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- The Milky Way has enjoyed a relatively quiet history in recent eons, but newcomers continue to stream in. Stargazers in the Southern Hemisphere can spot with the naked eye a pair of dwarf galaxies called the Large and Small Magellanic Clouds. Astronomers long believed the pair to be our steadfast orbiting companions, like moons of the Milky Way.
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- Then a series of Hubble Space Telescope observations between 2006 and 2013 found that they were more like incoming meteorites than orbiting moons. Astronomer clocked the clouds as coming in hot at about 330 kilometers per second, or , 738,200 miles per hour, early twice as fast as had been predicted.
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- The speedy clouds must be extremely massive, perhaps 10 times more than previously thought. Astronomers have predicted that the unexpectedly beefy dwarfs might be dragging parts of the Milky Way around.
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- How to neutralize the motions of the Earth and the sun, and how to average out the motion of halo stars so that the halo’s outer fringe could serve as a stationary backdrop.
When astronomers calibrated the data in this way, they found that the Earth, the sun, and the rest of the disk in which they sit are lurching in one direction. Not toward the Large Magellanic Cloud’s current position, but toward its position around a billion years ago.
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- The sliding of the disk against the halo undermines a fundamental assumption: that the Milky Way is an object in balance. It may spin and slip through space, but most astronomers assumed that after billions of years, the mature disk and the halo had settled into a stable configuration.
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- This analysis proves that assumption wrong. Even after 14 billion years, mergers continue to sculpt the overall shape of the galaxy. This realization is just the latest change in how we understand the great “stream of milk” across the sky.
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- When three galaxies collide, what happens to the central black holes growing at the cores of each? A new study using NASA's Chandra X-ray Observatory and several other telescopes reveals new information about how many black holes remain after these galactic smash ups.
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- Astronomers want to learn more about galactic collisions because the subsequent mergers are a key way that galaxies and the giant blackholes in their cores grow over cosmic time.
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- Astronomers have identified a triple galaxy merger systems by cross-matching the archives of data publicly available in NASA's “WISE mission” and the “Sloan Digital Sky Survey “(SDSS) to the “Chandra” archive. Using this method they found seven triple galaxy mergers located between 370 million and one billion light years from Earth.
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- What happens to their supermassive blackholes when galaxies collide? How about three galaxies swirling into one another until they collide and they all have supermassive blackholes at their core. Astronomers have found 7 separate systems that met those criteria.
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- The systems in question ranged between 370 million and one billion light years away, so any light coming from them is obviously very difficult to detect. X-rays are the wavelength most descriptive of blackhole interactions due to their ability to detect material being absorbed into the black hole (and being heated to millions of degrees in the process).
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- The Chandra X-ray observatory was used to observe these seven systems. It is in fact impossible for two supermassive blackholes to get close enough to one another to effectively merge. If they do in fact merge, there is the additional question of whether they emit gravitational waves.
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- The theory surrounding the merger of these colossal objects states that if there are only two in a system, they might not be able to get close enough to each other to merge. However, adding a third black hole to the system can destabilize the system, allowing two or more to merge, in a supermassive version of the “three-body problem“.
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- What they found was a mix of outcomes ranging from a system with three separate blackholes still existing to a system with no x-ray emissions from blackholes at all. Other results included four systems with two growing supermassive blackholes, and one that had a single supermassive blackhole in it.
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- It’s not clear yet what these findings will mean for understanding the further growth and evolution of black holes in such extreme environments as a trifecta or merging galaxies. As material falls toward a black hole, it gets heated to millions of degrees and produces X-rays.
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- Chandra, with its sharp X-ray vision, is ideal for detecting growing supermassive blackholes in mergers. The associated X-ray sources are challenging to detect because they are usually close together in images and are often faint.
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- Out of the seven triple galaxy mergers there is one with a single growing supermassive blackhole, four with double growing supermassive black holes, and one that is a triple. The final triple merger they studied seems to have struck out with no X-ray emission detected from the supermassive black holes. In the systems with multiple black holes, the separations between them range between about 10,000 and 30,000 light years.
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- Infrared data from the ‘WISE” mission, the “Infrared Astronomical Satellite“, and the “Two Micron All Sky Telescope” ere used to see how quickly stars are forming in the different galaxies in their survey. This allowed them to estimate how many of the detected X-rays are likely to come from X-ray emitting systems containing massive stars, rather than a growing supermassive black hole.
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- Studies of triple mergers can help scientists understand whether pairs of supermassive blackholes can approach so close to each other that they make ripples in spacetime called gravitational waves. The energy lost by these waves will inevitably cause the blackholes to merge.
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- The “Laser Interferometer Gravitational Wave Observatory” (LIGO) and “Virgo array” in Europe have shown astronomers that stellar-mass black holes create gravitational waves and merge, but it is not known if supermassive black holes do.
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- However, gravitational interactions from a third supermassive black hole may prevent this stalling process. Studies of supermassive black holes in systems where three galaxies are merging are therefore important for understanding whether the nightmare scenario might apply.
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------------- Other materials about the findings are available at: http://chandra.si.edu
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- 2998 - GALAXIES - when galaxies collide? What happens to the central black holes growing at the cores of each? A new study using the Chandra X-ray Observatory and several other telescopes reveals new information about how many black holes remain after these galactic smash ups.
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- 2930 - GALAXIES - how many galaxies is the Universe? The universe is an immensely large place. Even distances between the nearest objects are staggering, and the distances across the Milky Way Galaxy and certainly between galaxies in the universe are astonishingly huge to living beings stuck on a planet
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- 2926 - GALAXY ROTATION - can Dark Matter explain it? - ‘Dark’ matter was first proposed to explain the anomaly observed in the rotational velocities of galaxies. The observed rotational velocities of the gas and dust at the outer edges of a galaxy is rotating just as fast as the gas and dust near its center. Not what astronomers expected!
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- 2340 - About half the mass of the Milky is inside the Sun's orbit and half is outside. This fact has proven to be a real problem for astronomers. The inside orbit is much, much denser with visible light than the outside orbits. The mass of the outside orbits is larger than we can visible see. This fact has caused astronomers to invent another object in the Universe, Dark Matter
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- 2283 - Galaxies are structures of stars, and they in turn are structures of the Universe. The Universe forms itself into filaments and treads that are all interconnecting with galaxies at the nodes where the filaments come together
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- 2227 - How did galaxies form? Large galaxies devour smaller galaxies. The estimate of the total number of galaxies in the Observable Universe is 2 trillion in a volume 27.7 billion lightyears across.
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- 2062 - All disk galaxies complete one rotation about once every billion years. Stars orbiting the edges of galaxies have the same orbital periods. It is like a giant 78 RPM vinyl record. But, this structure is not following mathematical laws that require orbital periods sweep out equal areas in equal times. The planets orbiting the Sun follow this mathematical law, Kepler’s Law. But, galaxies do not?
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- 2060 - Galaxies are the structure of the Universe. Each individual galaxy has an ever expanding velocity, riding on a wave of space. This is the result of a fight between gravity and space expanding, but gravity falls off at the square of distance. Expanding space is constant mysterious force that is the same regardless of separating distances. It wins the battle.
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- 2046 - How do galaxies form and grow? Blackholes shape the cosmos. How does Dark Matter enter into this picture?
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January 24, 2021 GALAXIES - collide 3001
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