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Planets Could Travel Along with Rogue ‘Hypervelocity’ Stars, Spreading Life Throughout the Universe

An artist's conception of a hypervelocity star that has escaped the Milky Way. Credit: NASA

Back in 1988, astronomer Jack Hills predicted a type of “rogue”star might exist that is not bound to any particular galaxy. These stars, he reasoned, were periodically ejected from their host galaxy by some sort of mechanism to begin traveling through interstellar space.

Since that time, astronomers have made numerous discoveries that indicate these rogue, traveling stars indeed do exist, and far from being an occasional phenomenon, they are actually quite common. What’s more, some of these stars were found to be traveling at extremely high speeds, leading to the designation of hypervelocity stars (HVS).

And now, in a series of papers that published in arXiv Astrophysics, two Harvard researchers have argued that some of these stars may be traveling close to the speed of light. Known as semi-relativistic hypervelocity stars (SHS), these fast-movers are apparently caused by galactic mergers, where the gravitational effect is so strong that it fling stars out of a galaxy entirely. These stars, the researchers say, may have the potential to spread life throughout the Universe.

This finding comes on the heels of two other major announcements. The first occurred in early November when a paper published in the Astrophysical Journal reported that as many as 200 billion rogue stars have been detected in a cluster of galaxies some 4 billion light years away. These observations were made by the Hubble Space Telescope’s Frontier Fields program, which made ultra-deep multiwavelength observations of the Abell 2744 galaxy cluster.

This was followed by a study published in Science, where an international team of astronomers claimed that as many as half the stars in the entire universe live outside of galaxies.

Using ESO's Very Large Telescope, astronomers have recorded a massive star moving at more than 2.6 million kilometres per hour. Stars are not born with such large velocities. Its position in the sky leads to the suggestion that the star was kicked out from the Large Magellanic Cloud, providing indirect evidence for a massive black hole in the Milky Way's closest neighbour. Credit: ESO
Image of a moving star captured by the ESO Very Large Telescope, believed to have been ejected from the Large Magellanic Cloud. Credit: ESO

However, the recent observations made by Abraham Loeb and James Guillochon of Harvard University are arguably the most significant yet concerning these rogue celestial bodies. According to their research papers, these stars may also play a role in spreading life beyond the boundaries of their host galaxies.

In their first paper, the researchers trace these stars to galaxy mergers, which presumably lead to the formation of massive black hole binaries in their centers. According to their calculations, these supermassive black holes (SMBH) will occasionally slingshot stars to semi-relativistic speeds.

“We predict the existence of a new population of stars coasting through the Universe at nearly the speed of light,” Loeb told Universe Today via email. “The stars are ejected by slingshots made of pairs of massive black holes which form during mergers of galaxies.”

These findings have further reinforced that massive compact bodies, widely known as a supermassive black holes (SMBH), exist at the center of galaxies. Here, the fastest known stars exist, orbiting the SMBH and accelerating up to speeds of 10,000 km per second (3 percent the speed of light).

According to Leob and Guillochon, however, those that are ejected as a result of galactic mergers are accelerated to anywhere from one-tenth to one-third the speed of light (roughly 30,000 – 100,000 km per second).

Image of a hypervelocity star found in data from the Sloan Digital Sky Survey. Credit: Vanderbilt University
Image of a hypervelocity star found in data from the Sloan Digital Sky Survey. Credit: Vanderbilt University

Observing these semi-relativistic stars could tell us much about the distant cosmos, according to the Harvard researchers. Compared to conventional research, which relied on subatomic particles like photons, neutrinos, and cosmic rays from distant galaxies, studying ejected stars offers numerous advantages.

“Traditionally, cosmologists used light to study the Universe but objects moving less than the speed of light offer new possibilities,” said Loeb. “For example, stars moving at different speeds allow us to probe a distant source galaxy at different look-back times (since they must have been ejected at different times in order to reach us today), in difference from photons that give us just one snapshot of the galaxy.”

In their second paper, the researchers calculate that there are roughly a trillion of these stars out there to be studied. And given that these stars were detected thanks to the Spitzer Space Telescope, it is likely that future generations will be able to study them using more advanced equipment.

All-sky infrared surveys could locate thousands of these stars speeding through the cosmos. And spectrographic analysis could tell us much about the galaxies they came from.

But how could these fast moving stars be capable of spreading life throughout the cosmos?

Could an alien spore really travel light years between different star systems? Well, as long as your theory doesn't require it to still be alive when it arrives - sure it can.
The Theory of Panspermia argues that life is distributed throughout the universe by celestial objects. Credit: NASA/Jenny Mottar

“Tightly bound planets can join the stars for the ride,” said Loeb. “The fastest stars traverse billions of light years through the universe, offering a thrilling cosmic journey for extra-terrestrial civilizations. In the past, astronomers considered the possibility of transferring life between planets within the solar system and maybe through our Milky Way galaxy. But this newly predicted population of stars can transport life between galaxies across the entire universe.”

The possibility that traveling stars and planets could have been responsible for the spread of life throughout the universe is likely to have implications as a potential addition to the Theory of Panspermia, which states that life exists throughout the universe and is spread by meteorites, comets, asteroids.

But Loeb told Universe Today that a traveling planetary system could have potential uses for our species someday.

“Our descendants might contemplate boarding a related planetary system once the Milky Way will merge with its sister galaxy, Andromeda, in a few billion years,” he said.

Further Reading: arxiv.org/1411.5022, arxiv.org/1411.5030

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Black Holes, Fermi Bubbles and the Milky Way

Deep at the heart of our galaxy lurks a black hole. This isn’t exciting news, but neither is it a very exciting place. Or is it? While all might be quiet on the western front now, there may be evidence that our galactic center was once home to some pretty impressive activity – activity which may have included multiple collision events and mergers of black holes as it gorged on a satellite galaxies. Thanks to new insights from a pair of assistant professors, Kelly Holley-Bockelmann at Vanderbilt and Tamara Bogdanovic at Georgia Institute of Technology, we have more evidence which points to the Milky Way’s incredibly active past.

“Tamara and I had just attended an astronomy conference in Aspen, Colorado, where several of these new observations were announced,” said Holley-Bockelmann. “It was January 2010 and a snow storm had closed the airport. We decided to rent a car to drive to Denver. As we drove through the storm, we pieced together the clues from the conference and realized that a single catastrophic event – the collision between two black holes about 10 million years ago – could explain all the new evidence.”

Now, imagine a night sky illuminated by a a huge nebula, one that covers half the celestial sphere. This isn’t a dream, it’s a reality. These massive lobes of high-energy radiation are known as Fermi bubbles and they cover a region some 30,000 light years on either side of the Milky Way’s core. While we can’t observe them directly in visible light, these particles are moving along at close to 186,000 miles per second and glowing in x-ray and gamma ray wavelengths.

According to Fulai Guo and William G. Mathews of the University of California at Santa Cruz: “The Fermi bubbles provide plausible evidence for a recent powerful AGN jet activity in our Galaxy, shedding new insights into the origin of the halo CR population and the channel through which massive black holes in disk galaxies release feedback energy during their growth.”

However, our galactic center is home to more than just some incredible bubbles – it’s the location of three of the most massive clusters of young stars within the Milky Way’s realm. Known as the Central, Arches and Quintuplet clusters, each grouping houses several hundred hot, young stars which dwarf the Sun. They will live short, bright, violent lives… burning out in a scant few million years. Because they live fast and die young, these cluster stars must have formed within recent years during a eruption of star formation near the galactic center – another clue to this cosmic puzzle.

“Because of their high mass, and apparent top-heavy IMF, the Galactic Center clusters contain some of the most massive stars in the Galaxy. This is important, as massive stars are key ingredients and probes of astrophysical phenomena on all size and distance scales, from individual star formation sites, such as Orion, to the early Universe during the age of reionization when the first stars were born. As ingredients, they control the dynamical and chemical evolution of their local environs and individual galaxies through their influence on the energetics and composition of the interstellar medium.” says Donald F. Figer. “They likely play an important role in the early evolution of the first galaxies, and there is evidence that they are the progenitors of the most energetic explosions in the Universe, seen as gamma ray bursts. As probes, they define the upper limits of the star formation process and their presence likely ends further formation of nearby lower mass stars. They are also prominent output products of galactic mergers, starburst galaxies, and active galactic nuclei.”

To deepen the mystery, take a closer look at our central black hole. It spans about 40 light seconds in diameter and weighs about four million solar masses. According to what we know, this should produce intensive gravitational tides – ones that should be sucking in the surroundings. So how is it that astronomers have uncovered groups of new, bright stars closer than 3 light years from the event horizon? Of course, they could be on their way to oblivion, but the data shows these stars seem to have formed there. That’s quite a feat considering it would require a molecular cloud 10,000 times more dense than the one located at our galactic center! Shouldn’t there also be old stars located there as well? The answer is yes, there should be… but there are far fewer than what we can observe and what current theoretical models predict.

Holley-Bockelmann wasn’t about to let the problem rest. When she returned home, she enlisted the aid of Vanderbilt graduate student Meagan Lang to help solve the riddle. Then they recruited Pau Amaro-Seoane from the Max Planck Institute for Gravitational Physics in Germany, Alberto Sesana from the Institut de Ciències de l’Espai in Spain, and Vanderbilt Research Assistant Professor Manodeep Sinha to help. With so many bright minds to help solve this riddle, they soon arrived at a plausible explanation – one which matches observations and allows for testable predictions.

According to their theory, a Milky Way satellite galaxy began migrating towards our core. As it merged with our galaxy, its mass was torn away, leaving only its black hole and a small collection of gravitationally bound stars. After several million years, this “leftover” eventually reached the galactic center and the black holes began to merge. As the smaller black hole was swirled around the larger, it plowed up huge furrows of gas and dust, pushing it into the larger black hole and created the Fermi bubbles. The dueling gravitational forces weren’t gentle… these intense tides were quite capable of compressing the molecular clouds surrounding the core into the density required to produce fresh, young stars. Perhaps the very young stars we now observe at the galactic center?

However, there’s more to the picture than meets the eye. This same plowing of the cosmic turf would have also pushed out existing older stars from the vicinity of the massive central black hole. It’s a scene which fits current models where a black hole merger flings stars out into the galaxy at hyper velocities… a scene which fits the observation of a lack of old stars at the boundaries of our supermassive black hole.

“The gravitational pull of the satellite galaxy’s black hole could have carved nearly 1,000 stars out of the galactic centre,” said Bogdanovic. “Those stars should still be racing through space, about 10,000 light years away from their original orbits.”

Can any of this be proved? The answer is yes. Thanks to large scale surveys like the Sloan Digital Sky Survey, we should be able to pinpoint stars moving at a higher velocity than stars which haven’t been subjected to a similar interaction. If astronomers like Holley-Bockelmann and Bogdanovic look at the hard evidence, they are likely to discover a credible number of high velocity stars which will validate their Milky Way merger model.

Or are they just blowing bubbles?

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Galactic Gong – Milky Way Struck and Still Ringing After 100 Million Years

Small Magellanic Cloud
Small Magellanic Cloud

When galaxies collide, stars are thrown from orbits, spiral arms are stretched and twisted, and now scientists say galaxies ring like a bell long after the cosmic crash.

A team of astronomers from the United States and Canada say they have heard echoes of that ringing, possible evidence of a galactic encounter 100 million years ago when a small satellite galaxy or dark matter object passed through the Milky Way Galaxy; close to our position in the galaxy, as if a rock were thrown into a still pond causing the stars to bounce up and down on the waves. Their results were published in the Astrophysical Journal Letters.

“We have found evidence that our Milky Way had an encounter with a small galaxy or massive dark matter structure perhaps as recently as 100 million years ago,” said Larry Widrow, professor at Queen’s University in Canada. “We clearly observe unexpected differences in the Milky Way’s stellar distribution above and below the Galaxy’s midplane that have the appearance of a vertical wave — something that nobody has seen before.”

Astronomers took observations from about 300,000 nearby stars in the Sloan Digital Sky Survey. Stars move up and down at 20-30 kilometers per second while see-sawing around the galaxy at 220 kilometers per second. By comparison, the International Space Station putters around Earth at 7.71 kilometers per second; Voyager 1, the fastest man-made object, currently is leaving the solar system at about 17.46 kilometers per second. Widrow and colleagues at the University of Kentucky, The University of Chicago and Fermi National Accelerator Laboratory found that the positions of nearby stars is not quite as regular as previously thought. The team noticed a small but statistically significant difference in the distribution of stars above and below the midplane of the Milky Way.

“Our part of the Milky Way is ringing like a bell,” said Brian Yanny, of the Department of Energy’s Fermilab. “But we have not been able to identify the celestial object that passed through the Milky Way. It could have been one of the small satellite galaxies that move around the center of our galaxy, or an invisible structure such as a dark matter halo.”

Susan Gardner, professor of physics at the University of Kentucky added, “The perturbation need not have been a single isolated event in the past, and it may even be ongoing. Additional observations may well clarify its origin.”

Other possibilities considered for the variations were the effect of interstellar dust or simply the way the stars were selected in the survey. But as those events failed to explain fully the observations, the astronomers began to explore possible recent events in the history of the galaxy.

More than 20 visible satellite galaxies circle the Milky Way. Invisible satellites made up of dark matter, hypothetical matter that cannot be seen but is thought to make up a majority of the mass of the Universe, might also orbit our galaxy. Scientists believe that most of the mass orbiting the galaxy is in the form of dark matter. Using computer simulations to explore the effects of a small galaxy or dark matter structure passing through the disk of the Milky Way, the scientists developed a clearer picture of the see-saw effects they were seeing.

In terms of the nine-billion lifetime of the Milky Way Galaxy, the effects are short-lived. This part of the galaxy has been “ringing” for 100 million years and will continue for 100 million years more as the up-and-down motion dissipates, say the astronomers – unless we are hit again.

Image caption: The Small Magellanic Cloud is one of 20 visible satellite galaxies that orbit the Milky Way Galaxy. Astronomers report that a smaller counterpart or dark matter object passed through the Milky Way near our position about 100 million years ago.

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Galactic Archaeology: NGC 5907 – The Dragon Clash

NGC 5907 - Credit: R. Jay Gabany

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The sprawling northern constellation of Draco is home to a monumental galactic merger which left a singular spectacle – NGC 5907. Surrounded by an ethereal garment of wispy star trails and currents of stellar material, this spiral galaxy is the survivor of a “clash of the dragons” which may have occurred some 8 to 9 billion years ago. Recent theory suggests galaxies of this type may be the product of a larger galaxy encountering a smaller satellite – but this might not be the case. Not only is NGC 5907 a bit different in some respects, it’s a lot different in others… and peculiar motion is just the beginning.

“If the disc of many spirals is indeed rebuilt after a major merger, it is expected that tidal tails can be a fossil record and that there should be many loops and streams in their halos. Recently Martínez-Delgado et al. (2010) have conducted a pilot survey of isolated spiral galaxies in the Local Volume up to a low surface brightness sensitivity of ~28.5 mag/arcsec2 in V band. They find that many of these galaxies have loops or streams of various shapes and interpret these structures as evidence of minor merger or satellite infall.” says J. Wang of the Chinese Academy of Sciences. “However, if these loops are caused by minor mergers, the residual of the satellite core should be detected according to numerical simulations. Why is it hardly ever detected?”

The “why” is indeed the reason NGC 5907 is being intensively studied by a team of six scientists of the Observatoire de Paris, CNRS, Chinese Academy of Sciences, National Astronomical Observatories of China NAOC and Marseille Observatory. Even though NGC 5907 is a member of a galactic group, there are no galaxies near enough to it to be causing an interaction which could account for its streamers of stars. It is truly a warped galaxy with gaseous and stellar disks which extend beyond the nominal cut-off radius. But that’s not all… It also has a peculiar halo which includes a significant fraction of metal enriched stars. NGC 5907 just doesn’t fit the patterns.

“For some of our models, we assume a star formation history with a varying global efficiency in transforming gas to stars, in order to preserve enough gas from being consumed before fusion.” explains the research team. “Although this fine-tuned star formation history may have some physical motivations, its main role is also to ensure the formation of stars after the emergence of the gaseous disc just after fusion.”

On left, the NGC 5907 galaxy. It is compared to the simulations, on right. Both cases show an edge-on galactic disk surrounded by giant loops of old stars, which are witnessing of a former, gigantic collision. (Jay Gabany, cosmotography.com / Observatoire de Paris / CNRS / Pythéas / NAOC)

Now enter the 32- and 196-core computers at the Paris Observatory center and the 680-core Graphic Processor Unit supercomputer of Beijing NAOC with the capability to run 50000 billion operations per second. By employing several state of the art, hydrodynamical, and numerical simulations with particle numbers ranging from 200 000 to 6 millions, the team’s goal was to show the structure of NGC 5907 may have been the result of the clash of two dragon-sized galaxies… or was it?

“The exceptional features of NGC 5907 can be reproduced, together with the central galaxy properties, especially if we compare the observed loops to the high-order loops expected in a major merger model.” says Wang. “Given the extremely large number of parameters, as well as the very numerous constraints provided by the observations, we cannot claim that we have already identified the exact and unique model of NGC 5907 and its halo properties. We nevertheless succeeded in reproducing the loop geometry, and a disc-dominated, almost bulge-less galaxy.”

In the meantime, major galaxy merger events will continue to be a top priority in formation research. “Future work will include modelling other nearby spiral galaxies with large and faint, extended features in their halos.” concludes the team. “These distant galaxies are likely similar to the progenitors, six billion years ago, of present-day spirals, and linking them together could provide another crucial test for the spiral rebuilding disc scenario.”

And sleeping dragons may one day arise…

Original Story Source: Paris Observatory News. For Further Reading: Loops formed by tidal tails as fossil records of a major merger and Fossils of the Hierarchical Formation of the Nearby Spiral Galaxy NGC 5907.

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Galaxy Bets On A Pair Of Black Holes

How X-rays Work
This main image is a composite of X-rays from Chandra (blue) and optical data from the Hubble Space Telescope (gold) of the spiral galaxy NGC 3393. Meanwhile, the inset box shows the central region of NGC 3993 as observed just by Chandra.

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About 160 million light years away in the constellation of Hydra, spiral galaxy NGC 3393 has been keeping a billion year old secret. It might have a poker face, but it has a pair of black holes up its sleeve…

Using information obtained through NASA’s Chandra X-ray Observatory combined with Hubble Space Telescope imaging, scientists have uncovered first time evidence that NGC 3393 is harboring twin supermassive black holes. Residing only 490 light years apart, the duo may have been the product of a “minor merger” – where a small and large galaxy met. Although the hypothesis of two black holes within one galaxy isn’t new, it has been difficult to prove because the results of two galaxies combining material would result in a rather ordinary looking spiral.

“The current picture of galaxy evolution advocates co-evolution of galaxies and their nuclear massive black holes, through accretion and galactic merging.” says G. Fabbiano, lead author of a recent Nature paper. “Pairs of quasars, each with a massive black hole at the centre of its galaxy, have separations of 6,000 to 300,000 light years and exemplify the first stages of this gravitational interaction.”

If scientific calculations are correct, a smaller galaxy should have contained a smaller mass black hole. This leaves us with an odd situation. If both of these newly discovered black holes have similar mass, shouldn’t the merging pair also be of similar mass? If so, how could a minor merger be the answer?

“The final stages of the black-hole merging process, through binary black holes and final collapse into a single black hole with gravitational wave emission, are consistent with the sub-light-year separation inferred from the optical spectra and light-variability of two such quasars. The double active nuclei of a few nearby galaxies with disrupted morphology and intense star formation demonstrate the importance of major mergers of equal-mass spiral galaxies in this evolution.” says Fabbiano. “Minor mergers of a spiral galaxy with a smaller companion should be a more common occurrence, evolving into spiral galaxies with active massive black-hole pairs, but have hitherto not been seen. The regular spiral morphology and predominantly old circum-nuclear stellar population of this galaxy, and the closeness of the black holes embedded in the bulge, provide a hitherto missing observational point to the study of galaxy/black hole evolution.”

Lay down your bets, gentlemen… It seems the game changes each time it is played!

Original Story Source: Chandra News. For Further Reading: A close nuclear black-hole pair in the spiral galaxy NGC 3393.